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| 1 <!DOCTYPE html> |
| 2 <html lang="en"> |
| 3 <head> |
| 4 <meta http-equiv="Content-Type" content="text/html; charset=UTF-8"> |
| 5 <meta name="generator" content="AsciiDoc 8.6.8"> |
| 6 <title>Dart Cookbook</title> |
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| 635 |
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| 729 </script> |
| 730 </head> |
| 731 <body class="article"> |
| 732 <div id="header"> |
| 733 <h1>Dart Cookbook</h1> |
| 734 <span id="author">Shailen Tuli</span><br> |
| 735 <div id="toc"> |
| 736 <div id="toctitle">Table of Contents</div> |
| 737 <noscript><p><b>JavaScript must be enabled in your browser to display the tabl
e of contents.</b></p></noscript> |
| 738 </div> |
| 739 </div> |
| 740 <div id="content"> |
| 741 <div class="sect1"> |
| 742 <h2 id="_strings">Strings</h2> |
| 743 <div class="sectionbody"> |
| 744 <div class="sect2"> |
| 745 <h3 id="_concatenating_strings">Concatenating Strings</h3> |
| 746 <div class="sect3"> |
| 747 <h4 id="_problem">Problem</h4> |
| 748 <div class="paragraph"><p>You want to concatenate strings in Dart. You tried usi
ng <span class="monospaced">+</span>, but that |
| 749 resulted in an error.</p></div> |
| 750 </div> |
| 751 <div class="sect3"> |
| 752 <h4 id="_solution">Solution</h4> |
| 753 <div class="paragraph"><p>Use adjacent string literals:</p></div> |
| 754 <div class="listingblock"> |
| 755 <div class="content monospaced"> |
| 756 <pre>var fact = 'Dart' 'is' ' fun!'; // 'Dart is fun!'</pre> |
| 757 </div></div> |
| 758 </div> |
| 759 <div class="sect3"> |
| 760 <h4 id="_discussion">Discussion</h4> |
| 761 <div class="paragraph"><p>Adjacent literals work over multiple lines:</p></div> |
| 762 <div class="listingblock"> |
| 763 <div class="content monospaced"> |
| 764 <pre>var fact = 'Dart' |
| 765 'is' |
| 766 'fun!'; // 'Dart is fun!'</pre> |
| 767 </div></div> |
| 768 <div class="paragraph"><p>They also work when using multiline strings:</p></div> |
| 769 <div class="listingblock"> |
| 770 <div class="content monospaced"> |
| 771 <pre>var lunch = '''Peanut |
| 772 butter''' |
| 773 '''and |
| 774 jelly'''; // 'Peanut\nbutter and\njelly'</pre> |
| 775 </div></div> |
| 776 <div class="paragraph"><p>You can concatenate adjacent single line literals with
multiline |
| 777 strings:</p></div> |
| 778 <div class="listingblock"> |
| 779 <div class="content monospaced"> |
| 780 <pre>var funnyGuys = 'Dewey ' 'Cheatem' |
| 781 ''' and |
| 782 Howe'''; // 'Dewey Cheatem and\n Howe'</pre> |
| 783 </div></div> |
| 784 <div class="sect4"> |
| 785 <h5 id="_alternatives_to_adjacent_string_literals">Alternatives to adjacent stri
ng literals</h5> |
| 786 <div class="paragraph"><p>You can use the <span class="monospaced">concat()</spa
n> method on a string to concatenate it to |
| 787 another string:</p></div> |
| 788 <div class="listingblock"> |
| 789 <div class="content monospaced"> |
| 790 <pre>var film = filmToWatch(); |
| 791 film = film.concat('\n'); // 'The Big Lebowski\n'</pre> |
| 792 </div></div> |
| 793 <div class="paragraph"><p>Because <span class="monospaced">concat()</span> creat
es a new string every time it is invoked, a long |
| 794 chain of <span class="monospaced">concat()</span> s can be expensive. Avoid thos
e. Use a StringBuffer |
| 795 instead (see <em>Incrementally building a string efficiently using a |
| 796 StringBuffer</em>, below).</p></div> |
| 797 <div class="paragraph"><p>Use <span class="monospaced">join()</span> to combine
a sequence of strings:</p></div> |
| 798 <div class="listingblock"> |
| 799 <div class="content monospaced"> |
| 800 <pre>var film = ['The', 'Big', 'Lebowski']).join(' '); // 'The Big Lebowski'</pr
e> |
| 801 </div></div> |
| 802 <div class="paragraph"><p>You can also use string interpolation to concatenate s
trings (see |
| 803 <em>Interpolating expressions inside strings</em>, below).</p></div> |
| 804 </div> |
| 805 </div> |
| 806 </div> |
| 807 <div class="sect2"> |
| 808 <h3 id="_interpolating_expressions_inside_strings">Interpolating expressions ins
ide strings</h3> |
| 809 <div class="sect3"> |
| 810 <h4 id="_problem_2">Problem</h4> |
| 811 <div class="paragraph"><p>You want to embed Dart code inside strings.</p></div> |
| 812 </div> |
| 813 <div class="sect3"> |
| 814 <h4 id="_solution_2">Solution</h4> |
| 815 <div class="paragraph"><p>You can put the value of an expression inside a string
by using |
| 816 ${expression}.</p></div> |
| 817 <div class="listingblock"> |
| 818 <div class="content monospaced"> |
| 819 <pre>var favFood = 'sushi'; |
| 820 var whatDoILove = 'I love ${favFood.toUpperCase()}'; // 'I love SUSHI'</pre> |
| 821 </div></div> |
| 822 <div class="paragraph"><p>You can skip the {} if the expression is an identifier
:</p></div> |
| 823 <div class="listingblock"> |
| 824 <div class="content monospaced"> |
| 825 <pre>var whatDoILove = 'I love $favFood'; // 'I love sushi'</pre> |
| 826 </div></div> |
| 827 </div> |
| 828 <div class="sect3"> |
| 829 <h4 id="_discussion_2">Discussion</h4> |
| 830 <div class="paragraph"><p>An interpolated string, <span class="monospaced">'stri
ng ${expression}</span> is equivalent to the |
| 831 concatenation of the strings <span class="monospaced">string</span> and <span cl
ass="monospaced">expression.toString()</span>. |
| 832 Consider this code:</p></div> |
| 833 <div class="listingblock"> |
| 834 <div class="content monospaced"> |
| 835 <pre>var four = 4; |
| 836 var seasons = 'The $four seasons'; // 'The 4 seasons'</pre> |
| 837 </div></div> |
| 838 <div class="paragraph"><p>This is functionally equivalent to the following:</p><
/div> |
| 839 <div class="listingblock"> |
| 840 <div class="content monospaced"> |
| 841 <pre>var seasons = 'The '.concat(4.toString()).concat(' seasons'); |
| 842 // 'The 4 seasons'</pre> |
| 843 </div></div> |
| 844 <div class="paragraph"><p>You should consider implementing a <span class="monosp
aced">toString()</span> method for user-defined |
| 845 objects. Here’s what happens if you don’t:</p></div> |
| 846 <div class="listingblock"> |
| 847 <div class="content monospaced"> |
| 848 <pre>class Point { |
| 849 num x, y; |
| 850 Point(this.x, this.y); |
| 851 } |
| 852 |
| 853 var point = new Point(3, 4); |
| 854 print('Point: $point'); // "Point: Instance of 'Point'"</pre> |
| 855 </div></div> |
| 856 <div class="paragraph"><p>Probably not what you wanted. Here is the same example
with an explicit |
| 857 <span class="monospaced">toString()</span>:</p></div> |
| 858 <div class="listingblock"> |
| 859 <div class="content monospaced"> |
| 860 <pre>class Point { |
| 861 ... |
| 862 |
| 863 String toString() => 'x: $x, y: $y'; |
| 864 } |
| 865 |
| 866 print('Point: $point'); // 'Point: x: 3, y: 4'</pre> |
| 867 </div></div> |
| 868 </div> |
| 869 </div> |
| 870 <div class="sect2"> |
| 871 <h3 id="_handling_special_characters_within_strings">Handling special characters
within strings</h3> |
| 872 <div class="sect3"> |
| 873 <h4 id="_problem_3">Problem</h4> |
| 874 <div class="paragraph"><p>You want to put newlines, dollar signs, or other speci
al characters in strings.</p></div> |
| 875 </div> |
| 876 <div class="sect3"> |
| 877 <h4 id="_solution_3">Solution</h4> |
| 878 <div class="paragraph"><p>Prefix special characters with a <span class="monospac
ed">\</span>.</p></div> |
| 879 <div class="listingblock"> |
| 880 <div class="content monospaced"> |
| 881 <pre> print(Wile\nCoyote'); |
| 882 // Wile |
| 883 // Coyote</pre> |
| 884 </div></div> |
| 885 </div> |
| 886 <div class="sect3"> |
| 887 <h4 id="_discussion_3">Discussion</h4> |
| 888 <div class="paragraph"><p>Dart designates a few characters as special, and these
can be escaped:</p></div> |
| 889 <div class="ulist"><ul> |
| 890 <li> |
| 891 <p> |
| 892 \n for newline, equivalent to \x0A. |
| 893 </p> |
| 894 </li> |
| 895 <li> |
| 896 <p> |
| 897 \r for carriage return, equivalent to \x0D. |
| 898 </p> |
| 899 </li> |
| 900 <li> |
| 901 <p> |
| 902 \f for form feed, equivalent to \x0C. |
| 903 </p> |
| 904 </li> |
| 905 <li> |
| 906 <p> |
| 907 \b for backspace, equivalent to \x08. |
| 908 </p> |
| 909 </li> |
| 910 <li> |
| 911 <p> |
| 912 \t for tab, equivalent to \x09. |
| 913 </p> |
| 914 </li> |
| 915 <li> |
| 916 <p> |
| 917 \v for vertical tab, equivalent to \x0B. |
| 918 </p> |
| 919 </li> |
| 920 </ul></div> |
| 921 <div class="paragraph"><p>If you prefer, you can use <span class="monospaced">\x
</span> or <span class="monospaced">\u</span> notation to indicate the special |
| 922 character:</p></div> |
| 923 <div class="listingblock"> |
| 924 <div class="content monospaced"> |
| 925 <pre>print('Wile\x0ACoyote'); // Same as print('Wile\nCoyote') |
| 926 print('Wile\u000ACoyote'); // Same as print('Wile\nCoyote')</pre> |
| 927 </div></div> |
| 928 <div class="paragraph"><p>You can also use <span class="monospaced">\u{}</span>
notation:</p></div> |
| 929 <div class="listingblock"> |
| 930 <div class="content monospaced"> |
| 931 <pre>print('Wile\u{000A}Coyote'); // same as print('Wile\nCoyote')</pre> |
| 932 </div></div> |
| 933 <div class="paragraph"><p>You can also escape the <span class="monospaced">$</sp
an> used in string interpolation:</p></div> |
| 934 <div class="listingblock"> |
| 935 <div class="content monospaced"> |
| 936 <pre>var superGenius = 'Wile Coyote'; |
| 937 print('$superGenius and Road Runner'); // 'Wile Coyote and Road Runner' |
| 938 print('\$superGenius and Road Runner'); // '$superGenius and Road Runner'</pre> |
| 939 </div></div> |
| 940 <div class="paragraph"><p>If you escape a non-special character, the <span class
="monospaced">\</span> is ignored:</p></div> |
| 941 <div class="listingblock"> |
| 942 <div class="content monospaced"> |
| 943 <pre>print('Wile \E Coyote'); // 'Wile E Coyote'</pre> |
| 944 </div></div> |
| 945 </div> |
| 946 </div> |
| 947 <div class="sect2"> |
| 948 <h3 id="_incrementally_building_a_string_using_a_stringbuffer">Incrementally bui
lding a string using a StringBuffer</h3> |
| 949 <div class="sect3"> |
| 950 <h4 id="_problem_4">Problem</h4> |
| 951 <div class="paragraph"><p>You want to collect string fragments and combine them
in an efficient |
| 952 manner.</p></div> |
| 953 </div> |
| 954 <div class="sect3"> |
| 955 <h4 id="_solution_4">Solution</h4> |
| 956 <div class="paragraph"><p>Use a StringBuffer to programmatically generate a stri
ng. Consider this code |
| 957 below for assembling a series of urls from fragments:</p></div> |
| 958 <div class="listingblock"> |
| 959 <div class="content monospaced"> |
| 960 <pre>var data = [{'scheme': 'https', 'domain': 'news.ycombinator.com'}, |
| 961 {'domain': 'www.google.com'}, |
| 962 {'domain': 'reddit.com', 'path': 'search', 'params': 'q=dart'} |
| 963 ]; |
| 964 |
| 965 String assembleUrlsUsingStringBuffer(entries) { |
| 966 StringBuffer sb = new StringBuffer(); |
| 967 for (final item in entries) { |
| 968 sb.write(item['scheme'] != null ? item['scheme'] : 'http'); |
| 969 sb.write("://"); |
| 970 sb.write(item['domain']); |
| 971 sb.write('/'); |
| 972 sb.write(item['path'] != null ? item['path'] : ''); |
| 973 if (item['params'] != null) { |
| 974 sb.write('?'); |
| 975 sb.write(item['params']); |
| 976 } |
| 977 sb.write('\n'); |
| 978 } |
| 979 return sb.toString(); |
| 980 } |
| 981 |
| 982 // https://news.ycombinator.com/ |
| 983 // http://www.google.com/ |
| 984 // http://reddit.com/search?q=dart</pre> |
| 985 </div></div> |
| 986 <div class="paragraph"><p>A StringBuffer collects string fragments, but does not
generate a new string |
| 987 until <span class="monospaced">toString()</span> is called.</p></div> |
| 988 </div> |
| 989 <div class="sect3"> |
| 990 <h4 id="_discussion_4">Discussion</h4> |
| 991 <div class="paragraph"><p>Using a StringBuffer is vastly more efficient than con
catenating fragments |
| 992 at each step: Consider this rewrite of the above code:</p></div> |
| 993 <div class="listingblock"> |
| 994 <div class="content monospaced"> |
| 995 <pre>String assembleUrlsUsingConcat(entries) { |
| 996 var urls = ''; |
| 997 for (final item in entries) { |
| 998 urls = urls.concat(item['scheme'] != null ? item['scheme'] : 'http'); |
| 999 urls = urls.concat("://"); |
| 1000 urls = urls.concat(item['domain']); |
| 1001 urls = urls.concat('/'); |
| 1002 urls = urls.concat(item['path'] != null ? item['path'] : ''); |
| 1003 if (item['params'] != null) { |
| 1004 urls = urls.concat('?'); |
| 1005 urls = urls.concat(item['params']); |
| 1006 } |
| 1007 urls = urls.concat('\n'); |
| 1008 } |
| 1009 return urls; |
| 1010 }</pre> |
| 1011 </div></div> |
| 1012 <div class="paragraph"><p>This approach produces the exact same result, but incu
rs the cost of |
| 1013 joining strings multiple times.</p></div> |
| 1014 <div class="paragraph"><p>See the <em>Concatenating Strings</em> recipe for a de
scription of <span class="monospaced">concat()</span>.</p></div> |
| 1015 <div class="sect4"> |
| 1016 <h5 id="_other_stringbuffer_methods">Other StringBuffer methods</h5> |
| 1017 <div class="paragraph"><p>In addition to <span class="monospaced">write()</span>
, the StringBuffer class provides methods to |
| 1018 write a list of strings (<span class="monospaced">writeAll()</span>), write a nu
merical character code |
| 1019 (<span class="monospaced">writeCharCode()</span>), write with an added newline (
<span class="monospaced">writeln()</span>), and |
| 1020 more. The example below shows how to use these methods:</p></div> |
| 1021 <div class="listingblock"> |
| 1022 <div class="content monospaced"> |
| 1023 <pre>var sb = new StringBuffer(); |
| 1024 sb.writeln('The Beatles:'); |
| 1025 sb.writeAll(['John, ', 'Paul, ', 'George, and Ringo']); |
| 1026 sb.writeCharCode(33); // charCode for '!'. |
| 1027 var beatles = sb.toString(); // 'The Beatles:\nJohn, Paul, George, and Ringo!'</
pre> |
| 1028 </div></div> |
| 1029 </div> |
| 1030 </div> |
| 1031 </div> |
| 1032 <div class="sect2"> |
| 1033 <h3 id="_determining_whether_a_string_is_empty">Determining whether a string is
empty</h3> |
| 1034 <div class="sect3"> |
| 1035 <h4 id="_problem_5">Problem</h4> |
| 1036 <div class="paragraph"><p>You want to know whether a string is empty. You tried
<span class="monospaced">if (string) {...}</span>, but |
| 1037 that did not work.</p></div> |
| 1038 </div> |
| 1039 <div class="sect3"> |
| 1040 <h4 id="_solution_5">Solution</h4> |
| 1041 <div class="paragraph"><p>Use <span class="monospaced">string.isEmpty</span>:</p
></div> |
| 1042 <div class="listingblock"> |
| 1043 <div class="content monospaced"> |
| 1044 <pre>var emptyString = ''; |
| 1045 print(emptyString.isEmpty); // true</pre> |
| 1046 </div></div> |
| 1047 <div class="paragraph"><p>You can also just use <span class="monospaced">==</spa
n>:</p></div> |
| 1048 <div class="listingblock"> |
| 1049 <div class="content monospaced"> |
| 1050 <pre>if (string == '') {...} // True if string is empty.</pre> |
| 1051 </div></div> |
| 1052 <div class="paragraph"><p>A string with a space is not empty:</p></div> |
| 1053 <div class="listingblock"> |
| 1054 <div class="content monospaced"> |
| 1055 <pre>var space = ' '; |
| 1056 print(space.isEmpty); // false</pre> |
| 1057 </div></div> |
| 1058 </div> |
| 1059 <div class="sect3"> |
| 1060 <h4 id="_discussion_5">Discussion</h4> |
| 1061 <div class="paragraph"><p>Don’t use <span class="monospaced">if (string)</
span> to test the emptiness of a string. In Dart, all objects |
| 1062 except the boolean true evaluate to false, so <span class="monospaced">if(string
)</span> is always false. You |
| 1063 will see a warning in the editor if you use an <em>if</em> statement with a non-
boolean |
| 1064 in checked mode.</p></div> |
| 1065 </div> |
| 1066 </div> |
| 1067 <div class="sect2"> |
| 1068 <h3 id="_removing_leading_and_trailing_whitespace">Removing leading and trailing
whitespace</h3> |
| 1069 <div class="sect3"> |
| 1070 <h4 id="_problem_6">Problem</h4> |
| 1071 <div class="paragraph"><p>You want to remove spaces, tabs, and other whitespace
from the beginning and |
| 1072 end of strings.</p></div> |
| 1073 </div> |
| 1074 <div class="sect3"> |
| 1075 <h4 id="_solution_6">Solution</h4> |
| 1076 <div class="paragraph"><p>Use <span class="monospaced">string.trim()</span>:</p>
</div> |
| 1077 <div class="listingblock"> |
| 1078 <div class="content monospaced"> |
| 1079 <pre>var space = '\n\r\f\t\v'; // A variety of space characters. |
| 1080 var string = '$space X $space'; |
| 1081 var newString = string.trim(); // 'X'</pre> |
| 1082 </div></div> |
| 1083 <div class="paragraph"><p>The String class has no methods to remove only leading
or only trailing |
| 1084 whitespace. You can always use a RegExp.</p></div> |
| 1085 <div class="paragraph"><p>Remove only leading whitespace:</p></div> |
| 1086 <div class="listingblock"> |
| 1087 <div class="content monospaced"> |
| 1088 <pre>var newString = string.replaceFirst(new RegExp(r'^\s+'), ''); // 'X \n\r\f\
t\v'</pre> |
| 1089 </div></div> |
| 1090 <div class="paragraph"><p>Remove only trailing whitespace:</p></div> |
| 1091 <div class="listingblock"> |
| 1092 <div class="content monospaced"> |
| 1093 <pre>var newString = string.replaceFirst(new RegExp(r'\s+$'), ''); // '\n\r\f\t\
v X'</pre> |
| 1094 </div></div> |
| 1095 </div> |
| 1096 </div> |
| 1097 <div class="sect2"> |
| 1098 <h3 id="_changing_string_case">Changing string case</h3> |
| 1099 <div class="sect3"> |
| 1100 <h4 id="_problem_7">Problem</h4> |
| 1101 <div class="paragraph"><p>You want to change the case of strings.</p></div> |
| 1102 </div> |
| 1103 <div class="sect3"> |
| 1104 <h4 id="_solution_7">Solution</h4> |
| 1105 <div class="paragraph"><p>Use String’s <span class="monospaced">toUpperCas
e()</span> and <span class="monospaced">toLowerCase()</span> methods:</p></div> |
| 1106 <div class="listingblock"> |
| 1107 <div class="content monospaced"> |
| 1108 <pre>var theOneILove = 'I love Lucy'; |
| 1109 theOneILove.toUpperCase(); // 'I LOVE LUCY!' |
| 1110 theOneILove.toLowerCase(); // 'i love lucy!' |
| 1111 |
| 1112 // Zeus in modern Greek. |
| 1113 var zeus = '\u0394\u03af\u03b1\u03c2'; // 'Δίας' |
| 1114 zeus.toUpperCase(); // 'ΔΊΑΣ' |
| 1115 |
| 1116 var resume = '\u0052\u00e9\u0073\u0075\u006d\u00e9'; // 'Résumé' |
| 1117 resume.toLowerCase(); // 'résumé'</pre> |
| 1118 </div></div> |
| 1119 <div class="paragraph"><p>The <span class="monospaced">toUpperCase()</span> and
<span class="monospaced">toLowerCase()</span> methods don’t affect the cha
racters of |
| 1120 scripts such as Devanagri that don’t have distinct letter cases.</p></div> |
| 1121 <div class="listingblock"> |
| 1122 <div class="content monospaced"> |
| 1123 <pre>var chickenKebab = '\u091a\u093f\u0915\u0928 \u0915\u092c\u093e\u092c'; |
| 1124 // 'चिकन कबाब' (in Devanagari) |
| 1125 chickenKebab.toLowerCase(); // 'चिकन कबाब' |
| 1126 chickenKebab.toUpperCase(); // 'चिकन कबाब'</pre> |
| 1127 </div></div> |
| 1128 <div class="paragraph"><p>If a character’s case does not change when using
<span class="monospaced">toUpperCase()</span> and |
| 1129 <span class="monospaced">toLowerCase()</span>, it is most likely because the cha
racter only has one |
| 1130 form.</p></div> |
| 1131 </div> |
| 1132 </div> |
| 1133 <div class="sect2"> |
| 1134 <h3 id="_handling_extended_characters_that_are_composed_of_multiple_code_units">
Handling extended characters that are composed of multiple code units</h3> |
| 1135 <div class="sect3"> |
| 1136 <h4 id="_problem_8">Problem</h4> |
| 1137 <div class="paragraph"><p>You want to use emoticons and other special symbols th
at don’t fit into 16 |
| 1138 bits. How can you create such strings and use them correctly in your code?</p></
div> |
| 1139 </div> |
| 1140 <div class="sect3"> |
| 1141 <h4 id="_solution_8">Solution</h4> |
| 1142 <div class="paragraph"><p>You can create an extended character using <span class
="monospaced">'\u{}'</span> syntax:</p></div> |
| 1143 <div class="listingblock"> |
| 1144 <div class="content monospaced"> |
| 1145 <pre>var clef = '\u{1F3BC}'; // 🎼</pre> |
| 1146 </div></div> |
| 1147 </div> |
| 1148 <div class="sect3"> |
| 1149 <h4 id="_discussion_6">Discussion</h4> |
| 1150 <div class="paragraph"><p>Most UTF-16 strings are stored as two-byte (16 bit) co
de sequences. |
| 1151 Since two bytes can only contain the 65,536 characters in the 0x0 to 0xFFFF |
| 1152 range, a pair of strings is used to store values in the 0x10000 to 0x10FFFF |
| 1153 range. These strings only have semantic meaning as a pair. Individually, they |
| 1154 are invalid UTF-16 strings. The term <em>surrogate pair</em> is often used to |
| 1155 describe these strings.</p></div> |
| 1156 <div class="paragraph"><p>The clef glyph <span class="monospaced">'\u{1F3BC}'</s
pan> is composed of the <span class="monospaced">'\uD83C'</span> and <span class
="monospaced">'\uDFBC'</span> |
| 1157 surrogate pair.</p></div> |
| 1158 <div class="paragraph"><p>You can get an extended string’s surrogate pair
through its <span class="monospaced">codeUnits</span> |
| 1159 property:</p></div> |
| 1160 <div class="listingblock"> |
| 1161 <div class="content monospaced"> |
| 1162 <pre>clef.codeUnits.map((codeUnit) => codeUnit.toRadixString(16)); |
| 1163 // ['d83c', 'dfbc']</pre> |
| 1164 </div></div> |
| 1165 <div class="paragraph"><p>Accessing a surrogate pair member leads to errors, and
you should avoid |
| 1166 properties and methods that expose it:</p></div> |
| 1167 <div class="listingblock"> |
| 1168 <div class="content monospaced"> |
| 1169 <pre>print('\ud83c'); // Error: '\ud83c' is not a valid string. |
| 1170 print('\udfbc'); // Error: '\udfbc' is not a valid string either. |
| 1171 clef.split()[1]; // Error: accessing half of a surrogate pair. |
| 1172 print(clef[i];) // Again, error: accessing half of a surrogate pair.</pre> |
| 1173 </div></div> |
| 1174 <div class="paragraph"><p>When dealing with strings containing extended characte
rs, you should use the |
| 1175 <span class="monospaced">runes</span> getter.</p></div> |
| 1176 <div class="paragraph"><p>To get the string’s length, use <span class="mon
ospaced">string.runes.length</span>. Don’t use |
| 1177 <span class="monospaced">string.length</span>:</p></div> |
| 1178 <div class="listingblock"> |
| 1179 <div class="content monospaced"> |
| 1180 <pre>print(clef.runes.length); // 1 |
| 1181 print(clef.length); // 2 |
| 1182 print(clef.codeUnits.length); // 2</pre> |
| 1183 </div></div> |
| 1184 <div class="paragraph"><p>To get an individual character or its numeric equivale
nt, index the rune list:</p></div> |
| 1185 <div class="listingblock"> |
| 1186 <div class="content monospaced"> |
| 1187 <pre>print(clef.runes.toList()[0]); // 127932 ('\u{1F3BC}')</pre> |
| 1188 </div></div> |
| 1189 <div class="paragraph"><p>To get the string’s characters as a list, map th
e string runes:</p></div> |
| 1190 <div class="listingblock"> |
| 1191 <div class="content monospaced"> |
| 1192 <pre>var clef = '\u{1F3BC}'; // 🎼 |
| 1193 var title = '$clef list:' |
| 1194 print(subject.runes.map((rune) => new String.fromCharCode(rune)).toList()); |
| 1195 // ['🎼', ' ', 'l', 'i', 's', 't', ':']</pre> |
| 1196 </div></div> |
| 1197 </div> |
| 1198 </div> |
| 1199 <div class="sect2"> |
| 1200 <h3 id="_converting_between_characters_and_numerical_codes">Converting between c
haracters and numerical codes</h3> |
| 1201 <div class="sect3"> |
| 1202 <h4 id="_problem_9">Problem</h4> |
| 1203 <div class="paragraph"><p>You want to convert string characters into numerical c
odes and vice versa. |
| 1204 You want to do this because sometimes you need to compare characters in a string |
| 1205 to numerical values coming from another source. Or, maybe you want to split a |
| 1206 string and then operate on each character.</p></div> |
| 1207 </div> |
| 1208 <div class="sect3"> |
| 1209 <h4 id="_solution_9">Solution</h4> |
| 1210 <div class="paragraph"><p>Use the <span class="monospaced">runes</span> getter t
o get a string’s code points:</p></div> |
| 1211 <div class="listingblock"> |
| 1212 <div class="content monospaced"> |
| 1213 <pre>'Dart'.runes.toList(); // [68, 97, 114, 116] |
| 1214 |
| 1215 var smileyFace = '\u263A'; // ☺ |
| 1216 print(smileyFace.runes.toList()); // [9786], (equivalent to ['\u263A']). |
| 1217 |
| 1218 var clef = '\u{1F3BC}'; // 🎼 |
| 1219 print(clef.runes.toList()); // [127932], (equivalent to ['\u{1F3BC}']).</p
re> |
| 1220 </div></div> |
| 1221 <div class="paragraph"><p>Use <span class="monospaced">string.codeUnits</span> t
o get a string’s UTF-16 code units:</p></div> |
| 1222 <div class="listingblock"> |
| 1223 <div class="content monospaced"> |
| 1224 <pre>'Dart'.codeUnits.toList(); // [68, 97, 114, 116] |
| 1225 smileyFace.codeUnits.toList(); // [9786] |
| 1226 clef.codeUnits.toList(); // [55356, 57276]</pre> |
| 1227 </div></div> |
| 1228 <div class="sect4"> |
| 1229 <h5 id="_using_codeunitat_to_get_individual_code_units">Using codeUnitAt() to ge
t individual code units</h5> |
| 1230 <div class="paragraph"><p>To get the code unit at a particular index, use <span
class="monospaced">codeUnitAt()</span>:</p></div> |
| 1231 <div class="listingblock"> |
| 1232 <div class="content monospaced"> |
| 1233 <pre>'Dart'.codeUnitAt(0); // 68 |
| 1234 smileyFace.codeUnitAt(0); // 9786 (the decimal value of '\u263A') |
| 1235 clef.codeUnitAt(0); // 55356 (does not represent a legal string)</pre> |
| 1236 </div></div> |
| 1237 </div> |
| 1238 </div> |
| 1239 <div class="sect3"> |
| 1240 <h4 id="_converting_numerical_codes_to_strings">Converting numerical codes to st
rings</h4> |
| 1241 <div class="paragraph"><p>You can generate a new string from numerical codes usi
ng the factory |
| 1242 <span class="monospaced">String.fromCharCodes(charCodes)</span>. You can pass ei
ther runes or code units and |
| 1243 <span class="monospaced">String.fromCharCodes(charCodes)</span> can tell the dif
ference and do the right |
| 1244 thing automatically:</p></div> |
| 1245 <div class="listingblock"> |
| 1246 <div class="content monospaced"> |
| 1247 <pre>print(new String.fromCharCodes([68, 97, 114, 116])); // 'D
art' |
| 1248 |
| 1249 print(new String.fromCharCodes([73, 32, 9825, 32, 76, 117, 99, 121])); |
| 1250 // 'I ♡ Lucy' |
| 1251 |
| 1252 // Passing code units representing the surrogate pair. |
| 1253 print(new String.fromCharCodes([55356, 57276])); // 🎼 |
| 1254 |
| 1255 // Passing runes. |
| 1256 print(new String.fromCharCodes([127932])); // 🎼</pre> |
| 1257 </div></div> |
| 1258 <div class="paragraph"><p>You can use the <span class="monospaced">String.fromCh
arCode()</span> factory to convert a single rune |
| 1259 or code unit to a string:</p></div> |
| 1260 <div class="listingblock"> |
| 1261 <div class="content monospaced"> |
| 1262 <pre>new String.fromCharCode(68); // 'D' |
| 1263 new String.fromCharCode(9786); // ☺ |
| 1264 new String.fromCharCode(127932); // 🎼</pre> |
| 1265 </div></div> |
| 1266 <div class="paragraph"><p>Creating a string with only one half of a surrogate pa
ir is permitted, |
| 1267 but not recommended.</p></div> |
| 1268 </div> |
| 1269 </div> |
| 1270 <div class="sect2"> |
| 1271 <h3 id="_calculating_the_length_of_a_string">Calculating the length of a string<
/h3> |
| 1272 <div class="sect3"> |
| 1273 <h4 id="_problem_10">Problem</h4> |
| 1274 <div class="paragraph"><p>You want to get the length of a string, but are not su
re how to calculate the |
| 1275 length correctly when working with variable length Unicode characters.</p></div> |
| 1276 </div> |
| 1277 <div class="sect3"> |
| 1278 <h4 id="_solution_10">Solution</h4> |
| 1279 <div class="paragraph"><p>Use <span class="monospaced">string.runes.length</span
> to get the number of characters in a string.</p></div> |
| 1280 <div class="listingblock"> |
| 1281 <div class="content monospaced"> |
| 1282 <pre>print('I love music'.runes.length); // 12</pre> |
| 1283 </div></div> |
| 1284 <div class="paragraph"><p>You can safely use <span class="monospaced">string.run
es.length</span> to get the length of strings that |
| 1285 contain extended characters:</p></div> |
| 1286 <div class="listingblock"> |
| 1287 <div class="content monospaced"> |
| 1288 <pre>var clef = '\u{1F3BC}'; // 🎼 |
| 1289 var subject = '$clef list:'; // |
| 1290 var music = 'I $hearts $clef'; // 'I ♡ 🎼 ' |
| 1291 |
| 1292 clef.runes.length; // 1 |
| 1293 music.runes.length // 5</pre> |
| 1294 </div></div> |
| 1295 </div> |
| 1296 <div class="sect3"> |
| 1297 <h4 id="_discussion_7">Discussion</h4> |
| 1298 <div class="paragraph"><p>You can directly use a string’s <span class="mon
ospaced">length</span> property (minus <span class="monospaced">runes</span>). T
his returns |
| 1299 the string’s code unit length. Using <span class="monospaced">string.lengt
h</span> produces the same length |
| 1300 as <span class="monospaced">string.runes.length</span> for most unicode characte
rs.</p></div> |
| 1301 <div class="paragraph"><p>For extended characters, the code unit length is one m
ore than the rune |
| 1302 length:</p></div> |
| 1303 <div class="listingblock"> |
| 1304 <div class="content monospaced"> |
| 1305 <pre>clef.length; // 2 |
| 1306 |
| 1307 var music = 'I $hearts $clef'; // 'I ♡ 🎼 ' |
| 1308 music.length; // 6</pre> |
| 1309 </div></div> |
| 1310 <div class="paragraph"><p>Unless you specifically need the code unit length of a
string, use |
| 1311 <span class="monospaced">string.runes.length</span>.</p></div> |
| 1312 <div class="sect4"> |
| 1313 <h5 id="_working_with_combined_characters">Working with combined characters</h5> |
| 1314 <div class="paragraph"><p>It is tempting to brush aside the complexity involved
in dealing with runes and |
| 1315 code units and base the length of the string on the number of characters it |
| 1316 appears to have. Anyone can tell that <em>Dart</em> has four characters, and <em
>Amelié</em> |
| 1317 has six, right? Almost. The length of <em>Dart</em> is indeed four, but the leng
th of |
| 1318 <em>Amelié</em> depends on how that string was constructed:</p></div> |
| 1319 <div class="listingblock"> |
| 1320 <div class="content monospaced"> |
| 1321 <pre>var name = 'Ameli\u00E9'; // 'Amelié' |
| 1322 var anotherName = 'Ameli\u0065\u0301'; // 'Amelié' |
| 1323 print(name.length); // 6 |
| 1324 print(anotherName.length); // 7</pre> |
| 1325 </div></div> |
| 1326 <div class="paragraph"><p>Both <span class="monospaced">name</span> and <span cl
ass="monospaced">anotherName</span> return strings that look the same, but where |
| 1327 the <em>é</em> is constructed using a different number of runes. This makes it |
| 1328 impossible to know the length of these strings by just looking at them.</p></div
> |
| 1329 </div> |
| 1330 </div> |
| 1331 </div> |
| 1332 <div class="sect2"> |
| 1333 <h3 id="_processing_a_string_one_character_at_a_time">Processing a string one ch
aracter at a time</h3> |
| 1334 <div class="sect3"> |
| 1335 <h4 id="_problem_11">Problem</h4> |
| 1336 <div class="paragraph"><p>You want to do something with each character in a stri
ng.</p></div> |
| 1337 </div> |
| 1338 <div class="sect3"> |
| 1339 <h4 id="_solution_11">Solution</h4> |
| 1340 <div class="paragraph"><p>Map the results of calling <span class="monospaced">st
ring.split('')</span>:</p></div> |
| 1341 <div class="listingblock"> |
| 1342 <div class="content monospaced"> |
| 1343 <pre>var lang= 'Dart'; |
| 1344 |
| 1345 // ['*D*', '*a*', '*r*', '*t*'] |
| 1346 print(lang.split('').map((char) => '*${char}*').toList()); |
| 1347 |
| 1348 var smileyFace = '\u263A'; |
| 1349 var happy = 'I am $smileyFace'; |
| 1350 print(happy.split('')); // ['I', ' ', 'a', 'm', ' ', '☺']</pre> |
| 1351 </div></div> |
| 1352 <div class="paragraph"><p>Or, loop over the characters of a string:</p></div> |
| 1353 <div class="listingblock"> |
| 1354 <div class="content monospaced"> |
| 1355 <pre>var list = []; |
| 1356 for(var i = 0; i < lang.length; i++) { |
| 1357 list.add('*${lang[i]}*'); |
| 1358 } |
| 1359 |
| 1360 print(list); // ['*D*', '*a*', '*r*', '*t*']</pre> |
| 1361 </div></div> |
| 1362 <div class="paragraph"><p>Or, map the string runes:</p></div> |
| 1363 <div class="listingblock"> |
| 1364 <div class="content monospaced"> |
| 1365 <pre>// ['*D*', '*a*', '*r*', '*t*'] |
| 1366 var charList = "Dart".runes.map((rune) { |
| 1367 return '*${new String.fromCharCode(rune)}*').toList(); |
| 1368 }); |
| 1369 |
| 1370 // [[73, 'I'], [32, ' '], [97, 'a'], [109, 'm'], [32, ' '], [9786, '☺']] |
| 1371 var runeList = happy.runes.map((rune) { |
| 1372 return [rune, new String.fromCharCode(rune)]).toList(); |
| 1373 });</pre> |
| 1374 </div></div> |
| 1375 <div class="paragraph"><p>When working with extended characters, you should alwa
ys map the string runes. |
| 1376 Don’t use <span class="monospaced">split('')</span> and avoid indexing an
extended string. See the <em>Handling |
| 1377 extended characters that are composed of multiple code units</em> recipe for |
| 1378 special considerations when working with extended strings.</p></div> |
| 1379 </div> |
| 1380 </div> |
| 1381 <div class="sect2"> |
| 1382 <h3 id="_splitting_a_string_into_substrings">Splitting a string into substrings<
/h3> |
| 1383 <div class="sect3"> |
| 1384 <h4 id="_problem_12">Problem</h4> |
| 1385 <div class="paragraph"><p>You want to split a string into substrings using a del
imiter or a pattern.</p></div> |
| 1386 </div> |
| 1387 <div class="sect3"> |
| 1388 <h4 id="_solution_12">Solution</h4> |
| 1389 <div class="paragraph"><p>Use the <span class="monospaced">split()</span> method
with a string or a RegExp as an argument.</p></div> |
| 1390 <div class="listingblock"> |
| 1391 <div class="content monospaced"> |
| 1392 <pre>var smileyFace = '\u263A'; |
| 1393 var happy = 'I am $smileyFace'; |
| 1394 happy.split(' '); // ['I', 'am', '☺']</pre> |
| 1395 </div></div> |
| 1396 <div class="paragraph"><p>Here is an example of using <span class="monospaced">s
plit()</span> with a RegExp:</p></div> |
| 1397 <div class="listingblock"> |
| 1398 <div class="content monospaced"> |
| 1399 <pre>var nums = '2/7 3 4/5 3~/5'; |
| 1400 var numsRegExp = new RegExp(r'(\s|/|~/)'); |
| 1401 nums.split(numsRegExp); // ['2', '7', '3', '4', '5', '3', '5']</pre> |
| 1402 </div></div> |
| 1403 <div class="paragraph"><p>In the code above, the string <span class="monospaced"
>nums</span> contains various numbers, some of which |
| 1404 are expressed as fractions or as int-divisions. A RegExp splits the string to |
| 1405 extract just the numbers.</p></div> |
| 1406 <div class="paragraph"><p>You can perform operations on the matched and unmatche
d portions of a string |
| 1407 when using <span class="monospaced">split()</span> with a RegExp:</p></div> |
| 1408 <div class="listingblock"> |
| 1409 <div class="content monospaced"> |
| 1410 <pre>var phrase = 'Eats SHOOTS leaves'; |
| 1411 |
| 1412 var newPhrase = phrase.splitMapJoin((new RegExp(r'SHOOTS')), |
| 1413 onMatch: (m) => '*${m.group(0).toLowerCase()}*', |
| 1414 onNonMatch: (n) => n.toUpperCase()); |
| 1415 |
| 1416 print(newPhrase); // 'EATS *shoots* LEAVES'</pre> |
| 1417 </div></div> |
| 1418 <div class="paragraph"><p>The RegExp matches the middle word (<em>SHOOTS</em>).
A pair of callbacks are |
| 1419 registered to transform the matched and unmatched substrings before the |
| 1420 substrings are joined together again.</p></div> |
| 1421 </div> |
| 1422 </div> |
| 1423 <div class="sect2"> |
| 1424 <h3 id="_determining_whether_a_string_contains_another_string">Determining wheth
er a string contains another string</h3> |
| 1425 <div class="sect3"> |
| 1426 <h4 id="_problem_13">Problem</h4> |
| 1427 <div class="paragraph"><p>You want to find out whether a string is the substring
of another string.</p></div> |
| 1428 </div> |
| 1429 <div class="sect3"> |
| 1430 <h4 id="_solution_13">Solution</h4> |
| 1431 <div class="paragraph"><p>Use <span class="monospaced">string.contains()</span>:
</p></div> |
| 1432 <div class="listingblock"> |
| 1433 <div class="content monospaced"> |
| 1434 <pre>var fact = 'Dart strings are immutable'; |
| 1435 print(fact.contains('immutable')); // True.</pre> |
| 1436 </div></div> |
| 1437 <div class="paragraph"><p>You can use a second argument to specify where in the
string to start looking:</p></div> |
| 1438 <div class="listingblock"> |
| 1439 <div class="content monospaced"> |
| 1440 <pre>print(fact.contains('Dart', 2)); // False</pre> |
| 1441 </div></div> |
| 1442 </div> |
| 1443 <div class="sect3"> |
| 1444 <h4 id="_discussion_8">Discussion</h4> |
| 1445 <div class="paragraph"><p>The String class provides a couple of shortcuts for te
sting whether a |
| 1446 string is a substring of another:</p></div> |
| 1447 <div class="listingblock"> |
| 1448 <div class="content monospaced"> |
| 1449 <pre>print(string.startsWith('Dart')); // True. |
| 1450 print(string.endsWith('e')); // True.</pre> |
| 1451 </div></div> |
| 1452 <div class="paragraph"><p>You can also use <span class="monospaced">string.index
Of()</span>, which returns -1 if the substring |
| 1453 is not found within a string, and otherwise returns the matching index:</p></div
> |
| 1454 <div class="listingblock"> |
| 1455 <div class="content monospaced"> |
| 1456 <pre>var found = string.indexOf('art') != -1; // True, `art` is found in `Dart`.
</pre> |
| 1457 </div></div> |
| 1458 <div class="paragraph"><p>You can also use a RegExp and <span class="monospaced"
>hasMatch()</span>:</p></div> |
| 1459 <div class="listingblock"> |
| 1460 <div class="content monospaced"> |
| 1461 <pre>var found = new RegExp(r'ar[et]').hasMatch(string); |
| 1462 // True, 'art' and 'are' match.</pre> |
| 1463 </div></div> |
| 1464 </div> |
| 1465 </div> |
| 1466 <div class="sect2"> |
| 1467 <h3 id="_finding_matches_of_a_regexp_pattern_in_a_string">Finding matches of a R
egExp pattern in a string</h3> |
| 1468 <div class="sect3"> |
| 1469 <h4 id="_problem_14">Problem</h4> |
| 1470 <div class="paragraph"><p>You want to use RegExp to match a pattern in a string,
and want to be |
| 1471 able to access the matches.</p></div> |
| 1472 </div> |
| 1473 <div class="sect3"> |
| 1474 <h4 id="_solution_14">Solution</h4> |
| 1475 <div class="paragraph"><p>Construct a regular expression using the RegExp class,
and find matches |
| 1476 using the <span class="monospaced">allMatches()</span> method:</p></div> |
| 1477 <div class="listingblock"> |
| 1478 <div class="content monospaced"> |
| 1479 <pre>var neverEatingThat = 'Not with a fox, not in a box'; |
| 1480 var regExp = new RegExp(r'[fb]ox'); |
| 1481 List matches = regExp.allMatches(neverEatingThat); |
| 1482 print(matches.map((match) => match.group(0)).toList()); // ['fox', 'box']</pr
e> |
| 1483 </div></div> |
| 1484 </div> |
| 1485 <div class="sect3"> |
| 1486 <h4 id="_discussion_9">Discussion</h4> |
| 1487 <div class="paragraph"><p>You can query the object returned by <span class="mono
spaced">allMatches()</span> to find out the |
| 1488 number of matches:</p></div> |
| 1489 <div class="listingblock"> |
| 1490 <div class="content monospaced"> |
| 1491 <pre>var howManyMatches = matches.length; // 2</pre> |
| 1492 </div></div> |
| 1493 <div class="paragraph"><p>To find the first match, use <span class="monospaced">
firstMatch()</span>:</p></div> |
| 1494 <div class="listingblock"> |
| 1495 <div class="content monospaced"> |
| 1496 <pre>var firstMatch = RegExp.firstMatch(neverEatingThat).group(0); // 'fox'</pre
> |
| 1497 </div></div> |
| 1498 <div class="paragraph"><p>To directly get the matched string, use <span class="m
onospaced">stringMatch()</span>:</p></div> |
| 1499 <div class="listingblock"> |
| 1500 <div class="content monospaced"> |
| 1501 <pre>print(regExp.stringMatch(neverEatingThat)); // 'fox' |
| 1502 print(regExp.stringMatch('I like bagels and lox')); // null</pre> |
| 1503 </div></div> |
| 1504 </div> |
| 1505 </div> |
| 1506 <div class="sect2"> |
| 1507 <h3 id="_substituting_strings_based_on_regexp_matches">Substituting strings base
d on RegExp matches</h3> |
| 1508 <div class="sect3"> |
| 1509 <h4 id="_problem_15">Problem</h4> |
| 1510 <div class="paragraph"><p>You want to match substrings within a string and make
substitutions |
| 1511 based on the matches.</p></div> |
| 1512 </div> |
| 1513 <div class="sect3"> |
| 1514 <h4 id="_solution_15">Solution</h4> |
| 1515 <div class="paragraph"><p>Construct a regular expression using the RegExp class
and make |
| 1516 replacements using <span class="monospaced">replaceAll()</span> method:</p></div
> |
| 1517 <div class="listingblock"> |
| 1518 <div class="content monospaced"> |
| 1519 <pre>var resume = 'resume'.replaceAll(new RegExp(r'e'), '\u00E9'); // 'résumé'</
pre> |
| 1520 </div></div> |
| 1521 <div class="paragraph"><p>If you want to replace just the first match, use <span
class="monospaced">replaceFirst()</span>:</p></div> |
| 1522 <div class="listingblock"> |
| 1523 <div class="content monospaced"> |
| 1524 <pre>// Replace the first match of one or more zeros with an empty string. |
| 1525 var smallNum = '0.0001'.replaceFirst(new RegExp(r'0+'), ''); // '.0001'</pre> |
| 1526 </div></div> |
| 1527 <div class="paragraph"><p>You can use <span class="monospaced">replaceAllMatched
()</span> to register a function that modifies the |
| 1528 matches:</p></div> |
| 1529 <div class="listingblock"> |
| 1530 <div class="content monospaced"> |
| 1531 <pre>var heart = '\u2661'; // '♡' |
| 1532 var string = 'I like Ike but I $heart Lucy'; |
| 1533 var regExp = new RegExp(r'[A-Z]\w+'); |
| 1534 var newString = string.replaceAllMapped(regExp, (match) { |
| 1535 return match.group(0).toUpperCase() |
| 1536 }); |
| 1537 print(newString); // 'I like IKE but I ♡ LUCY'</pre> |
| 1538 </div></div> |
| 1539 </div> |
| 1540 </div> |
| 1541 </div> |
| 1542 </div> |
| 1543 </div> |
| 1544 <div id="footnotes"><hr></div> |
| 1545 <div id="footer"> |
| 1546 <div id="footer-text"> |
| 1547 Last updated 2013-03-16 10:56:58 PDT |
| 1548 </div> |
| 1549 </div> |
| 1550 </body> |
| 1551 </html> |
| OLD | NEW |
|---|
Chromium Code Reviews
Issue 12335109:
Strings recipes for the Dart Cookbook (Closed)
Base URL: https://github.com/dart-lang/cookbook.git@master