Support for observable models, fixes #259

lib/observe.dart

+// Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+/**
+ * A library for observing changes to observable Dart objects.
+ * Similar in spirit to EcmaScript Harmony
+ * [Object.observe](http://wiki.ecmascript.org/doku.php?id=harmony:observe), but
+ * able to observe expressions and not just objects, so long as the expressions
+ * are computed from observable objects.
+ *
+ * See the [observable] annotation and the [observe] function.
+ */
+// Note: one intentional difference from Harmony Object.observe is that our
+// change batches are tracked on a per-observed expression basis, instead of
+// per-observer basis.
+// We do this because there is no cheap way to store a pointer on a Dart
+// function (Expando uses linear search on the VM: http://dartbug.com/7558).
+// This difference means that a given observer will be called with one batch of
+// changes for each object it is observing.
+library observe;
+
+import 'dart:collection';
+// TODO(jmesserly): see if we can switch to Future.immediate. We need it to be
+// fast (next microtask) like our version, though.
+import 'src/utils.dart' show setImmediate;
+
+// TODO(jmesserly): support detailed change records on our collections, such as
+// INSERT/REMOVE, so we can use them from templates. Unlike normal objects,
+// list/map/set can add or remove new observable things at runtime, so it's
+// important to provide a way to listen for that.
+export 'observe/list.dart';
+export 'observe/map.dart';
+export 'observe/reference.dart';
+export 'observe/set.dart';
+
+// TODO(jmesserly): notifyRead/notifyWrite are only used by people
+// implementating advanced observable functionality. They need to be public, but
+// ideally they would not be in the top level "observe" library.
+
+/**
+ * Use `@observable` to make a class observable. All fields in the class will
+ * be transformed to track changes. The overhead will be minimal unless they are
+ * actually being observed.
+ */
+const observable = const Object();
+
+/** Callback fired when an expression changes. */
+typedef void ChangeObserver(ChangeNotification e);
+
+/** A function that unregisters the [ChangeObserver]. */
+typedef void ChangeUnobserver();
+
+/** A function that computes a value. */
+typedef Object ObservableExpression();
+
+/**
+ * Test for equality of two objects. For example [Object.==] and [identical]
+ * are two kinds of equality tests.
+ */
+typedef bool EqualityTest(Object a, Object b);
+
+/**
+ * A notification of a change to an [ObservableExpression] that is passed to a
+ * [ChangeObserver].
+ */
+class ChangeNotification {
+
+  /** Previous value seen on the watched expression. */
+  final oldValue;
+
+  /** New value seen on the watched expression. */
+  final newValue;
+
+  ChangeNotification(this.oldValue, this.newValue);
+
+  // Note: these two methods are here mainly to make testing easier.
+  bool operator ==(other) {
+    return other is ChangeNotification && oldValue == other.oldValue &&
+        newValue == other.newValue;
+  }
+
+  String toString() => 'change from $oldValue to $newValue';
+}
+
+/**
+ * Observes the [expression] and delivers asynchronous notifications of changes
+ * to the [callback].
+ *
+ * The expression is considered to have changed if the values no longer compare
+ * equal via the equality operator. You can perform additional comparisons in
+ * the [callback] if desired.
+ *
+ * Returns a function that can be used to stop observation.
+ * Calling this makes it possible for the garbage collector to reclaim memory
+ * associated with the observation and prevents further calls to [callback].
+ *
+ * Because notifications are delivered asynchronously and batched, only a single
+ * notification is provided for all changes that were made prior to running
+ * callback. Intermediate values of the expression are not saved. Instead,
+ * [ChangeNotification.oldValue] represents the value before any changes, and
+ * [ChangeNotification.newValue] represents the current value of [expression]
+ * at the time that [callback] is called.
+ *
+ * You can force a synchronous change delivery at any time by calling
+ * [deliverChangesSync]. Calling this method if there are no changes has no
+ * effect. If changes are delivered by deliverChangesSync, they will not be
+ * delivered again asynchronously, unless the value is changed again.
+ *
+ * Any errors thrown by [expression] and [callback] will be caught and sent to
+ * [onObserveUnhandledError].
+ */
+// TODO(jmesserly): debugName is here to workaround http://dartbug.com/8419.
+ChangeUnobserver observe(ObservableExpression expression,
+    ChangeObserver callback, [String debugName]) {
+
+  var observer = new _ExpressionObserver(expression, callback, debugName);
+  if (!observer._observe()) {
+    // If we didn't actually read anything, return a pointer to a no-op
+    // function so the observer can be reclaimed immediately.
+    return _doNothing;
+  }
+
+  return observer._unobserve;
+}
+
+// Optimizations to avoid extra work if observing const/final data.
+void _doNothing() {}
+
+/**
+ * The current observer that is tracking reads, or null if we aren't tracking
+ * reads. Reads are tracked when executing [_ExpressionObserver._observe].
+ */
+_ExpressionObserver _activeObserver;
+
+/**
+ * True if we are observing reads. This should be checked before calling
+ * [notifyRead].
+ *
+ * Note: this type is used by objects implementing observability.
+ * You should not need it if your type is marked `@observable`.
+ */
+bool get observeReads => _activeObserver != null;
+
+/**
+ * Notify the system of a new read. This will add the current change observer
+ * to the set of observers for this field.  This should *only* be called when
+ * [observeReads] is true, and it will initialize [observers] if it is null.
+ * For example:
+ *
+ *     get foo {
+ *       if (observeReads) _fooObservers = notifyRead(_fooObservers);
+ *       return _foo;
+ *     }
+ *
+ * Note: this function is used to implement observability.
+ * You should not need it if your type is marked `@observable`.
+ *
+ * See also: [notifyWrite]
+ */
+Object notifyRead(fieldObservers) {
+  // Note: fieldObservers starts null, then a single observer, then a List.
+
+  _activeObserver._wasRead = true;
+
+  // Note: there's some optimization here to avoid allocating an observer list
+  // unless we really need it.
+  if (fieldObservers == null) {
+    return _activeObserver;
+  }
+  if (fieldObservers is _ExpressionObserver) {
+    if (identical(fieldObservers, _activeObserver) || fieldObservers._dead) {
+      return _activeObserver;
+    }
+    return [fieldObservers, _activeObserver];
+  }
+  return fieldObservers..add(_activeObserver);
+}
+
+/**
+ * Notify the system of a new write. This will deliver a change notification
+ * to the set of observers for this field. This should *only* be called for a
+ * non-null list of [observers]. For example:
+ *
+ *     set foo(value) {
+ *       if (_fooObservers != null && _foo != value) {
+ *         _fooObservers = notifyWrite(_fooObservers);
+ *       }
+ *       _foo = value;
+ *     }
+ *
+ * Note: this function is used to implement observability.
+ * You should not need it if your type is marked `@observable`.
+ *
+ * See also: [notifyRead]
+ */
+Object notifyWrite(Object fieldObservers) {
+  if (_pendingWrites == null) {
+    _pendingWrites = [];
+    setImmediate(deliverChangesSync);
+  }
+  _pendingWrites.add(fieldObservers);
+
+  // Clear fieldObservers. This will prevent a second notification for this
+  // same set of observers on the current event loop. It also frees associated
+  // memory. If the item needs to be observed again, that will happen in
+  // _ExpressionObserver._deliver.
+
+  // NOTE: ObservableMap depends on this returning null.
+  return null;
+}
+
+List _pendingWrites;
+
+/**
+ * The limit of times we will attempt to deliver a set of pending changes.
+ *
+ * [deliverChangesSync] will attempt to deliver pending changes until there are
+ * no more. If one of the pending changes causes another batch of changes, it
+ * will iterate again and increment the iteration counter. Once it reaches
+ * this limit it will call [onCircularNotifyLimit].
+ *
+ * Note that there is no limit to the number of changes per batch, only to the
+ * number of iterations.
+ */
+int circularNotifyLimit = 100;
+
+/**
+ * Delivers observed changes immediately. Normally you should not call this
+ * directly, but it can be used to force synchronous delivery, which helps in
+ * certain cases like testing.
+ */
+void deliverChangesSync() {
+  int iterations = 0;
+  while (_pendingWrites != null) {
+    var pendingWrites = _pendingWrites;
+    _pendingWrites = null;
+
+    // Sort pending observers by order added.
+    // TODO(jmesserly): this is here to help our template system, which relies
+    // on earlier observers removing later ones to prevent them from firing.
+    // See if we can find a better solution at the template level.
+    var pendingObservers = new SplayTreeMap<int, _ExpressionObserver>();
+    for (var pending in pendingWrites) {
+      if (pending is _ExpressionObserver) {
+        pendingObservers[pending._id] = pending;
+      } else {
+        for (var observer in pending) {
+          pendingObservers[observer._id] = observer;
+        }
+      }
+    }
+
+    if (iterations++ == circularNotifyLimit) {
+      _diagnoseCircularLimit(pendingObservers);
+      return;
+    }
+
+    // TODO(jmesserly): we are avoiding SplayTreeMap.values because it performs
+    // an unnecessary copy. If that gets fixed we can use .values here.
+    // https://code.google.com/p/dart/issues/detail?id=8516
+    pendingObservers.forEach((id, obs) { obs._deliver(); });
+  }
+}
+
+/**
+ * Attempt to provide diagnostics about what change is causing a loop in
+ * observers. Unfortunately it is hard to help the programmer unless they have
+ * provided a `debugName` to [observe], as callbacks are hard to debug
+ * because of <http://dartbug.com/8419>. However we can print the records that
+ * changed which has proved helpful.
+ */
+void _diagnoseCircularLimit(Map<int, _ExpressionObserver> pendingObservers) {
+  // TODO(jmesserly,sigmund): we could do purity checks when running "observe"
+  // itself, to detect if it causes writes to happen. I think that case is less
+  // common than cycles caused by the notifications though.
+
+  var trace = new StringBuffer('exceeded notifiction limit of '
+      '${circularNotifyLimit}, possible '
+      'circular reference in observers: ');
+
+  int i = 0;
+  pendingObservers.forEach((id, obs) {
+    var change = obs._deliver();
+    if (change == null) return;
+
+    if (i < 10) {

[Siggi Cherem (dart-lang), 2013/02/13 19:28:54]

nit: combine with previous condition?

if (change == null || i >= 10) return;

[Jennifer Messerly, 2013/02/14 00:38:09]

Done.

+      if (i != 0) trace.add(', ');
+      trace.add('$obs $change');
+    }
+    i++;
+  });
+
+  // Throw away pending changes to prevent repeating this error.
+  _pendingWrites = null;
+
+  onCircularNotifyLimit(trace.toString());
+}
+
+
+class _ExpressionObserver {
+  static int _nextId = 0;
+
+  /**
+   * The ID indicating creation order. We will call observers in ID order.
+   * See the TODO in [deliverChangesSync].
+   */
+  final int _id = ++_ExpressionObserver._nextId;
+
+  // Note: fields in this class are private because instances of this class are
+  // exposed via notifyRead.
+  ObservableExpression _expression;
+
+  ChangeObserver _callback;
+
+  /** The last value of this observable. */
+  Object _value;
+
+  /**
+   * Whether this observer was read at all.
+   * If it wasn't read, we can free it immediately

[Siggi Cherem (dart-lang), 2013/02/13 19:28:54]

missing ending '.', same in the next comment.

[Jennifer Messerly, 2013/02/14 00:38:09]

Done.

+   */
+  bool _wasRead;
+
+  /**
+   * The name used for debugging. This will be removed once Dart has
+   * better debugging of callbacks
+   */
+  String _debugName;
+
+  _ExpressionObserver(this._expression, this._callback, this._debugName);
+
+  /** True if this observer has been unobserved. */
+  // Note: any time we call out to user-provided code, they might call
+  // unobserve, so we need to guard against that.
+  bool get _dead => _callback == null;
+
+  String toString() =>
+      _debugName != null ? '<observer $_id: $_debugName>' : '<observer $_id>';
+
+  bool _observe() {
+    // If an observe call starts another observation, we need to make sure that
+    // the outer observe is tracked correctly.
+    var parent = _activeObserver;
+    _activeObserver = this;
+
+    _wasRead = false;
+    try {
+      _value = _expression();
+    } catch (e, trace) {
+      onObserveUnhandledError(e, trace, _expression);
+      _value = null;
+    }
+
+    // TODO(jmesserly): should the parent also observe us?
+    assert(_activeObserver == this);
+    _activeObserver = parent;
+
+    return _wasRead;
+  }
+
+  void _unobserve() {
+    if (_dead) return;
+
+    // Note: we don't remove ourselves from objects that we are observing.
+    // That will happen automatically when those fields are written.
+    // Instead, we release our own memory and wait for notifyWrite and
+    // deliverChangesSync to do the rest.
+    // TODO(jmesserly): this is probably too over-optimized. We'll need to
+    // revisit this to provide detailed change records.
+    _expression = null;
+    _callback = null;
+    _value = null;
+    _wasRead = null;
+    _debugName = null;
+  }
+
+  /**
+   * _deliver does two things:
+   * 1. Evaluate the expression to compute the new value.
+   * 2. Invoke observer for this expression.
+   *
+   * Note: if you mutate a shared value from one observer, future
+   * observers will see the updated value. Essentially, we collapse
+   * the two change notifications into one.
+   *
+   * We could split _deliver into two methods, one to compute the new value
+   * and another to call observers. But the current order has benefits too: it
+   * preserves the invariant that ChangeNotification.newValue equals the current
+   * value of the expression.
+   */
+  ChangeNotification _deliver() {
+    if (_dead) return null;
+
+    // Call the expression again to compute the new value, and to get the new
+    // list of dependencies.
+    var oldValue = _value;
+    _observe();
+
+    // Note: whenever we run code we don't control, we need to check _dead again
+    // in case they have unobserved this object. This means `_observe`, `==`,
+    // need to check.
+    if (_dead) return null;
+
+    bool equal;
+    try {
+      equal = oldValue == _value;
+    } catch (e, trace) {
+      onObserveUnhandledError(e, trace, null);
+      return;
+    }
+
+    if (equal || _dead) return null;
+
+    var change = new ChangeNotification(oldValue, _value);
+    try {
+      _callback(change);
+    } catch (e, trace) {
+      onObserveUnhandledError(e, trace, _callback);
+    }
+    return change;
+  }
+
+  // TODO(jmesserly): workaround for terrible VM hash code performance.
+  int get hashCode => _id;
+}
+
+typedef void CircularNotifyLimitHandler(String message);
+
+/**
+ * Function that is called when change notifications get stuck in a circular
+ * loop, which can happen if one [ChangeObserver] causes another change to
+ * happen, and that change causes another, etc.
+ *
+ * This is called when [circularNotifyLimit] is reached by
+ * [deliverChangesSync]. Circular references are commonly the result of not
+ * correctly implementing equality for objects.
+ *
+ * The default behavior is to print the message.
+ */
+// TODO(jmesserly): using Logger seems better, but by default it doesn't do
+// anything, which leads to unobserved errors.
+CircularNotifyLimitHandler onCircularNotifyLimit = (message) => print(message);
+
+/**
+ * A function that handles an [error] given the [stackTrace] and [callback] that
+ * caused the error.
+ */
+typedef void ObserverErrorHandler(error, stackTrace, Function callback);
+
+/**
+ * Callback to intercept unhandled errors in evaluating an observable.
+ * Includes the error, stack trace, and the callback that caused the error.
+ * By default it will use [defaultObserveUnhandledError], which prints the
+ * error.
+ */
+ObserverErrorHandler onObserveUnhandledError = defaultObserveUnhandledError;
+
+/** The default handler for [onObserveUnhandledError]. Prints the error. */
+void defaultObserveUnhandledError(error, trace, callback) {
+  // TODO(jmesserly): using Logger seems better, but by default it doesn't do
+  // anything, which leads to unobserved errors.
+  // Ideally we could make this show up as an error in the browser's console.
+  print('web_ui.observe: unhandled error in callback $callback.\n'
+      'error:\n$error\n\nstack trace:\n$trace');
+}