[Android] Implement back key functionality in download manager

chrome/android/java/src/org/chromium/chrome/browser/download/DownloadActivity.java

Index: chrome/android/java/src/org/chromium/chrome/browser/download/DownloadActivity.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/download/DownloadActivity.java b/chrome/android/java/src/org/chromium/chrome/browser/download/DownloadActivity.java
index bc72f2ee633516f5aa283abf2e66626e408c122b..cde8c8f08428e028c23f0c142039a0d93bf24fbc 100644
--- a/chrome/android/java/src/org/chromium/chrome/browser/download/DownloadActivity.java
+++ b/chrome/android/java/src/org/chromium/chrome/browser/download/DownloadActivity.java
@@ -10,9 +10,15 @@ import android.os.Bundle;
 import org.chromium.base.VisibleForTesting;
 import org.chromium.chrome.browser.IntentHandler;
 import org.chromium.chrome.browser.SnackbarActivity;
+import org.chromium.chrome.browser.UrlConstants;
+import org.chromium.chrome.browser.download.ui.DownloadFilter;
 import org.chromium.chrome.browser.download.ui.DownloadManagerUi;
+import org.chromium.chrome.browser.download.ui.DownloadManagerUi.DownloadUiObserver;
 import org.chromium.chrome.browser.util.IntentUtils;
 
+import java.util.Deque;
+import java.util.LinkedList;
+
 /**
  * Activity for managing downloads handled through Chrome.
  */
@@ -20,6 +26,22 @@ public class DownloadActivity extends SnackbarActivity {
     private DownloadManagerUi mDownloadManagerUi;
     private boolean mIsOffTheRecord;
 
+    /** Caches the stack of filters applied to let the user backtrack through their history. */
+    private final Deque<String> mBackStack = new LinkedList<>();
+
+    private final DownloadUiObserver mUiObserver = new DownloadUiObserver() {
+        @Override
+        public void onManagerDestroyed() { }
+
+        @Override
+        public void onFilterChanged(int filter) {
+            String url = DownloadFilter.getUrlForFilter(filter);
+            if (mBackStack.isEmpty() || !mBackStack.peek().equals(url)) {
+                mBackStack.push(url);
+            }
+        }
+    };
+
     @Override
     public void onCreate(Bundle savedInstanceState) {
         super.onCreate(savedInstanceState);
@@ -30,6 +52,9 @@ public class DownloadActivity extends SnackbarActivity {
         mDownloadManagerUi = new DownloadManagerUi(this, isOffTheRecord, parentComponent);
         setContentView(mDownloadManagerUi.getView());
         mIsOffTheRecord = isOffTheRecord;
+        mDownloadManagerUi.addObserver(mUiObserver);
+        // Call updateForUrl() to align with how DownloadPage interacts with DownloadManagerUi.
+        mDownloadManagerUi.updateForUrl(UrlConstants.DOWNLOADS_URL);
     }
 
     @Override
@@ -41,7 +66,17 @@ public class DownloadActivity extends SnackbarActivity {
 
     @Override
     public void onBackPressed() {
-        if (!mDownloadManagerUi.onBackPressed()) super.onBackPressed();
+        if (mDownloadManagerUi.onBackPressed()) return;
+        // The top of the stack always represents the current filter. When back is pressed,
+        // the top is popped off and the new top indicates what filter to use. If there are
+        // no filters remaining, the Activity itself is closed.
+        if (mBackStack.size() > 1) {
+            mBackStack.pop();
+            mDownloadManagerUi.updateForUrl(mBackStack.peek());
+        } else {
+            if (!mBackStack.isEmpty()) mBackStack.pop();
+            super.onBackPressed();
+        }
     }
 
     @Override