[Android] Implement back key functionality in download manager

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

 // Copyright 2016 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 package org.chromium.chrome.browser.download;
 
 import android.content.ComponentName;
 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.
  */
 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);
 
         boolean isOffTheRecord = DownloadUtils.shouldShowOffTheRecordDownloads(getIntent());
         ComponentName parentComponent = IntentUtils.safeGetParcelableExtra(
                 getIntent(), IntentHandler.EXTRA_PARENT_COMPONENT);
         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
     public void onResume() {
         super.onResume();
         DownloadUtils.checkForExternallyRemovedDownloads(mDownloadManagerUi.getBackendProvider(),
                 mIsOffTheRecord);
     }
 
     @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
     protected void onDestroy() {
         mDownloadManagerUi.onDestroyed();
         super.onDestroy();
     }
 
     @VisibleForTesting
     DownloadManagerUi getDownloadManagerUiForTests() {
         return mDownloadManagerUi;
     }
 }