📰 Add visibility change triggers for bottom sheet content

chrome/android/java/src/org/chromium/chrome/browser/suggestions/SuggestionsSheetVisibilityChangeObserver.java

Index: chrome/android/java/src/org/chromium/chrome/browser/suggestions/SuggestionsSheetVisibilityChangeObserver.java
diff --git a/chrome/android/java/src/org/chromium/chrome/browser/suggestions/SuggestionsSheetVisibilityChangeObserver.java b/chrome/android/java/src/org/chromium/chrome/browser/suggestions/SuggestionsSheetVisibilityChangeObserver.java
new file mode 100644
index 0000000000000000000000000000000000000000..d8b23a0f0e66a4a69ae1b7b9cf3ad727897f7437
--- /dev/null
+++ b/chrome/android/java/src/org/chromium/chrome/browser/suggestions/SuggestionsSheetVisibilityChangeObserver.java
@@ -0,0 +1,147 @@
+// Copyright 2017 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.suggestions;
+
+import android.app.Activity;
+import android.support.annotation.CallSuper;
+
+import org.chromium.base.ActivityState;
+import org.chromium.base.ApplicationStatus;
+import org.chromium.chrome.browser.ChromeActivity;
+import org.chromium.chrome.browser.widget.bottomsheet.BottomSheet;
+import org.chromium.chrome.browser.widget.bottomsheet.BottomSheet.BottomSheetContent;
+import org.chromium.chrome.browser.widget.bottomsheet.EmptyBottomSheetObserver;
+
+/**
+ * Notifies of events dedicated to changes in visibility of a
+ * {@link SuggestionsBottomSheetContent}.
+ */
+public abstract class SuggestionsSheetVisibilityChangeObserver
+        extends EmptyBottomSheetObserver implements ApplicationStatus.ActivityStateListener {
+    @BottomSheet.SheetState
+    private int mCurrentContentState;
+    private boolean mCurrentVisibility;
+
+    private final ChromeActivity mActivity;
+    private final BottomSheet.BottomSheetContent mContentObserved;
+    private final BottomSheet mBottomSheet;
+
+    /**
+     * Creates and register the observer to receive events related to changes to the provided
+     * {@link BottomSheetContent}'s visibility.
+     * @param bottomSheetContent BottomSheetContent to observe visibility for.
+     * @param chromeActivity The BottomSheet the observed content is registered with. Note: the
+     *                    constructor does not register the object as observer!
+     */
+    public SuggestionsSheetVisibilityChangeObserver(
+            BottomSheetContent bottomSheetContent, ChromeActivity chromeActivity) {
+        mActivity = chromeActivity;
+        mContentObserved = bottomSheetContent;
+        mBottomSheet = chromeActivity.getBottomSheet();
+        assert mBottomSheet != null;
+
+        ApplicationStatus.registerStateListenerForActivity(this, chromeActivity);
+        mBottomSheet.addObserver(this);
+
+        // This event is swallowed when the observer is registered after the sheet is opened.
+        // (e.g. Chrome starts on the NTP). This allows taking it into account.
+        if (mBottomSheet.isSheetOpen()) onSheetOpened();
+    }
+
+    public void onDestroy() {
+        ApplicationStatus.unregisterActivityStateListener(this);
+        mBottomSheet.removeObserver(this);
+    }
+
+    /** Called when the observed sheet content becomes visible. */
+    public abstract void onContentShown();
+
+    /** Called when the observed sheet content becomes invisible. */
+    public abstract void onContentHidden();
+
+    /**
+     * Called when the visible state of the observed sheet content changes.
+     * @param contentState The new state, restricted to stable ones:
+     *                     {@link BottomSheet#SHEET_STATE_FULL},{@link BottomSheet#SHEET_STATE_HALF}
+     *                     or {@link BottomSheet#SHEET_STATE_PEEK}
+     */
+    public abstract void onContentStateChanged(@BottomSheet.SheetState int contentState);
+
+    @Override
+    @CallSuper
+    public void onSheetOpened() {
+        onStateChange();
+    }
+
+    @Override
+    @CallSuper
+    public void onSheetClosed() {
+        onStateChange();
+    }
+
+    @Override
+    @CallSuper
+    public void onSheetContentChanged(BottomSheetContent newContent) {
+        onStateChange();
+    }
+
+    @Override
+    @CallSuper
+    public void onSheetStateChanged(int newState) {
+        onStateChange();
+    }
+
+    @Override
+    @CallSuper
+    public void onActivityStateChange(Activity activity, @ActivityState int newState) {
+        if (newState == ActivityState.DESTROYED) {
+            onDestroy();
+            return;
+        }
+
+        if (!mBottomSheet.isSheetOpen()) return;
+
+        onStateChange();
+    }
+
+    /**
+     * Compares the current state of the bottom sheet and activity with the ones recorded at the
+     * previous call and generates events based on the difference.
+     * @see #onContentShown()
+     * @see #onContentHidden()
+     * @see #onContentStateChanged(int)
+     */
+    private void onStateChange() {
+        boolean newVisibility = mBottomSheet.isSheetOpen()
+                && mBottomSheet.getCurrentSheetContent() == mContentObserved
+                && ApplicationStatus.getStateForActivity(mActivity) == ActivityState.RESUMED;
+
+        // As the visibility we track is the one for a specific sheet content rather than the
+        // whole BottomSheet, we also need to reflect that in the state, marking it "peeking" here
+        // even though the BottomSheet itself is not.
+        @BottomSheet.SheetState
+        int newContentState =
+                newVisibility ? mBottomSheet.getSheetState() : BottomSheet.SHEET_STATE_PEEK;
+
+        // Flag overall changes to the visible state of the content, while ignoring transient states
+        // like |STATE_SCROLLING|.
+        boolean hasMeaningfulStateChange = BottomSheet.isStateStable(newContentState)
+                && (mCurrentContentState != newContentState || mCurrentVisibility != newVisibility);
+
+        if (newVisibility != mCurrentVisibility) {
+            if (newVisibility) {
+                onContentShown();
+            } else {
+                onContentHidden();
+            }
+            mCurrentVisibility = newVisibility;
+        }
+
+        if (hasMeaningfulStateChange) {
+            onContentStateChanged(newContentState);
+            mCurrentContentState = newContentState;
+        }
+    }
+}