[Android] Allow text handles to observe position of "parent" view

content/public/android/java/src/org/chromium/content/browser/ViewPositionObserver.java

+// Copyright 2013 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.content.browser;
+
+import android.view.View;
+import android.view.ViewTreeObserver;
+
+import java.util.ArrayList;
+
+/**
+ * Used to register listeners that can be notified of changes to the position of a view.
+ */
+public class ViewPositionObserver implements PositionObserver {
+    private View mView;
+    // Absolute position of the container view relative to its parent window.
+    private final int[] mPosition = new int[2];
+
+    private final ArrayList<Listener> mListeners;
+    private ViewTreeObserver.OnPreDrawListener mPreDrawListener;
+
+    /**
+     * @param view The view to observe.
+     */
+    public ViewPositionObserver(View view) {
+        mView = view;
+        mListeners = new ArrayList<Listener>();
+        updatePosition();
+        mPreDrawListener = new ViewTreeObserver.OnPreDrawListener() {
+            @Override
+            public boolean onPreDraw() {
+                updatePosition();
+                return true;
+            }
+        };
+    }
+
+    /**
+     * @return The current x position of the observed view.
+     */
+    public int getPositionX() {
+        // The stored position may be out-of-date. Get the real current position.
+        updatePosition();
+        return mPosition[0];
+    }
+
+    /**
+     * @return The current y position of the observed view.
+     */
+    public int getPositionY() {
+        // The stored position may be out-of-date. Get the real current position.
+        updatePosition();
+        return mPosition[1];
+    }
+
+    /**
+     * Register a listener to be called when the position of the underlying view changes.
+     */
+    public void addListener(Listener listener) {
+        if (mListeners.contains(listener)) return;
+
+        if (mListeners.isEmpty()) {
+            mView.getViewTreeObserver().addOnPreDrawListener(mPreDrawListener);
+            updatePosition();
+        }
+
+        mListeners.add(listener);
+    }
+
+    /**
+     * Remove a previously installed listener.
+     */
+    public void removeListener(Listener listener) {
+        if (!mListeners.contains(listener)) return;
+
+        mListeners.remove(listener);
+
+        if (mListeners.isEmpty()) {
+            mView.getViewTreeObserver().removeOnPreDrawListener(mPreDrawListener);
+        }
+    }
+
+    private void notifyListeners() {
+        for (int i = 0; i < mListeners.size(); i++) {
+            mListeners.get(i).onPositionChanged(mPosition[0], mPosition[1]);
+        }
+    }
+
+    private void updatePosition() {
+        int previousPositionX = mPosition[0];
+        int previousPositionY = mPosition[1];
+        mView.getLocationInWindow(mPosition);
+        if (mPosition[0] != previousPositionX || mPosition[1] != previousPositionY) {
+            notifyListeners();
+        }
+    }
+}
+