Use Event to render feedback animations.

remoting/android/java/src/org/chromium/chromoting/FeedbackAnimator.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.chromoting;
+
+import android.graphics.Color;
+import android.graphics.Paint;
+import android.graphics.Point;
+import android.graphics.RadialGradient;
+import android.graphics.Shader;
+import android.os.SystemClock;
+
+/** Helper class for displaying the press feedback animations. */

[Lambros, 2016/06/01 23:51:36]

Is this class now single-threaded?

[Hzj_jie, 2016/06/02 02:21:37]

Yes, this class is not necessarily thread-safe. Since the onPaint event will
only be called by DesktopView.paint function.

This class is still thread-safe, though not necessary. All fields in this class
are final, if two run function calls concurrently, the instance will draw two
same circles on two (or one, depends on the caller) canvases.

I removed the comment about the thread-safety, simply because it's not a
must-have anymore.

+public final class FeedbackAnimator
+        implements Event.ParameterCallback<Boolean, EventParameters.Paint> {
+    /** Total duration of the animation, in milliseconds. */
+    private static final float TOTAL_DURATION_MS = 220;
+
+    /** Start time of the animation, from {@link SystemClock#uptimeMillis()}. */
+    private final long mStartTimeInMs;
+
+    /** Contains the size of the feedback animation for the most recent request. */
+    private final float mFeedbackSizeInPixels;
+
+    private final Paint mPaint = new Paint();
+
+    private final Point mPos;
+
+    private FeedbackAnimator(float feedbackSizeInPixels, Point pos) {
+        mStartTimeInMs = SystemClock.uptimeMillis();
+        mFeedbackSizeInPixels = feedbackSizeInPixels;
+        mPos = pos;
+    }
+
+    /** Begins a new animation sequence at position (|x|, |y|). */

[Lambros, 2016/06/01 23:51:36]

position |pos|.

[Hzj_jie, 2016/06/02 02:21:37]

Done.

+    public static void startAnimation(DesktopView view,
+                                      Point pos,
+                                      DesktopView.InputFeedbackType feedbackType) {
+        Preconditions.notNull(view);

[Lambros, 2016/06/01 23:51:36]

Remove this?
If view == null, then view.onPaint() below will already throw a NPE, so this
check seems to add no value.

Checking pos != null is perhaps more useful (because 'pos' isn't immediately
dereferenced here). Though I'd prefer to avoid going overboard with
these null-checks everywhere.

[Hzj_jie, 2016/06/02 02:21:37]

My fault, I recently wrote more C++ code than Java. :)

+        if (feedbackType == DesktopView.InputFeedbackType.NONE) {
+            return;
+        }
+
+        view.onPaint().addSelfRemovable(new FeedbackAnimator(
+                getInputFeedbackSizeInPixels(feedbackType), pos));
+    }
+
+    private static float getInputFeedbackSizeInPixels(DesktopView.InputFeedbackType feedbackType) {
+        switch (feedbackType) {
+            case SMALL_ANIMATION:
+                return 40.0f;
+
+            case LARGE_ANIMATION:
+                return 160.0f;
+
+            default:
+                // Unreachable, but required by Google Java style and findbugs.
+                assert false : "Unreached";
+                return 0.0f;
+        }
+    }
+
+    @Override
+    public Boolean run(EventParameters.Paint parameter) {
+        float elapsedTimeInMs = SystemClock.uptimeMillis() - mStartTimeInMs;
+        if (elapsedTimeInMs < 1) {
+            return true;
+        }
+
+        // |progress| is 0 at the beginning, 1 at the end.
+        float progress = elapsedTimeInMs / TOTAL_DURATION_MS;
+        if (progress >= 1) {
+            return false;
+        }
+
+        float size = mFeedbackSizeInPixels / parameter.scale;
+        // Animation grows from 0 to |size|, and goes from fully opaque to transparent for a

[Lambros, 2016/06/01 23:51:36]

Blank line above comment.

[Hzj_jie, 2016/06/02 02:21:37]

Done.

+        // seamless fading-out effect. The animation needs to have more than one color so it's
+        // visible over any background color.
+        float radius = size * progress;
+        int alpha = (int) ((1 - progress) * 0xff);
+
+        int transparentBlack = Color.argb(0, 0, 0, 0);
+        int white = Color.argb(alpha, 0xff, 0xff, 0xff);
+        int black = Color.argb(alpha, 0, 0, 0);
+        mPaint.setShader(new RadialGradient(mPos.x, mPos.y, radius,
+                new int[] {transparentBlack, white, black, transparentBlack},
+                new float[] {0.0f, 0.8f, 0.9f, 1.0f}, Shader.TileMode.CLAMP));
+        parameter.canvas.drawCircle(mPos.x, mPos.y, radius, mPaint);
+        return true;
+    }
+}