Optimize CoverView scrolling by only redrawing the dirty region

src/org/kreed/vanilla/CoverView.java

@@ -41,28 +41,50 @@ import android.widget.Scroller;
  * generated by CoverBitmap.
  */
 public final class CoverView extends View implements Handler.Callback {
+	/**
+	 * The system-provided snap velocity, used as a threshold for detecting
+	 * flings.
+	 */
 	private static int sSnapVelocity = -1;
-
 	/**
 	 * The Handler with which to do background work. Will be null until
 	 * setupHandler is called.
 	 */
 	private Handler mHandler;
+	/**
+	 * A handler running on the UI thread, for UI operations.
+	 */
+	private final Handler mUiHandler = new Handler(this);
 	/**
 	 * How to render cover art and metadata. One of
 	 * CoverBitmap.STYLE_*
 	 */
 	private int mCoverStyle;
-
+	/**
+	 * Interface to respond to CoverView motion actions.
+	 */
 	public interface Callback {
+		/**
+		 * Called after the view has scrolled to the next (right) cover.
+		 */
 		public void nextSong();
+		/**
+		 * Called after the view has scrolled to the previous (left) cover.
+		 */
 		public void previousSong();
+		/**
+		 * Called when the user has swiped up on the view.
+		 */
 		public void upSwipe();
+		/**
+		 * Called when the user had swiped down on the view.
+		 */
 		public void downSwipe();
 	}
-
+	/**
+	 * The instance of the callback.
+	 */
 	private Callback mCallback;
-
 	/**
 	 * The current set of songs: 0 = previous, 1 = current, and 2 = next.
 	 */
@@ -79,24 +101,49 @@ public final class CoverView extends View implements Handler.Callback {
 	 * Cover art to use when a song has no cover art in no info display styles.
 	 */
 	private Bitmap mDefaultCover;
-
+	/**
+	 * Computes scroll animations.
+	 */
 	private final Scroller mScroller;
+	/**
+	 * Computes scroll velocity to detect flings.
+	 */
 	private VelocityTracker mVelocityTracker;
+	/**
+	 * The x coordinate of the last touch down or move event.
+	 */
 	private float mLastMotionX;
+	/**
+	 * The y coordinate of the last touch down or move event.
+	 */
 	private float mLastMotionY;
+	/**
+	 * The x coordinate of the last touch down event.
+	 */
 	private float mStartX;
+	/**
+	 * The y coordinate of the last touch down event.
+	 */
 	private float mStartY;
+	/**
+	 * The index of the cover that is being scrolled to during a fling
+	 * animation, or -1 if the cover is the active (middle) cover.
+	 */
 	private int mTentativeCover = -1;
 	/**
 	 * Ignore the next pointer up event, for long presses.
 	 */
 	private boolean mIgnoreNextUp;
-
 	/**
 	 * If true, querySongs was called before the view initialized and should
 	 * be called when initialization finishes.
 	 */
 	private boolean mPendingQuery;
+	/**
+	 * If true, calls to invalidate() will do nothing. We use this so we can
+	 * only invalidate the dirty rect during scrolling.
+	 */
+	private boolean mSuppressInvalidate;
 
 	/**
 	 * Constructor intended to be called by inflating from XML.
@@ -151,21 +198,20 @@ public final class CoverView extends View implements Handler.Callback {
 	@Override
 	protected void onDraw(Canvas canvas)
 	{
-		super.onDraw(canvas);
-
 		int width = getWidth();
 		int height = getHeight();
+		int x = 0;
 		int scrollX = getScrollX();
 
 		canvas.drawColor(Color.BLACK);
 
-		for (int x = 0, i = 0; i != 3; ++i, x += width) {
+		for (Bitmap bitmap : mBitmaps) {
-			Bitmap bitmap = mBitmaps[i];
 			if (bitmap != null && scrollX + width > x && scrollX < x + width) {
 				int xOffset = (width - bitmap.getWidth()) / 2;
 				int yOffset = (height - bitmap.getHeight()) / 2;
 				canvas.drawBitmap(bitmap, x + xOffset, yOffset, null);
 			}
+			x += width;
 		}
 	}
 
@@ -198,7 +244,7 @@ public final class CoverView extends View implements Handler.Callback {
 			mLastMotionX = x;
 			mLastMotionY = y;
 
-			mHandler.sendEmptyMessageDelayed(MSG_LONG_CLICK, ViewConfiguration.getLongPressTimeout());
+			mUiHandler.sendEmptyMessageDelayed(MSG_LONG_CLICK, ViewConfiguration.getLongPressTimeout());
 			break;
 		case MotionEvent.ACTION_MOVE: {
 			float deltaX = mLastMotionX - x;
@@ -221,7 +267,7 @@ public final class CoverView extends View implements Handler.Callback {
 			break;
 		}
 		case MotionEvent.ACTION_UP: {
-			mHandler.removeMessages(MSG_LONG_CLICK);
+			mUiHandler.removeMessages(MSG_LONG_CLICK);
 
 			VelocityTracker velocityTracker = mVelocityTracker;
 			velocityTracker.computeCurrentVelocity(250);
@@ -266,7 +312,7 @@ public final class CoverView extends View implements Handler.Callback {
 			if (whichCover != 1)
 				mTentativeCover = whichCover;
 
-			postInvalidate();
+			mUiHandler.sendEmptyMessage(MSG_SCROLL);
 
 			if (mVelocityTracker != null) {
 				mVelocityTracker.recycle();
@@ -279,28 +325,6 @@ public final class CoverView extends View implements Handler.Callback {
 		return true;
 	}
 
-	/**
-	 * Update position for fling scroll animation and, when it is finished,
-	 * notify PlaybackService that the user has requested a track change and
-	 * update the cover art views.
-	 */
-	@Override
-	public void computeScroll()
-	{
-		if (mScroller.computeScrollOffset()) {
-			scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
-			postInvalidate();
-		} else if (mTentativeCover != -1) {
-			int delta = mTentativeCover - 1;
-			mTentativeCover = -1;
-			if (delta == 1)
-				mCallback.nextSong();
-			else
-				mCallback.previousSong();
-			resetScroll();
-		}
-	}
-
 	/**
 	 * Generates a bitmap for the given song if the cache does not contain one
 	 * for it, or moves the bitmap to the top of the cache if it does.
@@ -390,6 +414,13 @@ public final class CoverView extends View implements Handler.Callback {
 	 * @see View#performLongClick()
 	 */
 	private static final int MSG_LONG_CLICK = 2;
+	/**
+	 * Update position for fling scroll animation and, when it is finished,
+	 * notify PlaybackService that the user has requested a track change and
+	 * update the cover art views. Will resend message until scrolling is
+	 * finished.
+	 */
+	private static final int MSG_SCROLL = 3;
 
 	public boolean handleMessage(Message message)
 	{
@@ -403,6 +434,27 @@ public final class CoverView extends View implements Handler.Callback {
 				performLongClick();
 			}
 			break;
+		case MSG_SCROLL:
+			if (mScroller.computeScrollOffset()) {
+				// scrollTo calls invalidate(), however, we want to invalidate
+				// only the region where the covers are drawn, so we need to
+				// suppress this call.
+				mSuppressInvalidate = true;
+				scrollTo(mScroller.getCurrX(), mScroller.getCurrY());
+				mSuppressInvalidate = false;
+
+				invalidateCovers();
+				mUiHandler.sendEmptyMessage(MSG_SCROLL);
+			} else if (mTentativeCover != -1) {
+				int delta = mTentativeCover - 1;
+				mTentativeCover = -1;
+				if (delta == 1)
+					mCallback.nextSong();
+				else
+					mCallback.previousSong();
+				resetScroll();
+			}
+			break;
 		default:
 			return false;
 		}
@@ -430,4 +482,40 @@ public final class CoverView extends View implements Handler.Callback {
 			setMeasuredDimension(size, size);
 		}
 	}
+
+	/**
+	 * Overridden to allow redraws to be suppressed.
+	 */
+	@Override
+	public void invalidate()
+	{
+		if (!mSuppressInvalidate)
+			super.invalidate();
+	}
+
+	/**
+	 * Call {@link View#invalidate(int,int,int,int)} with the area
+	 * containing the visible cover(s).
+	 */
+	public void invalidateCovers()
+	{
+		int width = getWidth();
+		int height = getHeight();
+		int scrollX = getScrollX();
+		int x = 0;
+		int maxHeight = 0;
+
+		for (Bitmap bitmap : mBitmaps) {
+			if (bitmap != null && scrollX + width > x && scrollX < x + width) {
+				int bitmapHeight = bitmap.getHeight();
+				if (bitmapHeight > maxHeight) {
+					maxHeight = bitmapHeight;
+				}
+			}
+			x += width;
+		}
+
+		int offset = (height - maxHeight) / 2;
+		invalidate(scrollX, offset, scrollX + width, height - offset);
+	}
 }