ScrollabilityCache.java

    /**
     * <p>ScrollabilityCache holds various fields used by a View when scrolling
     * is supported. This avoids keeping too many unused fields in most
     * instances of View.</p>
     */
    private static class ScrollabilityCache implements Runnable {

        /**
         * Scrollbars are not visible
         */
        public static final int OFF = 0;

        /**
         * Scrollbars are visible
         */
        public static final int ON = 1;

        /**
         * Scrollbars are fading away
         */
        public static final int FADING = 2;

        public boolean fadeScrollBars;

        public int fadingEdgeLength;
        public int scrollBarDefaultDelayBeforeFade;
        public int scrollBarFadeDuration;

        public int scrollBarSize;
        public ScrollBarDrawable scrollBar;
        public float[] interpolatorValues;
        public View host;

        public final Paint paint;
        public final Matrix matrix;
        public Shader shader;

        public final Interpolator scrollBarInterpolator = new Interpolator(1, 2);

        private static final float[] OPAQUE = { 255 };
        private static final float[] TRANSPARENT = { 0.0f };

        /**
         * When fading should start. This time moves into the future every time
         * a new scroll happens. Measured based on SystemClock.uptimeMillis()
         */
        public long fadeStartTime;


        /**
         * The current state of the scrollbars: ON, OFF, or FADING
         */
        public int state = OFF;

        private int mLastColor;

        public ScrollabilityCache(ViewConfiguration configuration, View host) {
            fadingEdgeLength = configuration.getScaledFadingEdgeLength();
            scrollBarSize = configuration.getScaledScrollBarSize();
            scrollBarDefaultDelayBeforeFade = ViewConfiguration.getScrollDefaultDelay();
            scrollBarFadeDuration = ViewConfiguration.getScrollBarFadeDuration();

            paint = new Paint();
            matrix = new Matrix();
            // use use a height of 1, and then wack the matrix each time we
            // actually use it.
            shader = new LinearGradient(0, 0, 0, 1, 0xFF000000, 0, Shader.TileMode.CLAMP);
            paint.setShader(shader);
            paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_OUT));

            this.host = host;
        }

        public void setFadeColor(int color) {
            if (color != mLastColor) {
                mLastColor = color;

                if (color != 0) {
                    shader = new LinearGradient(0, 0, 0, 1, color | 0xFF000000,
                            color & 0x00FFFFFF, Shader.TileMode.CLAMP);
                    paint.setShader(shader);
                    // Restore the default transfer mode (src_over)
                    paint.setXfermode(null);
                } else {
                    shader = new LinearGradient(0, 0, 0, 1, 0xFF000000, 0, Shader.TileMode.CLAMP);
                    paint.setShader(shader);
                    paint.setXfermode(new PorterDuffXfermode(PorterDuff.Mode.DST_OUT));
                }
            }
        }

        public void run() {
            long now = AnimationUtils.currentAnimationTimeMillis();
            if (now >= fadeStartTime) {

                // the animation fades the scrollbars out by changing
                // the opacity (alpha) from fully opaque to fully
                // transparent
                int nextFrame = (int) now;
                int framesCount = 0;

                Interpolator interpolator = scrollBarInterpolator;

                // Start opaque
                interpolator.setKeyFrame(framesCount++, nextFrame, OPAQUE);

                // End transparent
                nextFrame += scrollBarFadeDuration;
                interpolator.setKeyFrame(framesCount, nextFrame, TRANSPARENT);

                state = FADING;

                // Kick off the fade animation
                host.invalidate(true);
            }
        }
    }

    /**
     * Resuable callback for sending
     * {@link AccessibilityEvent#TYPE_VIEW_SCROLLED} accessibility event.
     */
    private class SendViewScrolledAccessibilityEvent implements Runnable {
        public volatile boolean mIsPending;

        public void run() {
            sendAccessibilityEvent(AccessibilityEvent.TYPE_VIEW_SCROLLED);
            mIsPending = false;
        }
    }