智能座舱media widget跑马灯功能cpu loading太高优化

一、背景
智能座舱在播放音乐的时候，有歌曲名称的media widget，当歌曲名称比较长的情况下，会有走马灯Marquee的效果，然后在桌面待机状态下，测试发现cpu loading过高，总体cpu负载120%左右，针对这种场景，需要优化这一特定场景的cpu 负载。

二、问题分析以及修改方案

通过抓systrace分析可知，launcher3 的app刷新率是60fps，通过结合代码分析，是media widget跑马灯功能固定的60fps，导致cpu loading过高，要想办法将60fps降低到30fps。

通过代码跟踪，最后找到是TextView的tick()函数会根据屏幕刷新率申请app vsync实现60fps的帧率效果。

        void tick() {
            if (mStatus != MARQUEE_RUNNING) {
                return;
            }

            mChoreographer.removeFrameCallback(mTickCallback);

            final TextView textView = mView.get();
            if (textView != null && textView.isAggregatedVisible()
                    && (textView.isFocused() || textView.isSelected())) {
                long currentMs = mChoreographer.getFrameTime();
                long deltaMs = currentMs - mLastAnimationMs;
                mLastAnimationMs = currentMs;
                float deltaPx = deltaMs * mPixelsPerMs;
                mScroll += deltaPx;
                if (mScroll > mMaxScroll) {
                    mScroll = mMaxScroll;
                    mChoreographer.postFrameCallbackDelayed(mRestartCallback, MARQUEE_DELAY);
                } else {
                    mChoreographer.postFrameCallback(mTickCallback);
                }
                textView.invalidate();
            }
        }

修改方案：postFrameCallbackDelayed 错开16.6ms的vsync，就能实现32ms的vsync。

long delayTimeMs = 1000 / 60 + 2;
mChoreographer.postFrameCallbackDelayed(mTickCallback, delayTimeMs);

        void tick() {
            if (mStatus != MARQUEE_RUNNING) {
                return;
            }

            mChoreographer.removeFrameCallback(mTickCallback);

            final TextView textView = mView.get();
            if (textView != null && textView.isAggregatedVisible()
                    && (textView.isFocused() || textView.isSelected())) {
                long currentMs = mChoreographer.getFrameTime();
                long deltaMs = currentMs - mLastAnimationMs;
                mLastAnimationMs = currentMs;
                float deltaPx = deltaMs * mPixelsPerMs;
                mScroll += deltaPx;
                if (mScroll > mMaxScroll) {
                    mScroll = mMaxScroll;
                    mChoreographer.postFrameCallbackDelayed(mRestartCallback, MARQUEE_DELAY);
                } else {
                    long delayTimeMs = 1000 / 60 + 2;
                    mChoreographer.postFrameCallbackDelayed(mTickCallback, delayTimeMs);
                }
                textView.invalidate();
            }
        }

修改之后的systace：

三、实验对比数据
通过实验对比数据可以看到，30fps能够降低cpu loading 30%以上，还是比较可观的。