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Android滚动组件图片加载优化与滚动速度的精确监听

开发技术 502℃ 0 5个月前 (03-04)

摘要

Android滚动组件图片加载优化与滚动速度的精确监听，含源码实现。

背景

在Android应用中，ListView / RecyclerView / ScrollView 滚动时，如果有过多图片加载容易导致卡顿，特别是快速滚动时，bindView中大量图片加载操作，会导致系统频繁分配回收内存，不仅消耗大量CPU和网络流量资源，而且极端情况下还会因为内存来不及回收产生OOM。

一种最基本的优化策略是滚动时暂停加载、滚动停止才加载图片。但是这种做法很影响用户体验，用户在慢速滚动时图片完全不会加载。

因此希望实现快速滚动时暂停加载图片，慢速滚动时继续加载图片，从而平衡图片加载和滚动流畅度的体验。

速度计算

速度一般按1s时间内滚动的像素值计算。

V = diffPixels * 1000 / t_ms

滚动距离精确监听

计算速度需要知道滚动距离和时间，时间很容易计算，滚动距离相对复杂一点。

ScrollView获取滚动距离

ScrollView可以通过View.OnScrollChangeListener获取滚动距离，高版本系统可以直接调用View.setOnScrollChangeListener设置，低版本系统覆写View的onScrollChanged方法即可。

还可以参考：ScrollView滚动事件和滚动状态的监听实现

RecyclerView获取滚动距离

RecyclerView的OnScrollListener可以直接获取滚动像素值，不需要特殊处理。

ListView滚动距离的精确监听

ListView的滚动使用的不是基类View提供的滚动机制，因此不能使用View提供的onScrollChanged方法监听滚动的像素值；而ListView的OnScrollListener只能监听滚动状态、滚动到第几个Item，也不能直接取到滚动像素值。

方案1：近似实现

监听单位时间内滚动的Item数量。

存在的问题：某些Item特别长或者特别短，会导致很大的误差。例如ListView的Header可能会包含超过1屏的内容。

方案2：精确获取滚动距离

如果连续两次回调onScroll，firstVisibleItem都是同一个，则通过第一个可见View的getTop之差，就可以知道滚动距离。

如果两次firstVisibleItem差1，可在每次回调时记录下第一个、第二个可见View的Top，然后两次对同一个View的Top求差，即为滚动距离。

对于两次firstVisibleItem相差超过1的情况，即一帧时间内，滚动的距离超过了一个Item。通常是由于Item特别短，此时可以考虑丢弃数据。

速度抖动的解决

获取到滚动像素后，计算出时间，就可以计算速度了。实际使用ListView进行了尝试。

在Android开启硬件加速、不卡顿的情况下，通常每次调用onScroll的时间间隔约为16.7ms（FPS=60）。

实际测试发现，在ListView中Item布局较为复杂的情况下，可能发生卡顿，特别是在getView复用Item的时候。卡顿时会出现某些帧时间间隔偏差很大，例如只有不到10ms；滚动距离也会有较大偏差。

最后导致计算出来的速度有很大偏差。这可能导致速度在阈值附近波动，频繁暂停、启动图片加载，有可能导致一些性能问题，效果不理想。

平滑滤波

解决上述问题，可以考虑对速度做平滑滤波。例如一种简单的滤波方式如下：

/**
 * 平滑后的速度
 */
private int mSmoothedVelocity = 0;

/**
 * 速度变化
 */
public void onVelocityChanged(int velocity) {
    L.d("VelocityTracker", "onVelocityChanged, velocity = %d", velocity);
    final int smoothedVelocity = mSmoothedVelocity * 4 / 5 + velocity / 5;
    if (smoothedVelocity != mSmoothedVelocity) {
        mSmoothedVelocity = smoothedVelocity;
        onSmoothedVelocityChanged(mSmoothedVelocity);
    }
}

/**
 * 平滑处理后的速度变化
 */
public void onSmoothedVelocityChanged(int velocity) {
    L.d("VelocityTracker", "onSmoothedVelocityChanged, velocity = %d", velocity);
}

平滑前后的Log如下（启动滚动时的Log）。可以看出中间有几帧发生卡顿，原始速度从7000多减小到了1000，而平滑滤波后的速度，只是从3000多减小到2900，稳定性有了一定的提高。

D/VelocityTracker: diff = 150
D/VelocityTracker: onScrollBy, diff = 150, ms = 21
D/VelocityTracker: onVelocityChanged, velocity = 7142
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 2384
D/VelocityTracker: diff = 125
D/VelocityTracker: onScrollBy, diff = 125, ms = 16
D/VelocityTracker: onVelocityChanged, velocity = 7812
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 3469
D/VelocityTracker: onReachThreshold, reach = true
D/VelocityTracker: diff = 37
D/VelocityTracker: onScrollBy, diff = 37, ms = 12
D/VelocityTracker: onVelocityChanged, velocity = 3083
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 3391
D/VelocityTracker: diff = 7
D/VelocityTracker: onScrollBy, diff = 7, ms = 7
D/VelocityTracker: onVelocityChanged, velocity = 1000
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 2912
D/VelocityTracker: onReachThreshold, reach = false
D/VelocityTracker: diff = 96
D/VelocityTracker: onScrollBy, diff = 96, ms = 13
D/VelocityTracker: onVelocityChanged, velocity = 7384
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 3805
D/VelocityTracker: onReachThreshold, reach = true
D/VelocityTracker: diff = 117
D/VelocityTracker: onScrollBy, diff = 117, ms = 16
D/VelocityTracker: onVelocityChanged, velocity = 7312
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 4506

双阈值

用前面的平滑滤波，代码中final int smoothedVelocity = mSmoothedVelocity * 4 / 5 + velocity / 5，每次会把新速度的1/5和平滑速度的4/5相加。

这里的1/5如果取得太小，会导致平滑后的速度延迟很大；如果取得太大，则平滑效果不理想，速度波动仍然会比较大。

为了避免速度在固定阈值上下来回波动，可以使用双阈值的方式处理，例如速度下降到2000则启动图片加载，而上升到2500才暂停图片加载。当速度在2000~2500之间波动时，并不会反复切换图片加载。

延长采样周期

每一帧回调onScroll方法时都采样和计算速度，容易导致较大的速度抖动。采用了前面的平滑滤波、双阈值方法，效果还是不理想，尝试使用延长采样周期的方式处理。

实现思路是，每一帧都计算滚动距离并累加，但每8帧才做一次时间采样和速度计算。8帧会持续约0.13s，这样只要不出现连续很久的卡顿，速度的计算就是比较准确的。

下面是延长采样周期后，一次完整的滚动Log输出，可以看出，速度比较平稳的减小。即使不使用平滑滤波和双阈值，也能比较好的实现需要的效果。

D/VelocityTracker: count = 0, diff = -2147483648, mDiff = 0
D/VelocityTracker: count = 1, diff = 39
D/VelocityTracker: count = 2, diff = 45
D/VelocityTracker: count = 3, diff = 312
D/VelocityTracker: count = 4, diff = 0
D/VelocityTracker: count = 5, diff = 79
D/VelocityTracker: count = 6, diff = 97
D/VelocityTracker: count = 7, diff = 102
D/VelocityTracker: onScrollBy, diff = 776, ms = 152
D/VelocityTracker: onVelocityChanged, velocity = 5105
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 1021
D/VelocityTracker: count = 0, diff = 102
D/VelocityTracker: count = 1, diff = 96
D/VelocityTracker: count = 2, diff = 106
D/VelocityTracker: count = 3, diff = 94 
D/VelocityTracker: count = 4, diff = 98 
D/VelocityTracker: count = 5, diff = 90 
D/VelocityTracker: count = 6, diff = 95 
D/VelocityTracker: count = 7, diff = 119
D/VelocityTracker: onScrollBy, diff = 767, ms = 135
D/VelocityTracker: onVelocityChanged, velocity = 5681
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 1952
D/VelocityTracker: count = 0, diff = 69
D/VelocityTracker: count = 1, diff = 77
D/VelocityTracker: count = 2, diff = 85
D/VelocityTracker: count = 3, diff = 77
D/VelocityTracker: count = 4, diff = 75
D/VelocityTracker: count = 5, diff = 76
D/VelocityTracker: count = 6, diff = 73
D/VelocityTracker: count = 7, diff = 66
D/VelocityTracker: onScrollBy, diff = 596, ms = 131
D/VelocityTracker: onVelocityChanged, velocity = 4549
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 2470
D/VelocityTracker: count = 0, diff = 67
D/VelocityTracker: count = 1, diff = 64
D/VelocityTracker: count = 2, diff = 58
D/VelocityTracker: count = 3, diff = 58
D/VelocityTracker: count = 4, diff = 55
D/VelocityTracker: count = 5, diff = 49
D/VelocityTracker: count = 6, diff = 50
D/VelocityTracker: count = 7, diff = 45
D/VelocityTracker: onScrollBy, diff = 424, ms = 133
D/VelocityTracker: onVelocityChanged, velocity = 3187
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 2613
D/VelocityTracker: count = 0, diff = 45
D/VelocityTracker: count = 1, diff = 43
D/VelocityTracker: count = 2, diff = 38
D/VelocityTracker: count = 3, diff = 39
D/VelocityTracker: count = 4, diff = 34
D/VelocityTracker: count = 5, diff = 43
D/VelocityTracker: count = 6, diff = 27
D/VelocityTracker: count = 7, diff = 28
D/VelocityTracker: onScrollBy, diff = 282, ms = 134
D/VelocityTracker: onVelocityChanged, velocity = 2104
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 2510
D/VelocityTracker: count = 0, diff = 30
D/VelocityTracker: count = 1, diff = 28
D/VelocityTracker: count = 2, diff = 29
D/VelocityTracker: count = 3, diff = 24
D/VelocityTracker: count = 4, diff = 23
D/VelocityTracker: count = 5, diff = 22
D/VelocityTracker: count = 6, diff = 23
D/VelocityTracker: count = 7, diff = 22
D/VelocityTracker: onScrollBy, diff = 189, ms = 132
D/VelocityTracker: onVelocityChanged, velocity = 1431
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 2294
D/VelocityTracker: count = 0, diff = 18
D/VelocityTracker: count = 1, diff = 20
D/VelocityTracker: count = 2, diff = 17
D/VelocityTracker: count = 3, diff = 18
D/VelocityTracker: count = 4, diff = 16
D/VelocityTracker: count = 5, diff = 16
D/VelocityTracker: count = 6, diff = 20
D/VelocityTracker: count = 7, diff = 11
D/VelocityTracker: onScrollBy, diff = 130, ms = 133
D/VelocityTracker: onVelocityChanged, velocity = 977
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 2030
D/VelocityTracker: count = 0, diff = 12
D/VelocityTracker: count = 1, diff = 13
D/VelocityTracker: count = 2, diff = 14
D/VelocityTracker: count = 3, diff = 12
D/VelocityTracker: count = 4, diff = 11
D/VelocityTracker: count = 5, diff = 11
D/VelocityTracker: count = 6, diff = 11
D/VelocityTracker: count = 7, diff = 10
D/VelocityTracker: onScrollBy, diff = 92, ms = 133
D/VelocityTracker: onVelocityChanged, velocity = 691
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 1762
D/VelocityTracker: count = 0, diff = 10
D/VelocityTracker: count = 1, diff = 10
D/VelocityTracker: count = 2, diff = 8
D/VelocityTracker: count = 3, diff = 9
D/VelocityTracker: count = 4, diff = 8
D/VelocityTracker: count = 5, diff = 8
D/VelocityTracker: count = 6, diff = 8
D/VelocityTracker: count = 7, diff = 7
D/VelocityTracker: onScrollBy, diff = 65, ms = 133
D/VelocityTracker: onVelocityChanged, velocity = 488
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 1506
D/VelocityTracker: count = 0, diff = 7
D/VelocityTracker: count = 1, diff = 6
D/VelocityTracker: count = 2, diff = 7
D/VelocityTracker: count = 3, diff = 6
D/VelocityTracker: count = 4, diff = 6
D/VelocityTracker: count = 5, diff = 6
D/VelocityTracker: count = 6, diff = 5
D/VelocityTracker: count = 7, diff = 5
D/VelocityTracker: onScrollBy, diff = 46, ms = 133
D/VelocityTracker: onVelocityChanged, velocity = 345
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 1273
D/VelocityTracker: count = 0, diff = 5
D/VelocityTracker: count = 1, diff = 5
D/VelocityTracker: count = 2, diff = 5
D/VelocityTracker: count = 3, diff = 4
D/VelocityTracker: count = 4, diff = 4
D/VelocityTracker: count = 5, diff = 4
D/VelocityTracker: count = 6, diff = 4
D/VelocityTracker: count = 7, diff = 4
D/VelocityTracker: onScrollBy, diff = 33, ms = 134
D/VelocityTracker: onVelocityChanged, velocity = 246
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 1067
D/VelocityTracker: count = 0, diff = 3
D/VelocityTracker: count = 1, diff = 3
D/VelocityTracker: count = 2, diff = 4
D/VelocityTracker: count = 3, diff = 3
D/VelocityTracker: count = 4, diff = 3
D/VelocityTracker: count = 5, diff = 2
D/VelocityTracker: count = 6, diff = 3
D/VelocityTracker: count = 7, diff = 2
D/VelocityTracker: onScrollBy, diff = 23, ms = 132
D/VelocityTracker: onVelocityChanged, velocity = 174
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 887
D/VelocityTracker: count = 0, diff = 3
D/VelocityTracker: count = 1, diff = 2
D/VelocityTracker: count = 2, diff = 2
D/VelocityTracker: count = 3, diff = 2
D/VelocityTracker: count = 4, diff = 2
D/VelocityTracker: count = 5, diff = 2
D/VelocityTracker: count = 6, diff = 2
D/VelocityTracker: count = 7, diff = 1
D/VelocityTracker: onScrollBy, diff = 15, ms = 133
D/VelocityTracker: onVelocityChanged, velocity = 112
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 731
D/VelocityTracker: count = 0, diff = 2
D/VelocityTracker: count = 1, diff = 1
D/VelocityTracker: count = 2, diff = 2
D/VelocityTracker: count = 3, diff = 1
D/VelocityTracker: count = 4, diff = 1
D/VelocityTracker: count = 5, diff = 1
D/VelocityTracker: count = 6, diff = 1
D/VelocityTracker: count = 7, diff = 1
D/VelocityTracker: onScrollBy, diff = 9, ms = 133
D/VelocityTracker: onVelocityChanged, velocity = 67
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 597
D/VelocityTracker: count = 0, diff = 1
D/VelocityTracker: count = 1, diff = 1
D/VelocityTracker: count = 2, diff = 0
D/VelocityTracker: count = 3, diff = 1
D/VelocityTracker: count = 4, diff = 1
D/VelocityTracker: count = 5, diff = 0
D/VelocityTracker: count = 6, diff = 0
D/VelocityTracker: count = 7, diff = 1
D/VelocityTracker: onScrollBy, diff = 4, ms = 133
D/VelocityTracker: onVelocityChanged, velocity = 30
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 483
D/VelocityTracker: count = 0, diff = 0
D/VelocityTracker: count = 1, diff = 0
D/VelocityTracker: count = 2, diff = 1
D/VelocityTracker: count = 3, diff = 0
D/VelocityTracker: count = 4, diff = 0
D/VelocityTracker: count = 5, diff = 0
D/VelocityTracker: count = 6, diff = 0
D/VelocityTracker: count = 7, diff = 0
D/VelocityTracker: onSmoothedVelocityChanged, velocity = 0
D/VelocityTracker: onVelocityChanged, velocity = 0

结论

经过尝试，最后确定同时使用延长采样周期、双阈值两种方法，比较好的解决了速度抖动的问题。

代码实现

将时间采样、速度计算、阈值处理相关的逻辑，放在一个单独的类ScrollVelocityTracker里，ScrollView、ListView、RecyclerView的监听器分别调用这个类，每次传入位移像素即可。最后在回调中，可以设置图片库暂停、继续加载，从而优化图片加载性能。

完整的代码实现和Demo示例在此：

https://github.com/jzj1993/AndroidPlayground/tree/master/app/src/scrollvelocity

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本文由原创，转载请注明来源：https://www.paincker.com/android-scroll-velocity
（标注了原文链接的文章除外）

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