Android Seekbar高低版本兼容问题之拖拽按钮位置异常问题排查

工作需要对Seekbar的拖拽按钮设置一个自定义的icon，然后就遇到一个Seekbar拖拽按钮机型适配问题。

在Android 10等高版本系统机型上Seekbar的拖拽按钮展示正常，但是在比如Android5.1系统上展示异常。异常现象是拖拽按钮偏离了轨道中心位置，拖拽按钮顶部紧贴控件的顶部边界。

Seekbar拖拽按钮是个Drawable，它的位置由Drawable#setBounds设置。所以拖拽按钮异常意味着Drawable#getBounds的返回值有误。debug了一下bounds的值，发现错误的情况下，bounds的top值是0，而正确的情况下bounds的top值是一个计算出的数字，比如60px。从设置Seekbar拖拽按钮Thumb位置的代码，可以看出top值就是offset。而offset的值是从updateThumbAndTrackPos
中获取的。因此Thumb的top值取决于trackHeight和thumbHeight的大小关系。但是问题就出现在不同系统版本上计算top值的具体方法不一致，而且高低版本差异巨大。其实低版本是有bug的。

从Andorid 6.x版本开始，源码中对方法updateThumbAndTrackPos进行了彻底重构，算是解决了这个bug，因此6.x一直到现在的10.x这个方法的具体实现就没变化过。updateThumbAndTrackPos方法的实现简单直接，Thumb的top值取决于trackHeight和thumbHeight的大小关系，不论这两个值谁更大，都可以保证Thumb可以在竖直方向上中心点落到轨道上。从下面这段代码看，不会出现偏离轨道的情况，所以排除高版本的嫌疑，这也和测试结果相符合。

    private void setThumbPos(int w, Drawable thumb, float scale, int offset) {
        ...
        final int top, bottom;
        ...
            top = offset;
            bottom = offset + thumbHeight;
        ...
        // Canvas will be translated, so 0,0 is where we start drawing
        thumb.setBounds(left, top, right, bottom);
        updateGestureExclusionRects();
    }
    private void updateThumbAndTrackPos(int w, int h) {
        final int paddedHeight = h - mPaddingTop - mPaddingBottom;
        final Drawable track = getCurrentDrawable();
        final Drawable thumb = mThumb;
        // The max height does not incorporate padding, whereas the height
        // parameter does.
        final int trackHeight = Math.min(mMaxHeight, paddedHeight);
        final int thumbHeight = thumb == null ? 0 : thumb.getIntrinsicHeight();
        // Apply offset to whichever item is taller.
        final int trackOffset;
        final int thumbOffset;
        if (thumbHeight > trackHeight) {
            final int offsetHeight = (paddedHeight - thumbHeight) / 2;
            trackOffset = offsetHeight + (thumbHeight - trackHeight) / 2;
            thumbOffset = offsetHeight;
        } else {
            final int offsetHeight = (paddedHeight - trackHeight) / 2;
            trackOffset = offsetHeight;
            thumbOffset = offsetHeight + (trackHeight - thumbHeight) / 2;
        }
        ...
        if (thumb != null) {
            setThumbPos(w, thumb, getScale(), thumbOffset);
        }
    }

    
    public void setThumb(Drawable thumb) {
        final boolean needUpdate;
        if (mThumb != null && thumb != mThumb) {
            mThumb.setCallback(null);
            needUpdate = true;
        } else {
            needUpdate = false;
        }
        ...
        if (needUpdate) {
            updateThumbAndTrackPos(getWidth(), getHeight());
            ...
        }
    }
    private void updateThumbAndTrackPos(int w, int h) {
        final Drawable track = getCurrentDrawable();
        final Drawable thumb = mThumb;
        // The max height does not incorporate padding, whereas the height
        // parameter does.
        final int trackHeight = Math.min(mMaxHeight, h - mPaddingTop - mPaddingBottom);
        final int thumbHeight = thumb == null ? 0 : thumb.getIntrinsicHeight();
        // Apply offset to whichever item is taller.
        final int trackOffset;
        final int thumbOffset;
        if (thumbHeight > trackHeight) {
            trackOffset = (thumbHeight - trackHeight) / 2;
            thumbOffset = 0;
        } else {
            trackOffset = 0;
            thumbOffset = (trackHeight - thumbHeight) / 2;
        }
        ...
        if (thumb != null) {
            setThumbPos(w, thumb, getScale(), thumbOffset);
        }
    }

private void updateThumbPos(int w, int h) {
    if (thumbHeight > trackHeight) {
        if (thumb != null) {
            setThumbPos(w, thumb, scale, 0);
        }
        ...
    } else {
        ...
        int gap = (trackHeight - thumbHeight) / 2;
        if (thumb != null) {
            setThumbPos(w, thumb, scale, gap);
        }
    }
}

final int trackHeight = Math.min(mMaxHeight, h - mPaddingTop - mPaddingBottom);

private void initProgressBar() {
        mMax = 100;
        mProgress = 0;
        mSecondaryProgress = 0;
        mIndeterminate = false;
        mOnlyIndeterminate = false;
        mDuration = 4000;
        mBehavior = AlphaAnimation.RESTART;
        mMinWidth = 24;
        mMaxWidth = 48;
        mMinHeight = 24;
        mMaxHeight = 48;
    }

mMaxHeight = a.getDimensionPixelSize(R.styleable.ProgressBar_maxHeight, mMaxHeight);

android:maxHeight="180dip"