RecyclerView设置最大高度、宽度

当RecyclerView属性设置为wrap_content+maxHeight时，maxHeight没有效果。

查看源码时发现，当RecyclerView的LayoutManager#isAutoMeasureEnabled()返回true时，RecyclerView高度取决于children view的布局高度，并非取决于RecyclerView自身的测量高度。

@Override
protected void onMeasure(int widthSpec, int heightSpec) {
    ...
    if (mLayout.mAutoMeasure) {
        ...
        // now we can get the width and height from the children.
        mLayout.setMeasuredDimensionFromChildren(widthSpec, heightSpec);
        if (mLayout.shouldMeasureTwice()) {
            ...        
            // now we can get the width and height from the children.
            mLayout.setMeasuredDimensionFromChildren(widthSpec, heightSpec);
        }
    } else {
      ...
    }
}

下面是setMeasuredDimensionFromChildren(int widthSpec, int heightSpec)源码

void setMeasuredDimensionFromChildren(int widthSpec, int heightSpec) {
    final int count = getChildCount();
    if (count == 0) {
        mRecyclerView.defaultOnMeasure(widthSpec, heightSpec);
        return;
    }
    int minX = Integer.MAX_VALUE;
    int minY = Integer.MAX_VALUE;
    int maxX = Integer.MIN_VALUE;
    int maxY = Integer.MIN_VALUE;
    for (int i = 0; i < count; i++) {
        View child = getChildAt(i);
        final Rect bounds = mRecyclerView.mTempRect;
        getDecoratedBoundsWithMargins(child, bounds);
        if (bounds.left < minX) {
            minX = bounds.left;
        }
        if (bounds.right > maxX) {
            maxX = bounds.right;
        }
        if (bounds.top < minY) {
            minY = bounds.top;
        }
        if (bounds.bottom > maxY) {
            maxY = bounds.bottom;
        }
    }
    mRecyclerView.mTempRect.set(minX, minY, maxX, maxY);
    setMeasuredDimension(mRecyclerView.mTempRect, widthSpec, heightSpec);
}

方法计算了RecyclerView当前所有Child View的布局范围mRecyclerView.mTempRect，最后调用了public void setMeasuredDimension(Rect childrenBounds, int wSpec, int hSpec)，并将得出的布局范围mRecyclerView.mTempRect、RecyclerView的测量参数widthSpec、heightSpec作为参数传入，以此来决定RecyclerView最终宽高值。

public void setMeasuredDimension(Rect childrenBounds, int wSpec, int hSpec) {
    int usedWidth = childrenBounds.width() + getPaddingLeft() + getPaddingRight();
    int usedHeight = childrenBounds.height() + getPaddingTop() + getPaddingBottom();
    int width = chooseSize(wSpec, usedWidth, getMinimumWidth());
    int height = chooseSize(hSpec, usedHeight, getMinimumHeight());
    setMeasuredDimension(width, height);
}

在这个方法中，RecyclerView的宽高通过chooseSize()方法最终决定。

public static int chooseSize(int spec, int desired, int min) {
    final int mode = View.MeasureSpec.getMode(spec);
    final int size = View.MeasureSpec.getSize(spec);
    switch (mode) {
        case View.MeasureSpec.EXACTLY:
            return size;
        case View.MeasureSpec.AT_MOST:
            return Math.min(size, Math.max(desired, min));
        case View.MeasureSpec.UNSPECIFIED:
        default:
            return Math.max(desired, min);
    }
}

可以看到，当测量模式mode为EXACTLY（明确指定宽高）时，RecyclerView大小即为指定的大小size；当测量模式为UNSPECIFIED时，RecyclerView大小取Child View布局范围和RecyclerView宽高最小值中大的值；当测量模式为AT_MOST时，取测量高度size和Math.max(desired, min)(Child View布局范围和RecyclerView宽高最小值中大的值)的小值。

解决办法

因此，我们只需要重写LayoutManager的public void setMeasuredDimension(Rect childrenBounds, int wSpec, int hSpec)方法即可为RecyclerView设置最大宽高。

override fun setMeasuredDimension(childrenBounds: Rect, wSpec: Int, hSpec: Int) {
    super.setMeasuredDimension(childrenBounds, wSpec, View.MeasureSpec.makeMeasureSpec(maxHeight, AT_MOST))
}