Android Scrollview 嵌套 ListView 显

Scrollview 嵌套 ListView 显示不全的原因

public class ScrollView extends FrameLayout {
    ...
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
        ...
    }
    ...
}

ScrollView 继承 FrameLayout，FrameLayout -> onMeasure()

public class FrameLayout extends ViewGroup {
    ...
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        ...
        for (int i = 0; i < count; i++) {
            final View child = getChildAt(i);
            if (mMeasureAllChildren || child.getVisibility() != GONE) {
                ...
                measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
                ...
            }
        }
    ...
    }
}

ViewGroup -> measureChildWithMargins()

public abstract class ViewGroup extends View implements ViewParent, ViewManager {
    ...
    protected void measureChildWithMargins(View child,int parentWidthMeasureSpec, int widthUsed,int parentHeightMeasureSpec, int heightUsed) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin+ widthUsed, lp.width);
        final int childHeightMeasureSpec = getChildMeasureSpec(parentHeightMeasureSpec,mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin+ heightUsed, lp.height);
        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }
    ...
}

ScrollView 重写了 measureChildWithMargins() 方法

public class ScrollView extends FrameLayout {
    ...
    @Override
    protected void measureChildWithMargins(View child,int parentWidthMeasureSpec, int widthUsed,int parentHeightMeasureSpec, int heightUsed) {
        final MarginLayoutParams lp = (MarginLayoutParams) child.getLayoutParams();
        final int childWidthMeasureSpec = getChildMeasureSpec(parentWidthMeasureSpec,mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin+ widthUsed, lp.width);
        final int usedTotal = mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin +heightUsed;
        final int childHeightMeasureSpec = MeasureSpec.makeSafeMeasureSpec(
                  Math.max(0, MeasureSpec.getSize(parentHeightMeasureSpec)
                  - usedTotal) ,MeasureSpec.UNSPECIFIED);
        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }
    ...
}

ScrollView 在测量时会将子布局的 Mode 设置为 MeasureSpec.UNSPECIFIED

ListView -> onMeasure()

public class ListView extends AbsListView {
    ...
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        ...
        if (heightMode == MeasureSpec.UNSPECIFIED) {
            heightSize = mListPadding.top + mListPadding.bottom + childHeight +
                    getVerticalFadingEdgeLength() * 2;
        }
        ...
        setMeasuredDimension(widthSize, heightSize);
        ...
    }
    ...
}

当 heightMode 为 MeasureSpec.UNSPECIFIED 时，ListView 的真实高度为一个 Item 的高度，这就是 ScrollView 嵌套 ListView 显示不全的原因

Scrollview 嵌套 ListView 显示不全的解决方法

public class MyListView extends ListView {
    ...
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        heightMeasureSpec = MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2, 
                            MeasureSpec.AT_MOST);
        super.onMeasure(widthMeasureSpec, heightMeasureSpec);
    }
}

重写 onMeasure()，重新设置 heightMeasureSpec

为什么这样写能解决问题 ？

为什么将 Mode 设置成 MeasureSpec.AT_MOST ？

public class ListView extends AbsListView {
    ...
    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        ...
        if (heightMode == MeasureSpec.UNSPECIFIED) {
            heightSize = mListPadding.top + mListPadding.bottom + childHeight +
                    getVerticalFadingEdgeLength() * 2;
        }
        if (heightMode == MeasureSpec.AT_MOST) {
            heightSize = measureHeightOfChildren(widthMeasureSpec, 0, 
                         NO_POSITION, heightSize, -1);
        }
        ...
        setMeasuredDimension(widthSize, heightSize);
        ...
    }
    ...
}

上面我们将 Mode 设置成 MeasureSpec.AT_MOST，它不会执行 MeasureSpec.UNSPECIFIED 的条件判断，会执行后面的条件判断

Integer.MAX_VALUE >> 2 为什么会向右平移 2 位 ？

// MeasureSpec 是 View 中的一个静态内部类
public static class MeasureSpec {
    ...
    public static final int UNSPECIFIED = 0 << MODE_SHIFT;
    
    public static final int EXACTLY     = 1 << MODE_SHIFT;

    public static final int AT_MOST     = 2 << MODE_SHIFT;
    ...
    public static int makeMeasureSpec(@IntRange(from = 0, to = (1 << MeasureSpec.MODE_SHIFT) - 1) int size,
                                          @MeasureSpecMode int mode) {
        if (sUseBrokenMakeMeasureSpec) {
            return size + mode;
        } else {
            return (size & ~MODE_MASK) | (mode & MODE_MASK);
        }
    }
    ...
}

makeMeasureSpec 方法会根据 API 大小来执行判断，如果 API <= 17
sUseBrokenMakeMeasureSpec 就为 true，否则为 false，默认为 false

MeasureSpec 中模式只有 3 种 ：「UNSPECIFIED」「EXACTLY」「AT_MOST」
它们对应的值分别为 0、1、2，对应的二进制为 00、01、10

MeasureSpec 是 32 位的 int 值，刚才 makeMeasureSpec 设置了「AT_MOST」模式占 2 位，所以设置值时会向右平移 2 位。

ScrollView 嵌套 ListView 显示不全的原因和解决方法以及为什么这样设置介绍到这里。如果有什么写得不对的，可以在下方评论留言，我会第一时间改正。