从源码角度分析SrcollView嵌套ListView显示不全的问题

从源码角度分析SrcollView嵌套ListView显示不全的问题

Sunxin's Bolg Sunxin's Github

问题描述

在之前开发的时候会碰到列表滑动布局中ScrollView嵌套ListView的情况，当嵌套了之后发现ListView只能显示一行数据。碰到这种情况也是

真让人头大

于是乎开始Google，很快就找到解决方法，自定义view继承自ListView，重写onMeasure()方法，然后再加入一行代码即可解决问题。

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2, MeasureSpec.AT_MOST);
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}

But... why?

真让人摸不着头脑

问题分析

首先应该可以想到，ScrollView嵌套ListView，ListView相当于ScrollView的子View了，我们应该去看看父View如何去测量子View的，打开源码先看ScrollView的onMeasure方法。

    
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);

    if (!mFillViewport) {
        return;
    }

    final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
    if (heightMode == MeasureSpec.UNSPECIFIED) {
        return;
    }

    if (getChildCount() > 0) {
        final View child = getChildAt(0);
        final int widthPadding;
        final int heightPadding;
        final int targetSdkVersion = getContext().getApplicationInfo().targetSdkVersion;
        final FrameLayout.LayoutParams lp = (LayoutParams) child.getLayoutParams();
        if (targetSdkVersion >= VERSION_CODES.M) {
            widthPadding = mPaddingLeft + mPaddingRight + lp.leftMargin + lp.rightMargin;
            heightPadding = mPaddingTop + mPaddingBottom + lp.topMargin + lp.bottomMargin;
        } else {
            widthPadding = mPaddingLeft + mPaddingRight;
            heightPadding = mPaddingTop + mPaddingBottom;
        }

        final int desiredHeight = getMeasuredHeight() - heightPadding;
        if (child.getMeasuredHeight() < desiredHeight) {
            final int childWidthMeasureSpec = getChildMeasureSpec(
                    widthMeasureSpec, widthPadding, lp.width);
            final int childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(
                    desiredHeight, MeasureSpec.EXACTLY);
            child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
        }
    }
}

发现第一行就调用了父类的onMeasure方法，点进去看看做了什么，ScrollView的父类是FrameLayout

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    int count = getChildCount();

    final boolean measureMatchParentChildren =
            MeasureSpec.getMode(widthMeasureSpec) != MeasureSpec.EXACTLY ||
            MeasureSpec.getMode(heightMeasureSpec) != MeasureSpec.EXACTLY;
    mMatchParentChildren.clear();

    int maxHeight = 0;
    int maxWidth = 0;
    int childState = 0;

    for (int i = 0; i < count; i++) {
        final View child = getChildAt(i);
        if (mMeasureAllChildren || child.getVisibility() != GONE) {
            measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);
            final LayoutParams lp = (LayoutParams) child.getLayoutParams();
            maxWidth = Math.max(maxWidth,
                    child.getMeasuredWidth() + lp.leftMargin + lp.rightMargin);
            maxHeight = Math.max(maxHeight,
                    child.getMeasuredHeight() + lp.topMargin + lp.bottomMargin);
            childState = combineMeasuredStates(childState, child.getMeasuredState());
            if (measureMatchParentChildren) {
                if (lp.width == LayoutParams.MATCH_PARENT ||
                        lp.height == LayoutParams.MATCH_PARENT) {
                    mMatchParentChildren.add(child);
                }
            }
        }
    }
}

截取了一部分代码，看呀看，发现了一个很可疑的方法

measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);

看名称是测量Child的，跟进,发现进入了ViewGroup里面。

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);
}

一眼望去，感觉应该在里面计算子View的宽高，并且把padding值和margin值都算进去了。不管那么多了，ViewGroup是ScrollView的爷爷了吧，ScrollView里面应该会重写这个方法，因为ScrollView里面也能包裹其他的View，跟进。

@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);
}

果然，这样一来ListView被ScrollView包裹，它的宽高应该是在这里被计算的，看看到底为啥只有一个条目的高度。看到这里

final int childHeightMeasureSpec = MeasureSpec.makeSafeMeasureSpec(
            Math.max(0, MeasureSpec.getSize(parentHeightMeasureSpec) - usedTotal),
            MeasureSpec.UNSPECIFIED);

这里就是计算childHeight的地方，到这里我们就应该去了解一下MeasureSpec了.
MeasureSpec： 从字面意思翻译像是测量规格，它在很大程度上决定了一个View的尺寸规则。实际上MeasureSpec 是一个32位的int值，高2位代表SpecMode（测量模式），低30位代表SpecSize（某种测量模式下的规格大小）。重点就在于这个SpecMode。

SpecMode： 测量模式有3种

• UNSPECFIED： 父容器不对View有任何限制，要多大给多大。这种情况一般用于系统内部，表示一种测量状态。
• EXACTLY： 父容器已经检测出View所需要的精确大小，这个时候View的最终大小就是SpecSize所指定的值。它对应于LayoutParams中的match_parent和具体的数值这两种模式。
• AT_MOST： 父容器指定了一个可用大小即SpecSize，View的大小不能大于这个值，具体是什么值要看不同View的具体实现。它对应于LayoutParams中的wrap_content。

private static final int MODE_SHIFT = 30;
private static final int MODE_MASK  = 0x3 << MODE_SHIFT;

/** @hide */
@IntDef({UNSPECIFIED, EXACTLY, AT_MOST})
@Retention(RetentionPolicy.SOURCE)
public @interface MeasureSpecMode {}

/**
 * Measure specification mode: The parent has not imposed any constraint
 * on the child. It can be whatever size it wants.
 */
public static final int UNSPECIFIED = 0 << MODE_SHIFT;

/**
 * Measure specification mode: The parent has determined an exact size
 * for the child. The child is going to be given those bounds regardless
 * of how big it wants to be.
 */
public static final int EXACTLY     = 1 << MODE_SHIFT;

/**
 * Measure specification mode: The child can be as large as it wants up
 * to the specified size.
 */
public static final int AT_MOST     = 2 << MODE_SHIFT;

在了解了测量模式之后，在看这段代码

final int childHeightMeasureSpec = MeasureSpec.makeSafeMeasureSpec(
            Math.max(0, MeasureSpec.getSize(parentHeightMeasureSpec) - usedTotal),
            MeasureSpec.UNSPECIFIED);

在这里ScrollView对子View的高的测量模式都制定成了MeasureSpec.UNSPECIFIED,也就是说，ListView要多高就给多高，这时候我们就可以去看看ListView到底要了多高。源码转入ListView,看看onMesaure()方法。

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    // Sets up mListPadding
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);

    final int widthMode = MeasureSpec.getMode(widthMeasureSpec);
    final int heightMode = MeasureSpec.getMode(heightMeasureSpec);
    int widthSize = MeasureSpec.getSize(widthMeasureSpec);
    int heightSize = MeasureSpec.getSize(heightMeasureSpec);

    int childWidth = 0;
    int childHeight = 0;
    int childState = 0;

    mItemCount = mAdapter == null ? 0 : mAdapter.getCount();
    if (mItemCount > 0 && (widthMode == MeasureSpec.UNSPECIFIED
            || heightMode == MeasureSpec.UNSPECIFIED)) {
        final View child = obtainView(0, mIsScrap);

        // Lay out child directly against the parent measure spec so that
        // we can obtain exected minimum width and height.
        measureScrapChild(child, 0, widthMeasureSpec, heightSize);

        childWidth = child.getMeasuredWidth();
        childHeight = child.getMeasuredHeight();
        childState = combineMeasuredStates(childState, child.getMeasuredState());

        if (recycleOnMeasure() && mRecycler.shouldRecycleViewType(
                ((LayoutParams) child.getLayoutParams()).viewType)) {
            mRecycler.addScrapView(child, 0);
        }
    }

    if (widthMode == MeasureSpec.UNSPECIFIED) {
        widthSize = mListPadding.left + mListPadding.right + childWidth +
                getVerticalScrollbarWidth();
    } else {
        widthSize |= (childState & MEASURED_STATE_MASK);
    }

    if (heightMode == MeasureSpec.UNSPECIFIED) {
        heightSize = mListPadding.top + mListPadding.bottom + childHeight +
                getVerticalFadingEdgeLength() * 2;
    }

    if (heightMode == MeasureSpec.AT_MOST) {
        // TODO: after first layout we should maybe start at the first visible position, not 0
        heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);
    }

    setMeasuredDimension(widthSize, heightSize);

    mWidthMeasureSpec = widthMeasureSpec;
}

看到这一段代码

if (heightMode == MeasureSpec.UNSPECIFIED) {
    heightSize = mListPadding.top + mListPadding.bottom + childHeight +
            getVerticalFadingEdgeLength() * 2;
}

当测量模式是MeasureSpec.UNSPECIFIED的时候，我们只看到了 heightSize = ** + childHeight **..，oh my god, 难道ListView只向他爸爸要了一个孩子的高度，这就可以解释清楚了为什么ScrollView嵌套ListView只显示一行的高度了。

问题解决

解决问题的方法在开头已经给出了，但是为什么要这么做呢。

我们在观察ListView的onMesaure方法的时候发现，当heightMode == MeasureSpec.UNSPECIFIED时，ListView只测量了一个child的高度，下面看看当heightMode == MeasureSpec.AT_MOST的时候会发生什么。

if (heightMode == MeasureSpec.AT_MOST) {
    // TODO: after first layout we should maybe start at the first visible position, not 0
    heightSize = measureHeightOfChildren(widthMeasureSpec, 0, NO_POSITION, heightSize, -1);
}

看这个方法貌似又去测量每个孩子的高度，然后赋值给heightSize了，看看方法的具体实现。

/**
 * Measures the height of the given range of children (inclusive) and
 * returns the height with this ListView's padding and divider heights
 * included. If maxHeight is provided, the measuring will stop when the
 * current height reaches maxHeight.
 *
 * @return The height of this ListView with the given children.
 */
final int measureHeightOfChildren(int widthMeasureSpec, int startPosition, int endPosition,
        int maxHeight, int disallowPartialChildPosition) {
    final ListAdapter adapter = mAdapter;
    if (adapter == null) {
        return mListPadding.top + mListPadding.bottom;
    }

    // Include the padding of the list
    int returnedHeight = mListPadding.top + mListPadding.bottom;
    final int dividerHeight = mDividerHeight;
    // The previous height value that was less than maxHeight and contained
    // no partial children
    int prevHeightWithoutPartialChild = 0;
    int i;
    View child;

    // mItemCount - 1 since endPosition parameter is inclusive
    endPosition = (endPosition == NO_POSITION) ? adapter.getCount() - 1 : endPosition;
    final AbsListView.RecycleBin recycleBin = mRecycler;
    final boolean recyle = recycleOnMeasure();
    final boolean[] isScrap = mIsScrap;

    for (i = startPosition; i <= endPosition; ++i) {
        child = obtainView(i, isScrap);

        measureScrapChild(child, i, widthMeasureSpec, maxHeight);

        if (i > 0) {
            // Count the divider for all but one child
            returnedHeight += dividerHeight;
        }

        // Recycle the view before we possibly return from the method
        if (recyle && recycleBin.shouldRecycleViewType(
                ((LayoutParams) child.getLayoutParams()).viewType)) {
            recycleBin.addScrapView(child, -1);
        }

        returnedHeight += child.getMeasuredHeight();

        if (returnedHeight >= maxHeight) {
            // We went over, figure out which height to return.  If returnedHeight > maxHeight,
            // then the i'th position did not fit completely.
            return (disallowPartialChildPosition >= 0) // Disallowing is enabled (> -1)
                        && (i > disallowPartialChildPosition) // We've past the min pos
                        && (prevHeightWithoutPartialChild > 0) // We have a prev height
                        && (returnedHeight != maxHeight) // i'th child did not fit completely
                    ? prevHeightWithoutPartialChild
                    : maxHeight;
        }

        if ((disallowPartialChildPosition >= 0) && (i >= disallowPartialChildPosition)) {
            prevHeightWithoutPartialChild = returnedHeight;
        }
    }

    // At this point, we went through the range of children, and they each
    // completely fit, so return the returnedHeight
    return returnedHeight;
}

从注释我们就可以看出，这个方法return的是The height of this ListView with the given children. 也就是ListView包含孩子的高度。看代码中

returnedHeight += child.getMeasuredHeight();

returnHeight也是在不断的累加每个孩子的高度，所以最终会得到ListView的真实高度。

所以，现在的问题就是如何指定ListView的MeasureSpec.SpecMode == MeasureSpec.AT_MOST
再来看开头的解决方法

@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
    MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2, MeasureSpec.AT_MOST);
    super.onMeasure(widthMeasureSpec, heightMeasureSpec);
}

makeMeasureSpec方法就能重新指定测量规则，第一个参数提供一个30位的SpecSize,第二个参数提供一个2位的SpecMode，然后该方法将其合并成一个新的MeasureSpec.

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);
    }
}

走完这个方法之后在调用父类的也就是ListView的onMeasure方法，重新获取测量模式和测量尺寸，这次拿到的测量模式就是AT_MOST，就可以去测量每个孩子的高度累加了。

问题 为什么要使用Integer.MAX_VALUE。

这是整型的最大值，很大很大有32位，我们在上面 measureHeightOfChildren这个方法的时候，注意一段代码

if (returnedHeight >= maxHeight) {
    // We went over, figure out which height to return.  If returnedHeight > maxHeight,
    // then the i'th position did not fit completely.
    return (disallowPartialChildPosition >= 0) // Disallowing is enabled (> -1)
                && (i > disallowPartialChildPosition) // We've past the min pos
                && (prevHeightWithoutPartialChild > 0) // We have a prev height
                && (returnedHeight != maxHeight) // i'th child did not fit completely
            ? prevHeightWithoutPartialChild
            : maxHeight;
}
return returnedHeight;

当returnedHeight >= maxHeight的时候，它会返回一个我们并不想要的值，我们需要的是returnedHeight,所以我们不能让这个判断成立，就选择了整型最大值。这个maxHeight追溯一下，就是我们设置的Integer.MAX_VALUE >> 2, 右移两位就剩下30位刚好和2位的模式配对。

结束语

此致 敬礼

作者：sun_____xin
链接：http://www.jianshu.com/p/a58f9901795b
來源：简书
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