從源碼角度分析SrcollView嵌套ListView顯示不全的問題

Sunxin's Bolg
Sunxin's Github

問題描述

在之前開發(fā)的時(shí)候會(huì)碰到列表滑動(dòng)布局中ScrollView嵌套ListView的情況，當(dāng)嵌套了之后發(fā)現(xiàn)ListView只能顯示一行數(shù)據(jù)。碰到這種情況也是

真讓人頭大

于是乎開始Google，很快就找到解決方法，自定義view繼承自ListView，重寫onMeasure()方法，然后再加入一行代碼即可解決問題。

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

But... why?

真讓人摸不著頭腦

問題分析

首先應(yīng)該可以想到，ScrollView嵌套ListView，ListView相當(dāng)于ScrollView的子View了，我們應(yīng)該去看看父View如何去測(cè)量子View的，打開源碼先看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);
        }
    }
}

發(fā)現(xiàn)第一行就調(diào)用了父類的onMeasure方法，點(diǎn)進(jìn)去看看做了什么，ScrollView的父類是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);
                }
            }
        }
    }
}

截取了一部分代碼，看呀看，發(fā)現(xiàn)了一個(gè)很可疑的方法

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

看名稱是測(cè)量Child的，跟進(jìn),發(fā)現(xiàn)進(jìn)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，感覺應(yīng)該在里面計(jì)算子View的寬高，并且把padding值和margin值都算進(jìn)去了。不管那么多了，ViewGroup是ScrollView的爺爺了吧，ScrollView里面應(yīng)該會(huì)重寫這個(gè)方法，因?yàn)镾crollView里面也能包裹其他的View，跟進(jìn)。

@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，這樣一來ListView被ScrollView包裹，它的寬高應(yīng)該是在這里被計(jì)算的，看看到底為啥只有一個(gè)條目的高度?？吹竭@里

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

這里就是計(jì)算childHeight的地方，到這里我們就應(yīng)該去了解一下MeasureSpec了.
MeasureSpec： 從字面意思翻譯像是測(cè)量規(guī)格，它在很大程度上決定了一個(gè)View的尺寸規(guī)則。實(shí)際上MeasureSpec 是一個(gè)32位的int值，高2位代表SpecMode（測(cè)量模式），低30位代表SpecSize（某種測(cè)量模式下的規(guī)格大?。?。重點(diǎn)就在于這個(gè)SpecMode。

SpecMode： 測(cè)量模式有3種

• UNSPECFIED： 父容器不對(duì)View有任何限制，要多大給多大。這種情況一般用于系統(tǒng)內(nèi)部，表示一種測(cè)量狀態(tài)。
• EXACTLY： 父容器已經(jīng)檢測(cè)出View所需要的精確大小，這個(gè)時(shí)候View的最終大小就是SpecSize所指定的值。它對(duì)應(yīng)于LayoutParams中的match_parent和具體的數(shù)值這兩種模式。
• AT_MOST： 父容器指定了一個(gè)可用大小即SpecSize，View的大小不能大于這個(gè)值，具體是什么值要看不同View的具體實(shí)現(xiàn)。它對(duì)應(yīng)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;

在了解了測(cè)量模式之后，在看這段代碼

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

在這里ScrollView對(duì)子View的高的測(cè)量模式都制定成了MeasureSpec.UNSPECIFIED,也就是說，ListView要多高就給多高，這時(shí)候我們就可以去看看ListView到底要了多高。源碼轉(zhuǎn)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;
}

看到這一段代碼

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

當(dāng)測(cè)量模式是MeasureSpec.UNSPECIFIED的時(shí)候，我們只看到了 heightSize = ** + childHeight **..，oh my god, 難道ListView只向他爸爸要了一個(gè)孩子的高度，這就可以解釋清楚了為什么ScrollView嵌套ListView只顯示一行的高度了。

問題解決

解決問題的方法在開頭已經(jīng)給出了，但是為什么要這么做呢。

我們?cè)谟^察ListView的onMesaure方法的時(shí)候發(fā)現(xiàn)，當(dāng)heightMode == MeasureSpec.UNSPECIFIED時(shí)，ListView只測(cè)量了一個(gè)child的高度，下面看看當(dāng)heightMode == MeasureSpec.AT_MOST的時(shí)候會(huì)發(fā)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);
}

看這個(gè)方法貌似又去測(cè)量每個(gè)孩子的高度，然后賦值給heightSize了，看看方法的具體實(shí)現(xiàn)。

/**
 * 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;
}

從注釋我們就可以看出，這個(gè)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?？创a中

returnedHeight += child.getMeasuredHeight();

returnHeight也是在不斷的累加每個(gè)孩子的高度，所以最終會(huì)得到ListView的真實(shí)高度。

所以，現(xiàn)在的問題就是如何指定ListView的MeasureSpec.SpecMode == MeasureSpec.AT_MOST
再來看開頭的解決方法

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

makeMeasureSpec方法就能重新指定測(cè)量規(guī)則，第一個(gè)參數(shù)提供一個(gè)30位的SpecSize,第二個(gè)參數(shù)提供一個(gè)2位的SpecMode，然后該方法將其合并成一個(gè)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);
    }
}

走完這個(gè)方法之后在調(diào)用父類的也就是ListView的onMeasure方法，重新獲取測(cè)量模式和測(cè)量尺寸，這次拿到的測(cè)量模式就是AT_MOST，就可以去測(cè)量每個(gè)孩子的高度累加了。

問題 為什么要使用Integer.MAX_VALUE。

這是整型的最大值，很大很大有32位，我們?cè)谏厦?measureHeightOfChildren這個(gè)方法的時(shí)候，注意一段代碼

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;

當(dāng)returnedHeight >= maxHeight的時(shí)候，它會(huì)返回一個(gè)我們并不想要的值，我們需要的是returnedHeight,所以我們不能讓這個(gè)判斷成立，就選擇了整型最大值。這個(gè)maxHeight追溯一下，就是我們?cè)O(shè)置的Integer.MAX_VALUE >> 2, 右移兩位就剩下30位剛好和2位的模式配對(duì)。

結(jié)束語

此致 敬禮