背景

咣当咣当咣当，乘着北京的地铁上班，突然俩小伙谈话被我听到了。"今天我遇到了一个很奇怪的问题，一个ScrollView嵌套ListView的时候只显示了一行item"，我曹，这俩小伙搞Android的，有前途！又想到自己实习的时候也曾经遇到过这样的问题，后来虽然解决了，但是只知道是View测量的问题，但是并没有仔细看源码中怎么设计的。今天把它记录下来。

现象

代码很简单，先过一下

public class MainActivity extends AppCompatActivity {
    private ListView listView;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        initView();
        initData();
    }

    private void initData() {
        listView.setAdapter(new ArrayAdapter<String>(this,android.R.layout.simple_list_item_1,new String[]{
                "第一行", "第二行", "第三行", "第四行", "第五行", "第六行", "第七行", "第八行", "第九行", "第一行",
                "第十行", "第十一行", "第十二行", "第十三行", "第十四行", "第十五行", "第十六行", "第十七行", "第十八行", "第十九行",
        }));
    }

    private void initView() {
        listView = (ListView) findViewById(R.id.lv);
    }
}

布局文件：

<ScrollView xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        tools:context="cx.com.hellotinker.MainActivity">

        <ListView
            android:id="@+id/lv"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />
    </LinearLayout>
</ScrollView>

显示效果：

image

探究

ListView只显示一行，想一下自定义View的时候要重写onMeause,onLayout,onDraw(disPatchDraw)方法。认真想一下，就能知道肯定是onMeasure中的高度没有测量对。看下ListView中onMeause()的源码。

 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) {//测量模式为UNSPECIFIED时，只显示一个childHeight的高度
            heightSize = mListPadding.top + mListPadding.bottom + childHeight +
                    getVerticalFadingEdgeLength() * 2;
        }

        if (heightMode == MeasureSpec.AT_MOST) {//测量模式为AT_MOST时，调用了measureHeightOfChildren来获取到了高度
            // 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;
    }

在上面的代码中，measure只处理了两种测量模式，UNSPECIFIED和AT_MOST这两种方式。UNSPECIFIED的时候高度只显示了一行。AT_MOST的时候调用了一个方法。我们知道，我们一般只用EXACTLY和AT_MOST这两种测量模式，而UNSPECIFIED自己来用。看下AT_MOST模式下测量的高度

    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;
    }

代码很简单，用一个for循环将各个子View的高度测量出来，然后相加得到了最后的ListView的高度。那现在问题有点思路了，只显示一行的原因是因为ListView在测量时测量模式被打上了UNSPECIFIED。那谁给他打上的呢？当然是他的父布局了。接着看ScrollView的onMeasure方法

 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);
            }
        }
    }

从代码中并不能看到哪里打UNSPECIFIED这个标志。仔细看下逻辑，先调用了父类的measure方法，然后再做下面的事情，这个操作很可能就在父类中执行了。command+左键点击进入了FrameLayout中的onMeasure()方法中，继续看代码。

 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);//这里出现了问题
                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);
                    }
                }
            }
        }
        .....
  }

老套路，获取每个子view的宽和高进行测量，还是没看到哪里有问题，其实问题就在 measureChildWithMargins(child, widthMeasureSpec, 0, heightMeasureSpec, 0);看下这个方法：

   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);
    }

看了半天，感觉非常的正常这个逻辑，没问题啊，其实ScrollView把这个方法重写了，此时心里一万只草拟吗在崩腾。看下ScrollView的这个方法：

 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);//把高度的模式转换成UNSPECIFIED

        child.measure(childWidthMeasureSpec, childHeightMeasureSpec);
    }

结果出来了，这里把高度的测量模式换成了UNSPECIFIED。寻找真理的过程也是一把辛酸一把泪。总算找到了。哈哈

解决方案

知道了原因，解决方案就好搞多了，我们只要把ListView的测量模式再打回AT_MOST不就行了吗。思路：从写一个ListView，把测量模式打回AT_MOST，然后执行他本身的measure方法。写代码：

public class MyListView extends ListView {
    public MyListView(Context context) {
        super(context);
    }

    public MyListView(Context context, AttributeSet attrs) {
        super(context, attrs);
    }

    @Override
    protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
        //测量的大小由一个32位的数字表示，前两位表示测量模式，后30位表示大小，这里需要右移两位才能拿到测量的大小
        int heightSpec = MeasureSpec.makeMeasureSpec(Integer.MAX_VALUE >> 2, MeasureSpec.AT_MOST);
        super.onMeasure(widthMeasureSpec, heightSpec);
    }
}

搞定，看效果：

image

好了，我们的listView也显示出来了。知其然知其所以然,coding路漫漫。。。。。。。。