Scroll嵌套ListView的小问题

ListView在ScrollView嵌套问题

Dont put ListView inside of a ScrollView

如果一定要这么做的话，可能会涉及到如下两点问题

ListView只显示第一个item，没有展开

这里涉及到ListView的绘制过程，产生问题的原因在ListView#onMeasure方法中。

现在我们有一个ListView，配置如下

<ListView
    android:id="@+id/scroll_list"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"/>
           

一般来说，wrap_content对应的specMode是AT_MOST，但是在ScrollView中则是UNSPECIFIED，所以导致了ListView显示不正常

D/Measure Model: onMeasure widthMode: EXACTLY
D/Measure Model: onMeasure heightMode: UNSPECIFIED
D/Measure Model: onMeasure widthMode: EXACTLY
D/Measure Model: onMeasure heightMode: UNSPECIFIED
           

因此有第一种解决方法，指定ListView的layout_height，这时specMode是EXACTLY，ListView会按指定高度绘制

<ListView
	android:layout_width="match_parent"
	android:layout_height="xxxdp"/>
           

然后在去找ListView#onMeasure方法,看一下在specMode为UNSPECIFIED的情况下，ListView如何测算高度

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

根据onMeasure源码不难看出，specMode指定为UNSPECIFIED时，只使用了一个childHeight，并不是所有childHeight。

据此我们可以动态计算ListView高度，利用LayoutParams重新设定layout_height,贴一个解决方案

好像只需要把specMode换成AT_MOST就可以了

public static void setListViewHeightBasedOnChildren(ListView listView) {
    ListAdapter listAdapter = listView.getAdapter();
    if (listAdapter == null) {
        // pre-condition
        return;
    }

    int totalHeight = 0;
    int desiredWidth = MeasureSpec.makeMeasureSpec(listView.getWidth(), MeasureSpec.AT_MOST);
    for (int i = 0; i < listAdapter.getCount(); i++) {
        View listItem = listAdapter.getView(i, null, listView);
        listItem.measure(desiredWidth, MeasureSpec.UNSPECIFIED);
        totalHeight += listItem.getMeasuredHeight();
    }

    ViewGroup.LayoutParams params = listView.getLayoutParams();
    params.height = totalHeight + (listView.getDividerHeight() * (listAdapter.getCount() - 1));
    listView.setLayoutParams(params);
    listView.requestLayout();
}
           

ListView不能滑动

这个涉及到Android的事件分发，本来以为是由于parent layout拦截touch event，导致事件在到达ListView前就被消耗。但是实际上测试一下发现并不是这样

I/EVENT: onDispatchEvent ACTION_DOWN: deal with ScrollView
I/EVENT: onInterceptTouchEvent ACTION_DOWN: judge by ScrollView
I/EVENT: onDispatchEvent ACTION_DOWN: deal with ListView
I/EVENT: onInterceptTouchEvent ACTION_DOWN: judge by ListView
I/EVENT: onTouchEvent ACTION_DOWN: deal with ListView
I/EVENT: onDispatchEvent ACTION_MOVE: deal with ScrollView
I/EVENT: onInterceptTouchEvent ACTION_MOVE: judge by ScrollView
I/EVENT: onDispatchEvent ACTION_MOVE: deal with ListView
I/EVENT: onTouchEvent ACTION_MOVE: deal with ListView
I/EVENT: onDispatchEvent ACTION_MOVE: deal with ScrollView
I/EVENT: onInterceptTouchEvent ACTION_MOVE: judge by ScrollView
I/EVENT: onDispatchEvent ACTION_CANCEL: deal with ListView
I/EVENT: onTouchEvent ACTION_CANCEL: deal with ListView
I/EVENT: onDispatchEvent ACTION_MOVE: deal with ScrollView
I/EVENT: onTouchEvent ACTION_MOVE: deal with ScrollView
I/EVENT: onDispatchEvent ACTION_MOVE: deal with ScrollView
I/EVENT: onTouchEvent ACTION_MOVE: deal with ScrollView
           

可以看到一开始

ACTION_DOWN

确实发送到了ListView中，但是ListView并没有接收完整的touch event，随后的

ACTION_MOVE

被ScrollView改写为

ACTION_CANCEL

发送到了ListView，导致ListView touch event的异常结束，该touch event被ScrollView接管。这一过程在用户看来，表现为ListView没有滑动。

贴一个requestDisallow解决方案。

利用

getParent().requestDisallowInterceptTouchEvent(boolean)

控制父ViewGroup的事件分发。根据Android事件分发的过程，这段代码写在dispatchTouchEvent里确保执行，由于ACTION_DOWN一定会到ListView的onTouchEvent中执行，所以我把代码放在了onTouchEvent中。

@Override
public boolean onTouchEvent(MotionEvent ev) {
    final int action = ev.getAction();

    switch (action){
    case MotionEvent.ACTION_DOWN:{
        getParent().requestDisallowInterceptTouchEvent(true);
        ...
        break;
    }
    case MotionEvent.ACTION_MOVE:{
    	...
        break;
    }
    case MotionEvent.ACTION_UP:
    case MotionEvent.ACTION_CANCEL:
        getParent().requestDisallowInterceptTouchEvent(false);
        break;
    }

    return super.onTouchEvent(ev);
}
           

然后继续分析输出日志，对于

ACTION_DOWN

来说，传递过程就是标准的事件分发过程，但是对于

ACTION_MOVE

而言，ListView并没有对其拦截判断，而是直接调用onTouchEvent进行处理。

在ViewGroup#dispatchTouchEvent源码中可知，onInterceptTouchEvent并不是一定被调用的。只有满足

actionMasked == MotionEvent.ACTION_DOWN

和

mFirstTouchTarget != null

存在子view可以接收touch event时，才会判断是否拦截；否则该ViewGroup会持续拦截接受到的touch event

@Override
public boolean dispatchTouchEvent(MotionEvent ev) {
	...
    // Check for interception.
    final boolean intercepted;
    if (actionMasked == MotionEvent.ACTION_DOWN
    || mFirstTouchTarget != null) {
        final boolean disallowIntercept = (mGroupFlags & FLAG_DISALLOW_INTERCEPT) != 0;
        if (!disallowIntercept) {
            intercepted = onInterceptTouchEvent(ev);
            ev.setAction(action); // restore action in case it was changed
        } else {
            intercepted = false;
        }
    } else {
        // There are no touch targets and this action is not an initial down
        // so this view group continues to intercept touches.
        intercepted = true;
    }
}
           

具体到ListView来说，当

ACTION_MOVE

传递到ListView时，已经是传递链末端，故

mFirstTouchTarget

为null，所以ListView并没有调用onInterceptionTouchEvent，而是直接处理。