ListView item布局layout_width和layout_height属性

最新推荐文章于 2021-05-27 05:19:14 发布

原创最新推荐文章于 2021-05-27 05:19:14 发布 · 1k 阅读

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#listview #itemView

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本文深入解析了Android中ListView的工作原理，特别是其子项View的创建和测量过程。详细阐述了不同inflate方法对Item布局的影响及LayoutParams的作用。

首先了解些关于布局的知识：
LayoutParams类：LayoutParams are used by views to tell their parents how they want to be laid out，是告诉父布局它想如何展示；其属性match_parent, warp_content都是相对于父布局来说的，父布局在 measure 子布局的时候首先会获取子布局的LayoutParams： LayoutParams p = (LayoutParams) child.getLayoutParams();之后结合父布局的widthMeasureSpec和heightMeasureSpec得到childWidthSpec, childHeightSpec之后调用child.measure(childWidthSpec, childHeightSpec);对child进行measure；
ListView通过getView函数获得item view，具体是通过函数：public View inflate(@LayoutRes int resource, @Nullable ViewGroup root, boolean attachToRoot);的调用来生成view；

inflate函数有三种调用方式：

1.view = inflater.inflate(R.layout.view_item, viewGroup, false);
2.view = inflater.inflate(R.layout.view_item, null);
3.view = inflater.inflate(R.layout.view_item, viewGroup, false);

对于listview支持两种1和2；
对于方式3是不支持的，如果使用这种调用方式，程序会出错，在inflate执行到下面的addview时出错：

if (root != null && attachToRoot) {
    root.addView(temp, params);
}

root就是ListView也就是AdapterView，很遗憾不支持addview；

throw new UnsupportedOperationException("addView(View, LayoutParams) "
            + "is not supported in AdapterView");

所以只能使用方式1和2；
这里的讲解可以参考这里

使用方式1，那么item的顶层布局的layout_width和layout_height都将失效，请无视它们吧，运行时啥作用也不会起；原因看下面代码 ①：

final View temp = createViewFromTag(root, name, inflaterContext, attrs);

ViewGroup.LayoutParams params = null;

if (root != null) {
  ......
    // Create layout params that match root, if supplied
    params = root.generateLayoutParams(attrs);
    if (!attachToRoot) {
        // Set the layout params for temp if we are not
        // attaching. (If we are, we use addView, below)
        temp.setLayoutParams(params);
    }
}

......
// We are supposed to attach all the views we found (int temp)
// to root. Do that now.
if (root != null && attachToRoot) {
    root.addView(temp, params);
}

// Decide whether to return the root that was passed in or the
// top view found in xml.
if (root == null || !attachToRoot) {
    result = temp;
}
......
return result;

如上final View temp = createViewFromTag(root, name, inflaterContext, attrs);这里新鲜出炉的view的layoutParams是null；而如果root == null，则temp.setLayoutParams(params);不会执行，result = temp;之后就返回了，listview在measure的时候对LayoutParams为null这种情况作了判断：
ListView的layout_height为warp_content时：

private void measureScrapChild(View child, int position, int widthMeasureSpec) {
    LayoutParams p = (LayoutParams) child.getLayoutParams();
    if (p == null) {
        p = (AbsListView.LayoutParams) generateDefaultLayoutParams();
        child.setLayoutParams(p);
    }

ListView的layout_height为match_parent时，执行layout()时进行了判断：

 private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,
                        boolean selected, boolean recycled) {
    ......
    AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
    if (p == null) {
        p = (AbsListView.LayoutParams) generateDefaultLayoutParams();
    }
}

再来看下generateDefaultLayoutParams函数：

@Override
protected ViewGroup.LayoutParams generateDefaultLayoutParams() {
    return new AbsListView.LayoutParams(ViewGroup.LayoutParams.MATCH_PARENT,
            ViewGroup.LayoutParams.WRAP_CONTENT, 0);
}

ViewGroup.LayoutParams.WRAP_CONTENT就不用看了，很多时候没啥用，这也是root为null时我们的 item布局总是能横向铺满的原因了；

root不为null的情况，则执行params = root.generateLayoutParams(attrs);这里就获得了当初设置的layout_width和layout_height;之后执行了temp.setLayoutParams(params);用于addview以及addViewInner；所以item顶层width或height的具体数值，如：100dp是有作用的；

很遗憾，item的布局顶层以及之下的match_parent或者warp_content都没啥作用，因为ListView的childHeightSpec 完全是由子view的LayoutParams决定的；除非子view自己争气挣点高度空间，否则高度就是0，比如放个view，高度match_parent，布局预览的时候好好的，运行的时候就消失了，因为高度是0；例外是父层布局有具体的高度，则其子view的match_parent和warp_content仍然有意义；想一想也是这个道理，ListView本来的高度就是不固定，你match_parent，想match到哪里去；照例找下代码的证据：

private void setupChild(View child, int position, int y, boolean flowDown, int childrenLeft,
                        boolean selected, boolean recycled) {
    ......
    AbsListView.LayoutParams p = (AbsListView.LayoutParams) child.getLayoutParams();
    ......
    int lpHeight = p.height;
    int childHeightSpec;
    if (lpHeight > 0) {
        childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
    } else {
        childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);
    }
    child.measure(childWidthSpec, childHeightSpec);
}

private void measureScrapChild(View child, int position, int widthMeasureSpec) {
    LayoutParams p = (LayoutParams) child.getLayoutParams();
    ......
    int lpHeight = p.height;
    ......
    if (lpHeight > 0) {
        childHeightSpec = MeasureSpec.makeMeasureSpec(lpHeight, MeasureSpec.EXACTLY);
    } else {
        childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);
    }
    child.measure(childWidthSpec, childHeightSpec);
}

MATCH_PARENT值是-1WRAP_CONTENT值是-2，所以lpHeight < 0将执行childHeightSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);对于子view来说，就是看它自己的本事，自己有多大占多大；MeasureSpec.UNSPECIFIED可以传递，子view如果还是match_parent或者warp_content，那还会执行childHeightMeasureSpec = MeasureSpec.makeMeasureSpec(0, MeasureSpec.UNSPECIFIED);

对于view的绘制流程讲解，推荐看这里