【Android】【ViewGroup】【ListView】ListView的一些细节

来源：互联网发布：cf号怎么解封软件编辑：程序博客网时间：2024/05/22 15:42

为什么我们在Item根布局xml中设置LayoutParams无效?

<?xml version="1.0" encoding="utf-8"?><TextView    xmlns:android="http://schemas.android.com/apk/res/android"    android:id="@+id/textView1"    android:layout_width="match_parent"    android:background="#FFFF0000"    android:layout_height="100dp"    android:text="New Text"/>

类似于上面这种 layout_height =”100dp”,然后运行后发现高度还是wrap_content,这是为什么呢?
在我们的getView方法中

            final TextView tv;            if (convertView == null) {//                tv = new TextView(DiyViewActivity.this);//                tv.setLayoutParams(new ViewGroup.LayoutParams(-1,100));//                tv.setBackgroundColor(0xFFFF0000);                tv = (TextView) getLayoutInflater().inflate(R.layout.layout_item_diy_view, null);            } else {                tv = (TextView) convertView;            }

被注释的代码可以成功的设置Item的高度,但是没被注释的代码却不能.
是不是可以这么理解,tv = (TextView) getLayoutInflater().inflate(R.layout.layout_item_diy_view, null);
这句代码执行的时候 tv根本没有被加入到父布局中,我们在xml设置的 android:layout_是layout的属性是子View在父布局如何呈现的属性,所以这样设置根本是无效的,事实证明是的,在inflater的源码中

 public View inflate(XmlPullParser parser, @Nullable ViewGroup root, boolean attachToRoot)

也未发现我们设置的android:layout_属性,但是你会发现我们在TextView外面再嵌套一层布局,TextView中设置的android:layout_属性可以了,但是TextView外层的android:layout_属性又不行了.那为什么我们在代码中setLayoutParams(new ViewGroup.LayoutParams(-1,100)); 有可以呢? 还有你们也许还会发现不管Item的根布局的android:layout_属性如何设置高度都是wrap_content 宽度都是 match_parent,
这是因为在AbsListView中有这样一个方法

  View obtainView(int position, boolean[] isScrap)

在这个方法中就这样一段代码

 final View scrapView = mRecycler.getScrapView(position);        final View child = mAdapter.getView(position, scrapView, this);        if (scrapView != null) {            if (child != scrapView) {                // Failed to re-bind the data, return scrap to the heap.                mRecycler.addScrapView(scrapView, position);            } else {                isScrap[0] = true;                // Finish the temporary detach started in addScrapView().                child.dispatchFinishTemporaryDetach();            }        }        if (mCacheColorHint != 0) {            child.setDrawingCacheBackgroundColor(mCacheColorHint);        }        if (child.getImportantForAccessibility() == IMPORTANT_FOR_ACCESSIBILITY_AUTO) {            child.setImportantForAccessibility(IMPORTANT_FOR_ACCESSIBILITY_YES);        }        setItemViewLayoutParams(child, position);

final View child = mAdapter.getView(position, scrapView, this);
这个很眼熟不是吗?就是我们的getView方法啊
然后是这个方法
setItemViewLayoutParams(child, position);

 private void setItemViewLayoutParams(View child, int position) {        final ViewGroup.LayoutParams vlp = child.getLayoutParams();        LayoutParams lp;        if (vlp == null) {            lp = (LayoutParams) generateDefaultLayoutParams();        } else if (!checkLayoutParams(vlp)) {            lp = (LayoutParams) generateLayoutParams(vlp);        } else {            lp = (LayoutParams) vlp;        }        if (mAdapterHasStableIds) {            lp.itemId = mAdapter.getItemId(position);        }        lp.viewType = mAdapter.getItemViewType(position);        if (lp != vlp) {          child.setLayoutParams(lp);        }    }

答案一目了然啊
final ViewGroup.LayoutParams vlp = child.getLayoutParams();
LayoutParams lp;
if (vlp == null) {
lp = (LayoutParams) generateDefaultLayoutParams();
} else if (!checkLayoutParams(vlp)) {
lp = (LayoutParams) generateLayoutParams(vlp);
} else {
lp = (LayoutParams) vlp;
}
先获取 getLayoutParams,如果是null 调用generateDefaultLayoutParams(),
否则判断是否是 AbsListView.LayoutParams的实例或者其子类的实例,
这就是为什么我们在getView中通过代码设置LayoutParams可以成功;
在generateDefaultLayoutParams()方法中

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

可以看到这就是为什么默认宽度是match_parent ,高度是 wrap_content

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