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

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