介绍
--
ViewStub 是一个轻量级的View，没有尺寸，不绘制任何东西，因此绘制或者移除时更省时。(ViewStub不可见，大小为0)
优点
--
实现View的延迟加载，避免资源的浪费，减少渲染时间，在需要的时候才加载View
缺点
--
ViewStub所要替代的layout文件中不能有`<merge>`标签
ViewStub在加载完后会被移除，或者说是被加载进来的layout替换掉了
用法
```
<ViewStub
android:id="@+id/stub_import"
android:inflatedId="@+id/panel_import"
android:layout="@layout/progress_overlay"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_gravity="bottom" />
```
用ViewStub加载layout文件时，可以调用 setVisibility(View.VISIBLE) 或者 inflate()
```
((ViewStub) findViewById(R.id.stub_import)).setVisibility(View.VISIBLE);
// or
View importPanel = ((ViewStub) findViewById(R.id.stub_import)).inflate();
```
注意
--
- 一旦ViewStub visible/inflated，则ViewStub将从视图框架中移除，其id stub_import 也会失效
- ViewStub被绘制完成的layout文件取代，并且该layout文件的root view的id是android:inflatedId指定的id panel_import，root view的布局和ViewStub视图的布局保持一致
```
<!-- layout_viewstub.xml 要延迟加载的view -->
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout android:id="@+id/layout_viewstub_old"
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical">
<TextView
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_margin="5dp"
android:background="@android:color/darker_gray"
android:padding="5dp"
android:text="This is the layout instead of ViewStub view."/>
</LinearLayout>
```
```
<!-- act_test_viewstub.xml -->
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:gravity="center_horizontal"
android:orientation="vertical">
<TextView
android:id="@+id/act_test_viewstub_tv_show"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_margin="5dp"
android:background="@android:color/darker_gray"
android:padding="5dp"
android:text="Show ViewStub"/>
<ViewStub
android:id="@+id/act_test_viewstub_viewstub"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:inflatedId="@+id/act_layout_viewstub_new"
android:layout="@layout/layout_viewstub"/>
<!--<include-->
<!--android:layout_width="wrap_content"-->
<!--android:layout_height="wrap_content"-->
<!--layout="@layout/layout_viewstub"/>-->
</LinearLayout>
```
```
public class ViewStubTestActivity extends FragmentActivity {
private static final String TAG = "test_viewstub";
protected ViewStub mViewStub;
@Override
protected void onCreate(@Nullable Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.act_test_viewstub);
mViewStub = (ViewStub) findViewById(R.id.act_test_viewstub_viewstub);
Log.e(TAG, "viewstub: " + findViewById(R.id.act_test_viewstub_viewstub));
Log.e(TAG, "layout: " + findViewById(R.id.act_layout_viewstub_new));
findViewById(R.id.act_test_viewstub_tv_show).setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
View layoutView;
// mViewStub.setVisibility(View.VISIBLE);
layoutView = mViewStub.inflate();
Log.e(TAG, "mViewStub: " + mViewStub);
// ViewStub在visible/inflated后会被移除，所以此处为null
Log.e(TAG, "viewstub: " + findViewById(R.id.act_test_viewstub_viewstub));
// layoutView = findViewById(R.id.act_layout_viewstub_new);
Log.e(TAG, "layoutView equals finviewbyid(layout): " +
layoutView.equals(findViewById(R.id.act_layout_viewstub_new)));
Log.e(TAG, "layout: " + layoutView);
if (layoutView != null) {
// layoutView的root view id 是mViewStub inflatedId指定的ID
if (layoutView.getId() == R.id.act_layout_viewstub_new) {
Log.e(TAG, "layout root id is act_layout_viewstub_new");
} else if (layoutView.getId() == R.id.layout_viewstub_old) {
Log.e(TAG, "layout root id is layout_viewstub_old");
} else {
Log.e(TAG, "layout root id is anyone : " + layoutView.getId());
}
// layoutView的root view布局 和mViewStub的布局保持一致
int width = layoutView.getLayoutParams().width;
if (width == ViewGroup.LayoutParams.MATCH_PARENT) {
Log.e(TAG, "layout width is MATCH_PARENT");
} else if (width == ViewGroup.LayoutParams.WRAP_CONTENT) {
Log.e(TAG, "layout width is WRAP_CONTENT");
} else {
Log.e(TAG, "layout width is anyone : " + width);
}
}
}
});
}
}
```
