标签：inflate ViewStub layout 布局 AndroidRuntime Android android view
在Android开发中,View是我们必须要接触的用来展示的技术.通常情况下随着View视图的越来越复杂,整体布局的性能也会随之下降.这里介绍一个在某些场景下提升布局性能的View,它就是ViewStub.
ViewStub是什么
- ViewStub是View的子类
- 它不可见,大小为0
- 用来延迟加载布局资源
注,关于Stub的解释
A stub is a small program routine that substitutes for a longer program, possibly to be loaded later or that is located remotely
在Java中,桩是指用来代替关联代码或者未实现代码的代码.
ViewStub使用场景
如上图所示,
- 一个ListView包含了诸如
新闻,商业,科技等Item - 每个Item又包含了各自对应的子话题,
- 但是子话题的View(蓝色区域)只有在点击展开按钮才真正需要加载.
- 如果默认加载子话题的View,则会造成内存的占用和CPU的消耗
所以,这时候就ViewStub就派上用处了.使用ViewStub可以延迟加载布局资源.
ViewStub 怎么用
- 在布局文件中使用ViewStub标签
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin"
tools:context="com.droidyue.viewstubsample.MainActivity">
<Button
android:id="@+id/clickMe"
android:text="Hello World!"
android:layout_width="wrap_content"
android:layout_height="wrap_content"/>
<ViewStub
android:id="@+id/myViewStub"
android:inflatedId="@+id/myInflatedViewId"
android:layout="@layout/include_merge"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@id/clickMe"
/>
</RelativeLayout> |
2.在代码中inflate布局
ViewStub myViewStub = (ViewStub)findViewById(R.id.myViewStub);
if (myViewStub != null) {
myViewStub.inflate();
//或者是下面的形式加载
//myViewStub.setVisibility(View.VISIBLE);
} |
关于ViewStub的事
- 除了
inflate方法外,我们还可以调用setVisibility()方法加载布局文件 - 一旦加载布局完成后,ViewStub会从当前布局层级中删除
-
android:id指定ViewStub ID,用于查找ViewStub进行延迟加载 -
android:layout延迟加载布局的资源id -
android:inflatedId加载的布局被重写的id,这里为RelativeLayout的id
ViewStub的不足
官方的文档中有这样一段描述
Note: One drawback of ViewStub is that it doesn’t currently support the
tag in the layouts to be inflated.
意思是ViewStub不支持<merge>标签.
关于不支持<merge>标签的程度,我们进行一个简单的验证
验证一:直接
标签
如下,我们有布局文件名为merge_layout.xml
<merge xmlns:android="http://schemas.android.com/apk/res/android">
<Button
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="Yes"/>
<Button
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:text="No"/>
</merge> |
替换对应的ViewStub的android:layout属性值之后,运行后(点击Button按钮)得到产生了如下的崩溃
E AndroidRuntime: android.view.InflateException: Binary XML file line #1: <merge /> can be used only with a valid ViewGroup root and attachToRoot=true
E AndroidRuntime: at android.view.LayoutInflater.inflate(LayoutInflater.java:551)
E AndroidRuntime: at android.view.LayoutInflater.inflate(LayoutInflater.java:429)
E AndroidRuntime: at android.view.ViewStub.inflate(ViewStub.java:259)
E AndroidRuntime: at com.droidyue.viewstubsample.MainActivity$1.onClick(MainActivity.java:20)
E AndroidRuntime: at android.view.View.performClick(View.java:5697)
E AndroidRuntime: at android.widget.TextView.performClick(TextView.java:10815)
E AndroidRuntime: at android.view.View$PerformClick.run(View.java:22526)
E AndroidRuntime: at android.os.Handler.handleCallback(Handler.java:739)
E AndroidRuntime: at android.os.Handler.dispatchMessage(Handler.java:95)
E AndroidRuntime: at android.os.Looper.loop(Looper.java:158)
E AndroidRuntime: at android.app.ActivityThread.main(ActivityThread.java:7237)
E AndroidRuntime: at java.lang.reflect.Method.invoke(Native Method)
E AndroidRuntime: at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
E AndroidRuntime: at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
E AndroidRuntime: Caused by: android.view.InflateException: <merge /> can be used only with a valid ViewGroup root and attachToRoot=true
E AndroidRuntime: at android.view.LayoutInflater.inflate(LayoutInflater.java:491)
E AndroidRuntime: ... 13 more |
可见,直接的<merge>标签,ViewStub是不支持的.
验证二 间接的ViewStub
下面布局间接使用了merge标签.文件名为include_merge.xml
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent">
<include layout="@layout/merge_layout"/>
</LinearLayout> |
然后修改ViewStub的android:layout值,运行,一切正常.
除此之外,本例也验证了ViewStub也是对<include>标签支持良好的.
关于ViewStub的一点代码剖析
inflate vs setVisibility
inflate和setVisibility的共同点是都可以实现加载布局
/**
* When visibility is set to {@link #VISIBLE} or {@link #INVISIBLE},
* {@link #inflate()} is invoked and this StubbedView is replaced in its parent
* by the inflated layout resource.
*
* @param visibility One of {@link #VISIBLE}, {@link #INVISIBLE}, or {@link #GONE}.
*
* @see #inflate()
*/
@Override
public void setVisibility(int visibility) {
if (mInflatedViewRef != null) {
View view = mInflatedViewRef.get();
if (view != null) {
view.setVisibility(visibility);
} else {
throw new IllegalStateException("setVisibility called on un-referenced view");
}
} else {
super.setVisibility(visibility);
if (visibility == VISIBLE || visibility == INVISIBLE) {
inflate();
}
}
} |
setVisibility只是在ViewStub第一次延迟初始化时,并且visibility是非GONE时,调用了inflate方法.
inflate源码
通过阅读下面的inflate方法实现,我们将更加理解
- android:inflatedId的用途
- ViewStub在初始化后从视图层级中移除
- ViewStub的layoutParameters应用
- mInflatedViewRef通过弱引用形式,建立ViewStub与加载的View的联系.
/**
* Inflates the layout resource identified by {@link #getLayoutResource()}
* and replaces this StubbedView in its parent by the inflated layout resource.
*
* @return The inflated layout resource.
*
*/
public View inflate() {
final ViewParent viewParent = getParent();
if (viewParent != null && viewParent instanceof ViewGroup) {
if (mLayoutResource != 0) {
final ViewGroup parent = (ViewGroup) viewParent;
final LayoutInflater factory = LayoutInflater.from(mContext);
final View view = factory.inflate(mLayoutResource, parent,
false);
if (mInflatedId != NO_ID) {
view.setId(mInflatedId);
}
final int index = parent.indexOfChild(this);
parent.removeViewInLayout(this);
final ViewGroup.LayoutParams layoutParams = getLayoutParams();
if (layoutParams != null) {
parent.addView(view, index, layoutParams);
} else {
parent.addView(view, index);
}
mInflatedViewRef = new WeakReference<View>(view);
if (mInflateListener != null) {
mInflateListener.onInflate(this, view);
}
return view;
} else {
throw new IllegalArgumentException("ViewStub must have a valid layoutResource");
}
} else {
throw new IllegalStateException("ViewStub must have a non-null ViewGroup viewParent");
}
} |
关于ViewStub的研究就是这些,希望对大家关于优化视图有所帮助和启发.
标签：inflate,
ViewStub,
layout,
布局,
AndroidRuntime,
Android,
android,
view
From： https://blog.51cto.com/u_3987305/5842091