Android中使用ViewStub实现布局优化

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.