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

1. ViewStub是View的子类

2. 它不可见,大小为0

3. 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,

1. 一个ListView包含了诸如 新闻,商业,科技 等Item

2. 每个Item又包含了各自对应的子话题,

3. 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.

4. 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.