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

1.在布局文件中使用ViewStub标签

<?xml version="1.0" encoding="utf-8"?> 
<RelativeLayout 
        xmlns:andro 
        xmlns:tools="http://schemas.android.com/tools" 
        android:layout_ 
        android:layout_ 
        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: 
            android:text="Hello World!" 
            android:layout_ 
            android:layout_/> 
     
    <ViewStub 
            android: 
            android:inflated 
            android:layout="@layout/include_merge" 
            android:layout_ 
            android:layout_ 
            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 标签.

关于不支持 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:andro> 
 
    <Button            android:layout_ 
            android:layout_ 
            android:text="Yes"/> 
 
    <Button            android:layout_ 
            android:layout_ 
            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 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:andro 
              android:orientation="vertical" 
              android:layout_ 
              android:layout_> 
        <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 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"); 
        } 
    } 

来自：http://mobile.51cto.com/android-517729.htm

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