Android使用ViewStub提高布局性能

作者：佚名 2016-09-22 09:24:33

在Android開發(fā)中,View是我們必須要接觸的用來(lái)展示的技術(shù).通常情況下隨著View視圖的越來(lái)越復(fù)雜,整體布局的性能也會(huì)隨之下降.這里介紹一個(gè)在某些場(chǎng)景下提升布局性能的View,它就是ViewStub.

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ViewStub是什么

ViewStub是View的子類
它不可見,大小為0
用來(lái)延遲加載布局資源

注,關(guān)于Stub的解釋

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,樁是指用來(lái)代替關(guān)聯(lián)代碼或者未實(shí)現(xiàn)代碼的代碼.

ViewStub使用場(chǎng)景

如上圖所示,

一個(gè)ListView包含了諸如新聞,商業(yè),科技等Item
每個(gè)Item又包含了各自對(duì)應(yīng)的子話題,
但是子話題的View(藍(lán)色區(qū)域)只有在點(diǎn)擊展開按鈕才真正需要加載.
如果默認(rèn)加載子話題的View,則會(huì)造成內(nèi)存的占用和CPU的消耗

所以,這時(shí)候就ViewStub就派上用處了.使用ViewStub可以延遲加載布局資源.

ViewStub 怎么用

1.在布局文件中使用ViewStub標(biāo)簽

<?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.在代碼中inflate布局

ViewStub myViewStub = (ViewStub)findViewById(R.id.myViewStub); 
if (myViewStub != null) { 
    myViewStub.inflate(); 
    //或者是下面的形式加載 
    //myViewStub.setVisibility(View.VISIBLE); 
}

關(guān)于ViewStub的事

除了 inflate 方法外,我們還可以調(diào)用 setVisibility() 方法加載布局文件
一旦加載布局完成后,ViewStub會(huì)從當(dāng)前布局層級(jí)中刪除
android:id 指定ViewStub ID,用于查找ViewStub進(jìn)行延遲加載
android:layout 延遲加載布局的資源id
android:inflatedId 加載的布局被重寫的id,這里為RelativeLayout的id

ViewStub的不足

官方的文檔中有這樣一段描述

Note: One drawback of ViewStub is that it doesn’t currently support the tag in the layouts to be inflated.

意思是ViewStub不支持標(biāo)簽.

關(guān)于不支持標(biāo)簽的程度,我們進(jìn)行一個(gè)簡(jiǎn)單的驗(yàn)證

驗(yàn)證一:直接標(biāo)簽

如下,我們有布局文件名為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>

替換對(duì)應(yīng)的ViewStub的android:layout屬性值之后,運(yùn)行后(點(diǎn)擊Button按鈕)得到產(chǎn)生了如下的崩潰

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

可見,直接的標(biāo)簽,ViewStub是不支持的.

驗(yàn)證二間接的ViewStub

下面布局間接使用了merge標(biāo)簽.文件名為 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,運(yùn)行,一切正常.

除此之外,本例也驗(yàn)證了ViewStub也是對(duì) 標(biāo)簽支持良好的.

關(guān)于ViewStub的一點(diǎn)代碼剖析

inflate vs setVisibility

inflate和setVisibility的共同點(diǎn)是都可以實(shí)現(xiàn)加載布局

/**     * 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***次延遲初始化時(shí),并且visibility是非 GONE 時(shí),調(diào)用了 inflate 方法.

inflate源碼

通過(guò)閱讀下面的inflate方法實(shí)現(xiàn),我們將更加理解

android:inflatedId的用途
ViewStub在初始化后從視圖層級(jí)中移除
ViewStub的layoutParameters應(yīng)用
mInflatedViewRef通過(guò)弱引用形式,建立ViewStub與加載的View的聯(lián)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"); 
        } 
    }

責(zé)任編輯：趙寧寧來(lái)源：技術(shù)小黑屋

Android ViewStub

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Android使用ViewStub提高布局性能