Android中怎么利用ViewStub提高布局性能(android,viewstub,移动开发)

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

<?xmlversion="1.0"encoding="utf-8"?><RelativeLayoutxmlns: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"><Buttonandroid:id="@+id/clickMe"android:text="HelloWorld!"android:layout_width="wrap_content"android:layout_height="wrap_content"/><ViewStubandroid: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

ViewStubmyViewStub=(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

<mergexmlns:android="http://schemas.android.com/apk/res/android"><Buttonandroid:layout_width="fill_parent"android:layout_height="wrap_content"android:text="Yes"/><Buttonandroid: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

EAndroidRuntime:android.view.InflateException:BinaryXMLfileline#1:<merge/>canbeusedonlywithavalidViewGrouprootandattachToRoot=trueEAndroidRuntime:atandroid.view.LayoutInflater.inflate(LayoutInflater.java:551)EAndroidRuntime:atandroid.view.LayoutInflater.inflate(LayoutInflater.java:429)EAndroidRuntime:atandroid.view.ViewStub.inflate(ViewStub.java:259)EAndroidRuntime:atcom.droidyue.viewstubsample.MainActivity$1.onClick(MainActivity.java:20)EAndroidRuntime:atandroid.view.View.performClick(View.java:5697)EAndroidRuntime:atandroid.widget.TextView.performClick(TextView.java:10815)EAndroidRuntime:atandroid.view.View$PerformClick.run(View.java:22526)EAndroidRuntime:atandroid.os.Handler.handleCallback(Handler.java:739)EAndroidRuntime:atandroid.os.Handler.dispatchMessage(Handler.java:95)EAndroidRuntime:atandroid.os.Looper.loop(Looper.java:158)EAndroidRuntime:atandroid.app.ActivityThread.main(ActivityThread.java:7237)EAndroidRuntime:atjava.lang.reflect.Method.invoke(NativeMethod)EAndroidRuntime:atcom.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)EAndroidRuntime:atcom.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)EAndroidRuntime:Causedby:android.view.InflateException:<merge/>canbeusedonlywithavalidViewGrouprootandattachToRoot=trueEAndroidRuntime:atandroid.view.LayoutInflater.inflate(LayoutInflater.java:491)EAndroidRuntime:...13more

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

<?xmlversion="1.0"encoding="utf-8"?><LinearLayoutxmlns:android="http://schemas.android.com/apk/res/android"android:orientation="vertical"android:layout_width="match_parent"android:layout_height="match_parent"><includelayout="@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

/***Whenvisibilityissetto{@link#VISIBLE}or{@link#INVISIBLE},*{@link#inflate()}isinvokedandthisStubbedViewisreplacedinitsparent*bytheinflatedlayoutresource.**@paramvisibilityOneof{@link#VISIBLE},{@link#INVISIBLE},or{@link#GONE}.**@see#inflate()*/@OverridepublicvoidsetVisibility(intvisibility){if(mInflatedViewRef!=null){Viewview=mInflatedViewRef.get();if(view!=null){view.setVisibility(visibility);}else{thrownewIllegalStateException("setVisibilitycalledonun-referencedview");}}else{super.setVisibility(visibility);if(visibility==VISIBLE||visibility==INVISIBLE){inflate();}}}

setVisibility只是在ViewStub***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.

/***Inflatesthelayoutresourceidentifiedby{@link#getLayoutResource()}*andreplacesthisStubbedViewinitsparentbytheinflatedlayoutresource.**@returnTheinflatedlayoutresource.**/publicViewinflate(){finalViewParentviewParent=getParent();if(viewParent!=null&&viewParentinstanceofViewGroup){if(mLayoutResource!=0){finalViewGroupparent=(ViewGroup)viewParent;finalLayoutInflaterfactory=LayoutInflater.from(mContext);finalViewview=factory.inflate(mLayoutResource,parent,false);if(mInflatedId!=NO_ID){view.setId(mInflatedId);}finalintindex=parent.indexOfChild(this);parent.removeViewInLayout(this);finalViewGroup.LayoutParamslayoutParams=getLayoutParams();if(layoutParams!=null){parent.addView(view,index,layoutParams);}else{parent.addView(view,index);}mInflatedViewRef=newWeakReference<View>(view);if(mInflateListener!=null){mInflateListener.onInflate(this,view);}returnview;}else{thrownewIllegalArgumentException("ViewStubmusthaveavalidlayoutResource");}}else{thrownewIllegalStateException("ViewStubmusthaveanon-nullViewGroupviewParent");}}