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注,關于Stub的解釋：在Java中,樁(stub)是指用來代替關聯代碼或者未實現代碼的代碼.

ViewStub使用場景

如上圖所示,

• 一個ListView包含了諸如新聞,商業,科技等Item
• 每個Item又包含了各自對應的子話題,
• 但是子話題的View(藍色區域)只有在點擊展開按鈕才真正需要加載.
• 如果默認加載子話題的View,則會造成內存的占用和CPU的消耗

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

ViewStub 怎么用

在布局文件中使用ViewStub標簽

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<?xml version="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="Hello World!"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.在代碼中inflate布局

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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); }

關于ViewStub的事

• 除了inflate方法外,我們還可以調用setVisibility()方法加載布局文件
• 一旦加載布局完成后,ViewStub會從當前布局層級中刪除
• android:id指定ViewStub ID,用于查找ViewStub進行延遲加載
• 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<merge>標簽.

關于不支持<merge>標簽的程度,我們進行一個簡單的驗證

驗證一:直接標簽

如下,我們有布局文件名為merge_layout.xml

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<merge xmlns: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>

替換對應的ViewStub的android:layout屬性值之后,運行后(點擊Button按鈕)得到產生了如下的崩潰

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E AndroidRuntime: android.view.InflateException: Binary XML file line #1: <merge /> can be used only with a valid ViewGroup root and attachToRoot=trueE 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=trueE AndroidRuntime: at android.view.LayoutInflater.inflate(LayoutInflater.java:491)E AndroidRuntime: ... 13 more

可見,直接的<merge>標簽,ViewStub是不支持的.

驗證二 間接的ViewStub

下面布局間接使用了merge標簽.文件名為include_merge.xml

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<?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,運行,一切正常.

除此之外,本例也驗證了ViewStub也是對<include>標簽支持良好的.

關于ViewStub的一點代碼剖析

inflate vs setVisibility

inflate和setVisibility的共同點是都可以實現加載布局

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/*** 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() */@Overridepublic 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第一次延遲初始化時,并且visibility是非GONE時,調用了inflate方法.

inflate源碼

通過閱讀下面的inflate方法實現,我們將更加理解

• android:inflatedId的用途
• ViewStub在初始化后從視圖層級中移除
• ViewStub的layoutParameters應用
• mInflatedViewRef通過弱引用形式,建立ViewStub與加載的View的聯系.

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/*** 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");}}

關于ViewStub的研究就是這些,希望對大家關于優化視圖有所幫助和啟發.

轉載于:https://my.oschina.net/u/2517174/blog/846203

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