Android中怎么利用ViewStub提高布局性能

Android中怎么利用ViewStub提高布局性能，很多新手對(duì)此不是很清楚，為了幫助大家解決這個(gè)難題，下面小編將為大家詳細(xì)講解，有這方面需求的人可以來(lái)學(xué)習(xí)下，希望你能有所收獲。

ViewStub是什么

• ViewStub是View的子類

• 它不可見(jiàn),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(kāi)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(xiě)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(jiàn),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");         }     }

看完上述內(nèi)容是否對(duì)您有幫助呢？如果還想對(duì)相關(guān)知識(shí)有進(jìn)一步的了解或閱讀更多相關(guān)文章，請(qǐng)關(guān)注億速云行業(yè)資訊頻道，感謝您對(duì)億速云的支持。