Android中使用ViewStub實現布局優化

在Android開發中,View是我們必須要接觸的用來展示的技術.通常情況下隨著View視圖的越來越復雜,整體布局的性能也會隨之下降.這裡介紹一個在某些場景下提升布局性能的View,它就是ViewStub.

ViewStub是什麼

1. ViewStub是View的子類
2. 它不可見,大小為0
3. 用來延遲加載布局資源
   

注,關於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,

1. 一個ListView包含了諸如 新聞,商業,科技 等Item
2. 每個Item又包含了各自對應的子話題,
3. 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.
4. 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 怎麼用

在布局文件中使用ViewStub標簽

<?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);
}

關於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
一旦加載布局完成後,ViewStub會從當前布局層級中刪除
android:id 指定ViewStub ID,用於查找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,這裡為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.

關於不支持 <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

<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>

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

 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 <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

<?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>

然後修改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.

關於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

/**
  * 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第一次延遲初始化時,並且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在初始化後從視圖層級中移除
• ViewStub的layoutParameters應用
• mInflatedViewRef通過弱引用形式,建立ViewStub與加載的View的聯系.

/**
  * 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");
  }
 }

關於ViewStub的研究就是這些,希望對大家關於優化視圖有所幫助和啟發.