Android中使用ViewStub提高布局性能

   <p>在Android开发中,View是我们必须要接触的用来展示的技术.通常情况下随着View视图的越来越复杂,整体布局的性能也会随之下降.这里介绍一个在某些场景下提升布局性能的View,它就是ViewStub.</p>    <h2><strong>ViewStub是什么</strong></h2>    <ul>     <li> <p>ViewStub是View的子类</p> </li>     <li> <p>它不可见,大小为0</p> </li>     <li> <p>用来延迟加载布局资源</p> </li>    </ul>    <p>注,关于Stub的解释</p>    <p>A stub is a small program routine that substitutes for a longer program, possibly to be loaded later or that is located remotely</p>    <p>在Java中,桩是指用来代替关联代码或者未实现代码的代码.</p>    <h2><strong>ViewStub使用场景</strong></h2>    <p style="text-align:center"><img src="https://simg.open-open.com/show/b0867ddb5c9bc872ef8b30f0c79c5748.png"></p>    <p>如上图所示,</p>    <ul>     <li>一个ListView包含了诸如 新闻,商业,科技 等Item</li>     <li>每个Item又包含了各自对应的子话题,</li>     <li>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.</li>     <li>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</li>    </ul>    <p>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.</p>    <h2><strong>ViewStub 怎么用</strong></h2>    <p>1.在布局文件中使用ViewStub标签</p>    <pre>  <code class="language-java"><?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></code></pre>    <p>2.在代码中inflate布局</p>    <pre>  <code class="language-java">ViewStub myViewStub = (ViewStub)findViewById(R.id.myViewStub);  if (myViewStub != null) {       myViewStub.inflate();       //或者是下面的形式加载       //myViewStub.setVisibility(View.VISIBLE);  }</code></pre>    <h2><strong>关于ViewStub的事</strong></h2>    <ul>     <li>除了 inflate 方法外,我们还可以调用 setVisibility() 方法加载布局文件</li>     <li>一旦加载布局完成后,ViewStub会从当前布局层级中删除</li>     <li>android:id 指定ViewStub ID,用于查找ViewStub进行延迟加载</li>     <li>android:layout 延迟加载布局的资源id</li>     <li>android:inflatedId 加载的布局被重写的id,这里为RelativeLayout的id</li>    </ul>    <h2><strong>ViewStub的不足</strong></h2>    <p>官方的文档中有这样一段描述</p>    <p>Note: One drawback of ViewStub is that it doesn’t currently support the tag in the layouts to be inflated.</p>    <p>意思是ViewStub不支持 <merge> 标签.</p>    <p>关于不支持 <merge> 标签的程度,我们进行一个简单的验证</p>    <h3><strong>验证一:直接标签</strong></h3>    <p>如下,我们有布局文件名为</p>    <pre>  <code class="language-java">merge_layout.xml    </code></pre>    <pre>  <code class="language-java"><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></code></pre>    <p>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</p>    <pre>  <code class="language-java">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</code></pre>    <p>可见,直接的 <merge> 标签,ViewStub是不支持的.</p>    <h3><strong>验证二 间接的ViewStub</strong></h3>    <p>下面布局间接使用了merge标签.文件名为</p>    <pre>  <code class="language-java">include_merge.xml    </code></pre>    <pre>  <code class="language-java"><?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></code></pre>    <p>然后修改ViewStub的 android:layout 值,运行,一切正常.</p>    <p>除此之外,本例也验证了ViewStub也是对 <include> 标签支持良好的.</p>    <h2><strong>关于ViewStub的一点代码剖析</strong></h2>    <h3><strong>inflate vs setVisibility</strong></h3>    <p>inflate和setVisibility的共同点是都可以实现加载布局</p>    <pre>  <code class="language-java">/**   * 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();           }       }  }</code></pre>    <p>setVisibility只是在ViewStub第一次延迟初始化时,并且visibility是非 GONE 时,调用了 inflate 方法.</p>    <h3><strong>inflate源码</strong></h3>    <p>通过阅读下面的inflate方法实现,我们将更加理解</p>    <ul>     <li>android:inflatedId的用途</li>     <li>ViewStub在初始化后从视图层级中移除</li>     <li>ViewStub的layoutParameters应用</li>     <li>mInflatedViewRef通过弱引用形式,建立ViewStub与加载的View的联系.</li>    </ul>    <pre>  <code class="language-java">/**   * 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");       }   }</code></pre>    <p>关于ViewStub的研究就是这些,希望对大家关于优化视图有所帮助和启发.</p>    <p> </p>    <p>来自：http://www.androidchina.net/5550.html</p>    <p> </p>