Android使用ViewStub提高布局性能

   <p style="text-align:center"><img src="https://simg.open-open.com/show/92a7539ac651710ad6af4ed8bca52dbe.jpg"></p>    <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>    <h3><strong>ViewStub是什么</strong></h3>    <ul>     <li>ViewStub是View的子类</li>     <li>它不可见,大小为0</li>     <li>用来延迟加载布局资源</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>    <p>ViewStub使用场景</p>    <p style="text-align:center"><img src="https://simg.open-open.com/show/ce64c5903367d060a634fb4161b6a536.jpg"></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>    <h3><strong>ViewStub 怎么用</strong></h3>    <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>    <h3><strong>关于ViewStub的事</strong></h3>    <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>    <h3><strong>ViewStub的不足</strong></h3>    <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 标签.</p>    <p>关于不支持 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merge_layout.xml</p>    <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 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 include_merge.xml</p>    <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 标签支持良好的.</p>    <h3><strong>关于ViewStub的一点代码剖析</strong></h3>    <p><strong>inflate vs setVisibility</strong></p>    <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>    <p><strong>inflate源码</strong></p>    <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> </p>    <p> </p>    <p>来自：http://mobile.51cto.com/android-517729.htm</p>    <p> </p>