Android中使用ViewStub实现布局优化

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.

viewstub是什么

1. viewstub是view的子类
2. 它不可见,大小为0
3. 用来延迟加载布局资源
   

注,关于stub的解释

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,桩是指用来代替关联代码或者未实现代码的代码.

viewstub使用场景

 

如上图所示,

1. 一个listview包含了诸如 新闻,商业,科技 等item
2. 每个item又包含了各自对应的子话题,
3. 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.
4. 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
   

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.

viewstub 怎么用

在布局文件中使用viewstub标签

<?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.在代码中inflate布局

viewstub myviewstub = (viewstub)findviewbyid(r.id.myviewstub);
if (myviewstub != null) {
 myviewstub.inflate();
 //或者是下面的形式加载
 //myviewstub.setvisibility(view.visible);
}

关于viewstub的事

除了 inflate 方法外,我们还可以调用 setvisibility() 方法加载布局文件
一旦加载布局完成后,viewstub会从当前布局层级中删除
android:id 指定viewstub id,用于查找viewstub进行延迟加载
android:layout 延迟加载布局的资源id
android:inflatedid 加载的布局被重写的id,这里为relativelayout的id

viewstub的不足

官方的文档中有这样一段描述

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> 标签.

关于不支持 <merge> 标签的程度,我们进行一个简单的验证

验证一:直接 标签

如下,我们有布局文件名为 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

 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

可见,直接的 <merge> 标签,viewstub是不支持的.

验证二 间接的viewstub

下面布局间接使用了merge标签.文件名为 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,运行,一切正常.

除此之外,本例也验证了viewstub也是对 <include> 标签支持良好的.

关于viewstub的一点代码剖析

inflate vs setvisibility

inflate和setvisibility的共同点是都可以实现加载布局

/**
  * 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第一次延迟初始化时,并且visibility是非 gone 时,调用了 inflate 方法.

inflate源码

通过阅读下面的inflate方法实现,我们将更加理解

• android:inflatedid的用途
• viewstub在初始化后从视图层级中移除
• viewstub的layoutparameters应用
• minflatedviewref通过弱引用形式,建立viewstub与加载的view的联系.

/**
  * 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");
  }
 }

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.