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

  • ViewStub是View的子类
  • 它不可见,大小为0
  • 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

Android中使用ViewStub提高布局性能_android

如上图所示,

  • 一个ListView包含了诸如​​新闻,商业,科技​​等Item
  • 每个Item又包含了各自对应的子话题,
  • 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.
  • 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

  1. 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.