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

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

在布局文件中使用ViewStub标签

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">

android:id="@+id/clickMe"

android:text="Hello World!"

android:layout_width="wrap_content"

android:layout_height="wrap_content"/>

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"

/>

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 标签.

关于不支持 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

android:layout_width="fill_parent"

android:layout_height="wrap_content"

android:text="Yes"/>

android:layout_width="fill_parent"

android:layout_height="wrap_content"

android:text="No"/>

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: 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: 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 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

android:orientation="vertical"

android:layout_width="match_parent"

android:layout_height="match_parent">

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 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);

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.

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