在Android開發中,View是我們必須要接觸的用來展示的技術.通常情況下隨著View視圖的越來越復雜,整體布局的性能也會隨之下降.這里介紹一個在某些場景下提升布局性能的View,它就是ViewStub.

ViewStub是什么

ViewStub是View的子類

它不可見,大小為0

用來延遲加載布局資源

注,關于Stub的解釋

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,樁是指用來代替關聯代碼或者未實現代碼的代碼.

ViewStub使用場景

如上圖所示,

一個ListView包含了諸如新聞,商業,科技等Item

每個Item又包含了各自對應的子話題,

但是子話題的View(藍色區域)只有在點擊展開按鈕才真正需要加載.

如果默認加載子話題的View,則會造成內存的占用和CPU的消耗

所以,這時候就ViewStub就派上用處了.使用ViewStub可以延遲加載布局資源.

ViewStub 怎么用

1.在布局文件中使用ViewStub標簽

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.在代碼中inflate布局

ViewStub myViewStub = (ViewStub)findViewById(R.id.myViewStub);

if (myViewStub != null) {

myViewStub.inflate();

//或者是下面的形式加載

//myViewStub.setVisibility(View.VISIBLE);

}

關于ViewStub的事

除了inflate方法外,我們還可以調用setVisibility()方法加載布局文件

一旦加載布局完成后,ViewStub會從當前布局層級中刪除

android:id指定ViewStub ID,用于查找ViewStub進行延遲加載

android:layout延遲加載布局的資源id

android:inflatedId加載的布局被重寫的id,這里為RelativeLayout的id

ViewStub的不足

官方的文檔中有這樣一段描述

Note: One drawback of ViewStub is that it doesn’t currently support the?tag in the layouts to be inflated.

意思是ViewStub不支持標簽.

關于不支持標簽的程度,我們進行一個簡單的驗證

驗證一:直接標簽

如下,我們有布局文件名為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"/>

替換對應的ViewStub的android:layout屬性值之后,運行后(點擊Button按鈕)得到產生了如下的崩潰

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

可見,直接的標簽,ViewStub是不支持的.

驗證二 間接的ViewStub

下面布局間接使用了merge標簽.文件名為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,運行,一切正常.

除此之外,本例也驗證了ViewStub也是對標簽支持良好的.

關于ViewStub的一點代碼剖析

inflate vs setVisibility

inflate和setVisibility的共同點是都可以實現加載布局

/**

* 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第一次延遲初始化時,并且visibility是非GONE時,調用了inflate方法.

inflate源碼

通過閱讀下面的inflate方法實現,我們將更加理解

android:inflatedId的用途

ViewStub在初始化后從視圖層級中移除

ViewStub的layoutParameters應用

mInflatedViewRef通過弱引用形式,建立ViewStub與加載的View的聯系.

/**

* 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");

}

}

關于ViewStub的研究就是這些,希望對大家關于優化視圖有所幫助和啟發.

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