android listview viewstub,Android中使用ViewStub提高布局性能
在Android開(kāi)發(fā)中,View是我們必須要接觸的用來(lái)展示的技術(shù).通常情況下隨著View視圖的越來(lái)越復(fù)雜,整體布局的性能也會(huì)隨之下降.這里介紹一個(gè)在某些場(chǎng)景下提升布局性能的View,它就是ViewStub.
ViewStub是什么
ViewStub是View的子類(lèi)
它不可見(jiàn),大小為0
用來(lái)延遲加載布局資源
注,關(guān)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(lái)代替關(guān)聯(lián)代碼或者未實(shí)現(xiàn)代碼的代碼.
ViewStub使用場(chǎng)景
如上圖所示,
一個(gè)ListView包含了諸如新聞,商業(yè),科技等Item
每個(gè)Item又包含了各自對(duì)應(yīng)的子話(huà)題,
但是子話(huà)題的View(藍(lán)色區(qū)域)只有在點(diǎn)擊展開(kāi)按鈕才真正需要加載.
如果默認(rèn)加載子話(huà)題的View,則會(huì)造成內(nèi)存的占用和CPU的消耗
所以,這時(shí)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(biāo)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(guān)于ViewStub的事
除了inflate方法外,我們還可以調(diào)用setVisibility()方法加載布局文件
一旦加載布局完成后,ViewStub會(huì)從當(dāng)前布局層級(jí)中刪除
android:id指定ViewStub ID,用于查找ViewStub進(jìn)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(xiě)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(biāo)簽.
關(guān)于不支持標(biāo)簽的程度,我們進(jìn)行一個(gè)簡(jiǎn)單的驗(yàn)證
驗(yàn)證一:直接標(biāo)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(duì)應(yīng)的ViewStub的android:layout屬性值之后,運(yùn)行后(點(diǎn)擊Button按鈕)得到產(chǎn)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(jiàn),直接的標(biāo)簽,ViewStub是不支持的.
驗(yàn)證二 間接的ViewStub
下面布局間接使用了merge標(biāo)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(yùn)行,一切正常.
除此之外,本例也驗(yàn)證了ViewStub也是對(duì)標(biāo)簽支持良好的.
關(guān)于ViewStub的一點(diǎn)代碼剖析
inflate vs setVisibility
inflate和setVisibility的共同點(diǎn)是都可以實(shí)現(xiàn)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(shí),并且visibility是非GONE時(shí),調(diào)用了inflate方法.
inflate源碼
通過(guò)閱讀下面的inflate方法實(shí)現(xiàn),我們將更加理解
android:inflatedId的用途
ViewStub在初始化后從視圖層級(jí)中移除
ViewStub的layoutParameters應(yīng)用
mInflatedViewRef通過(guò)弱引用形式,建立ViewStub與加載的View的聯(lián)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(guān)于ViewStub的研究就是這些,希望對(duì)大家關(guān)于優(yōu)化視圖有所幫助和啟發(fā).
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