源码基于安卓8.0分析结果
ViewStub是一种不可见的并且大小为0的试图，它可以延迟到运行时才填充inflate布局资源，当Viewstub设为可见或者是inflate的时候，就会填充布局资源，这个布局和普通的试图就基本上没有任何区别，比如说，加载网络失败，或者是一个比较消耗性能的功能，需要用户去点击才可以加载!从而这样更加的节约了性能。对安卓布局很友好！ViewStub用法
<ViewStub android:padding="10dp" android:background="@color/colorPrimary" android:layout_gravity="center" android:inflatedId="@+id/view_stub_inflateid" android:id="@+id/view_stub" android:layout="@layout/view_stub_imageview" android:layout_width="wrap_content" android:layout_height="wrap_content" />
这篇文章安卓代码、图片、布局、网络和电量优化说如果这个根布局是个View，比如说是个ImagView，那么找出来的id为null，得必须注意这一点 -----2018.6.7修正这个说法，以前我说的是错误的，根本上的原因是ViewStub设置了 inflateid ，这才是更本身的原因
在这里记住一点，如果在
ViewStub标签中设置了android:inflatedId="@+id/view_stub_inflateid",在layout布局中的根布局在设置android:id="@+id/view_stub_layout",这个id永远找出来都是为null的，原因会在下面说明
<?xml version="1.0" encoding="utf-8"?><FrameLayout xmlns:android="http://schemas.android.com/apk/res/android" android:padding="10dp" android:id="@+id/view_stub_layout" android:class="lazyload" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB/AAffA0nNPuCLAAAAAElFTkSuQmCC" data-original="@drawable/ic_launcher_background" android:layout_width="match_parent" android:layout_height="match_parent"> <TextView android:text="如果这个根布局是个View，比如说是个ImagView，那么找出来的id为null，得必须注意这一点 -----2018.6.7修正这个说法，以前我说的是错误的，根本上的原因是ViewStub设置了 inflateid ，这才是更本身的原因" android:layout_width="wrap_content" android:layout_height="wrap_content" /> <ImageView android:layout_marginTop="20dp" android:id="@+id/imageview" android:padding="10dp" android:class="lazyload" src="data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAAAXNSR0IArs4c6QAAAARnQU1BAACxjwv8YQUAAAAJcEhZcwAADsQAAA7EAZUrDhsAAAANSURBVBhXYzh8+PB/AAffA0nNPuCLAAAAAElFTkSuQmCC" data-original="@drawable/ic_launcher_background" android:layout_width="match_parent" android:layout_height="match_parent"/></FrameLayout>
在
activity或者是fragment中的使用,mViewStub.getParent()==null就是说明没有被填充，需要填充，如果填充了，那么它的parent不会为null，具体的骚操作，后续我介绍View的绘制流程的时候在详细说明。第一种使用的方法
mViewStub = findViewById(R.id.view_stub); if (null!=mViewStub.getParent()){
View inflate = mViewStub.inflate();
....
}第二种方式:
mViewStub.setVisibility(View.VISIBLE);和inflate()方法一样。
mViewStub = findViewById(R.id.view_stub); if (null!=mViewStub.getParent()){
mViewStub.setVisibility(View.VISIBLE);
....
}第三种方式,
my_title_parent_id是layout的根布局的id
mViewStub = findViewById(R.id.view_stub); // 成员变量commLv2为空则代表未加载 commLv2 的id为ViewStub中的根布局的id
View commLv2=findViewById(R.id.my_title_parent_id); if ( commLv2 == null ) { // 加载评论列表布局, 并且获取评论ListView,inflate函数直接返回ListView对象
commLv2 = (View)mViewStub.inflate();
} else { // ViewStub已经加载
}ViewStub构造方法,注意获取了几个值mInflatedId就是android:inflatedId="@+id/find_view_stub"这个值，mLayoutResource就是layout的resId，ViewStub的mIDid。可以看出ViewStub是View的子类.
public final class ViewStub extends View { public ViewStub(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) { super(context); final TypedArray a = context.obtainStyledAttributes(attrs,
R.styleable.ViewStub, defStyleAttr, defStyleRes); // TODO: 2018/5/23 ViewStub 中设置的标签id 如果设置了 这里就一定有值 mInflatedId！=NO_Id
mInflatedId = a.getResourceId(R.styleable.ViewStub_inflatedId, NO_ID);
mLayoutResource = a.getResourceId(R.styleable.ViewStub_layout, 0);
mID = a.getResourceId(R.styleable.ViewStub_id, NO_ID);
a.recycle(); //不可见
setVisibility(GONE); // 设置不绘制
setWillNotDraw(true);
}
}在构造方法中:同时注意不可见
setVisibility(GONE);,设置不绘制setWillNotDraw(true);,同时通过下面的方法看出，ViewStub 是一个大小为0的视图。
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { // 宽高都为0 onMeasure的时候 宽高都为0
setMeasuredDimension(0, 0);
} //todo 为啥这个控件 是个大小为0的控件 ，那是因为他妈的这里更不就没有画
@Override
public void draw(Canvas canvas) {
} @Override
protected void dispatchDraw(Canvas canvas) {
}关于
inflate()方法
public View inflate() { // 1、获取ViewStub的parent view，也是目标布局根元素的parent view
final ViewParent viewParent = getParent(); if (viewParent != null && viewParent instanceof ViewGroup) { if (mLayoutResource != 0) { final ViewGroup parent = (ViewGroup) viewParent; /// 2、加载目标布局 牛逼的方法
final View view = inflateViewNoAdd(parent); // 3、将ViewStub自身从parent中移除
replaceSelfWithView(view, parent);
mInflatedViewRef = new WeakReference<>(view); if (mInflateListener != null) {
mInflateListener.onInflate(this, view);
} return view;
} else { // TODO: 2018/5/23 必须设置布局的文件
throw new IllegalArgumentException("ViewStub must have a valid layoutResource");
}
} else { // TODO: 2018/5/23 iewParent instanceof ViewGroup 不属于的话，就好比在一个TextView创建一个ViewStub直接爆炸
throw new IllegalStateException("ViewStub must have a non-null ViewGroup viewParent");
}
}第一点，ViewStup也只能在ViewGroup中使用，不能在View中去使用，要不然会抛出异常
IllegalStateException("ViewStub must have a non-null ViewGroup viewParent");第二点，也必须设置
layout属性，要不然也会抛出异常throw new IllegalArgumentException("ViewStub must have a valid layoutResource");;关于方法
inflateViewNoAdd(parent);
private View inflateViewNoAdd(ViewGroup parent) { final LayoutInflater factory; if (mInflater != null) {
factory = mInflater;
} else {
factory = LayoutInflater.from(mContext);
} final View view = factory.inflate(mLayoutResource, parent, false); //和 LayoutInflater一个道理，设置了，ViewStub 引用进来的根布局的id找出来为null 非常有些意思
if (mInflatedId != NO_ID) {
view.setId(mInflatedId);
} return view;
}第一点，底层调用的还是
LayoutInflater.from(mContext).inflate(mLayoutResource, parent, false);第二点，又看到这个方法，似曾相识，对，这也是为什么
ViewStub找不到根布局id的原因，因为mInflatedId != NO_ID,就会view.setId(mInflatedId);
if (mInflatedId != NO_ID) {
view.setId(mInflatedId);
}将ViewStub自身从parent中移除
replaceSelfWithView(view, parent);,具体的原因，这里不做分析，因为有点小复杂，这里就大概明白就行，对于理解这个ViewStub不困难，哈哈
private void replaceSelfWithView(View view, ViewGroup parent) { final int index = parent.indexOfChild(this); // 3、将ViewStub自身从parent中移除
parent.removeViewInLayout(this); final ViewGroup.LayoutParams layoutParams = getLayoutParams(); if (layoutParams != null) { // 4、将目标布局的根元素添加到parent中，有参数
parent.addView(view, index, layoutParams);
} else { // 5、将目标布局的根元素添加到parent中
parent.addView(view, index);
}
}这里使用到了弱引用，只有弱引用指向的对象的生命周期更短，当垃圾回收器扫描到只有具有弱引用的对象的时候，不论当前空间是否不足，都会对弱引用对象进行回收，当然弱引用也可以和一个队列配合着使用，为了更好地释放内存，安卓代码、图片、布局、网络和电量优化这篇文章有很好的解释，而且这个
mInflatedViewRef只在这里初始化，如果说没有调用inflate的方法的话，这个对象一定为null;
//更好的释放内存
private WeakReference<View> mInflatedViewRef;
mInflatedViewRef = new WeakReference<>(view); if (mInflateListener != null) {
mInflateListener.onInflate(this, view)
}为啥
setVisibility（View.VISIBLE）等同于inflate,原因是ViewStub进行了重写。可以看出代码的逻辑，只要没有调用过，inflate()方法，setVisibility(VISIBLE )和setVisibility(INVISIBLE)这个两个参数走的方法一样，只不过，一个看不到，实际上的位置已经确定了（INVISIBLE）。但是如果调用多次的话setVisibility()记得也得判断下null!=mViewStub.getParent()
@Override
@android.view.RemotableViewMethod(asyncImpl = "setVisibilityAsync") public void setVisibility(int visibility) { // TODO: 2018/5/23 弱引用的使用
//如果已经加载过则只设置Visibility属性
if (mInflatedViewRef != null) {
View view = mInflatedViewRef.get(); if (view != null) {
view.setVisibility(visibility);
} else { throw new IllegalStateException("setVisibility called on un-referenced view");
}
} else { // 如果未加载,这加载目标布局
super.setVisibility(visibility); if (visibility == VISIBLE || visibility == INVISIBLE) {
inflate();// 调用inflate来加载目标布局
}
}
}贴出全部的代码，有空的话，可以研究下。
@RemoteViewpublic final class ViewStub extends View { private int mInflatedId; private int mLayoutResource; // TODO: 2018/5/23 弱引用：弱引用是比软引用更弱的一种的引用的类型，
// 只有弱引用指向的对象的生命周期更短，当垃圾回收器扫描到只有具有弱引用的对象的时候，
// 不敢当前空间是否不足，都会对弱引用对象进行回收，当然弱引用也可以和一个队列配合着使用
//更好的释放内存
private WeakReference<View> mInflatedViewRef; private LayoutInflater mInflater; private OnInflateListener mInflateListener; public ViewStub(Context context) { this(context, 0);
} /**
* Creates a new ViewStub with the specified layout resource.
*
* @param context The application's environment.
* @param layoutResource The reference to a layout resource that will be inflated.
*/
public ViewStub(Context context, @LayoutRes int layoutResource) { this(context, null);
mLayoutResource = layoutResource;
} public ViewStub(Context context, AttributeSet attrs) { this(context, attrs, 0);
} public ViewStub(Context context, AttributeSet attrs, int defStyleAttr) { this(context, attrs, defStyleAttr, 0);
} public ViewStub(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) { super(context); final TypedArray a = context.obtainStyledAttributes(attrs,
R.styleable.ViewStub, defStyleAttr, defStyleRes); // TODO: 2018/5/23 ViewStub 中设置的标签id 如果设置了 这里就一定有值 mInflatedId！=NO_Id
mInflatedId = a.getResourceId(R.styleable.ViewStub_inflatedId, NO_ID);
mLayoutResource = a.getResourceId(R.styleable.ViewStub_layout, 0);
mID = a.getResourceId(R.styleable.ViewStub_id, NO_ID);
a.recycle(); //不可见
setVisibility(GONE); // 设置不绘制
setWillNotDraw(true);
} /**
* Returns the id taken by the inflated view. If the inflated id is
* {@link View#NO_ID}, the inflated view keeps its original id.
*
* @return A positive integer used to identify the inflated view or
* {@link #NO_ID} if the inflated view should keep its id.
*
* @see #setInflatedId(int)
* @attr ref android.R.styleable#ViewStub_inflatedId
*/
@IdRes
public int getInflatedId() { return mInflatedId;
} /**
* Defines the id taken by the inflated view. If the inflated id is
* {@link View#NO_ID}, the inflated view keeps its original id.
*
* @param inflatedId A positive integer used to identify the inflated view or
* {@link #NO_ID} if the inflated view should keep its id.
*
* @see #getInflatedId()
* @attr ref android.R.styleable#ViewStub_inflatedId
*/
@android.view.RemotableViewMethod(asyncImpl = "setInflatedIdAsync") public void setInflatedId(@IdRes int inflatedId) {
mInflatedId = inflatedId;
} /** @hide **/
public Runnable setInflatedIdAsync(@IdRes int inflatedId) {
mInflatedId = inflatedId; return null;
} /**
* Returns the layout resource that will be used by {@link #setVisibility(int)} or
* {@link #inflate()} to replace this StubbedView
* in its parent by another view.
*
* @return The layout resource identifier used to inflate the new View.
*
* @see #setLayoutResource(int)
* @see #setVisibility(int)
* @see #inflate()
* @attr ref android.R.styleable#ViewStub_layout
*/
@LayoutRes
public int getLayoutResource() { return mLayoutResource;
} /**
* Specifies the layout resource to inflate when this StubbedView becomes visible or invisible
* or when {@link #inflate()} is invoked. The View created by inflating the layout resource is
* used to replace this StubbedView in its parent.
*
* @param layoutResource A valid layout resource identifier (different from 0.)
*
* @see #getLayoutResource()
* @see #setVisibility(int)
* @see #inflate()
* @attr ref android.R.styleable#ViewStub_layout
*/
@android.view.RemotableViewMethod(asyncImpl = "setLayoutResourceAsync") public void setLayoutResource(@LayoutRes int layoutResource) {
mLayoutResource = layoutResource;
} /** @hide **/
public Runnable setLayoutResourceAsync(@LayoutRes int layoutResource) {
mLayoutResource = layoutResource; return null;
} /**
* Set {@link LayoutInflater} to use in {@link #inflate()}, or {@code null}
* to use the default.
*/
public void setLayoutInflater(LayoutInflater inflater) {
mInflater = inflater;
} /**
* Get current {@link LayoutInflater} used in {@link #inflate()}.
*/
public LayoutInflater getLayoutInflater() { return mInflater;
} @Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) { // 宽高都为0 onMeasure的时候 宽高都为0
setMeasuredDimension(0, 0);
} //todo 为啥这个控件 是个大小为0的控件 ，那是因为他妈的这里更不就没有画
@Override
public void draw(Canvas canvas) {
} @Override
protected void dispatchDraw(Canvas canvas) {
} /**
* 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. After that calls to this function are passed
* through to the inflated view.
*
* @param visibility One of {@link #VISIBLE}, {@link #INVISIBLE}, or {@link #GONE}.
*
* @see #inflate()
*/
@Override
@android.view.RemotableViewMethod(asyncImpl = "setVisibilityAsync") public void setVisibility(int visibility) { // TODO: 2018/5/23 弱引用的使用
//如果已经加载过则只设置Visibility属性
if (mInflatedViewRef != null) {
View view = mInflatedViewRef.get(); if (view != null) {
view.setVisibility(visibility);
} else { throw new IllegalStateException("setVisibility called on un-referenced view");
}
} else { // 如果未加载,这加载目标布局
super.setVisibility(visibility); if (visibility == VISIBLE || visibility == INVISIBLE) {
inflate();// 调用inflate来加载目标布局
}
}
} /** @hide **/
public Runnable setVisibilityAsync(int visibility) { if (visibility == VISIBLE || visibility == INVISIBLE) {
ViewGroup parent = (ViewGroup) getParent(); return new ViewReplaceRunnable(inflateViewNoAdd(parent));
} else { return null;
}
} private View inflateViewNoAdd(ViewGroup parent) { final LayoutInflater factory; if (mInflater != null) {
factory = mInflater;
} else {
factory = LayoutInflater.from(mContext);
} final View view = factory.inflate(mLayoutResource, parent, false); //和 LayoutInflater一个道理，设置了，ViewStub 引用进来的根布局的id找出来为null 非常有些意思
if (mInflatedId != NO_ID) {
view.setId(mInflatedId);
} return view;
} // TODO: 2018/5/23 关注他
private void replaceSelfWithView(View view, ViewGroup parent) { final int index = parent.indexOfChild(this); // 3、将ViewStub自身从parent中移除
parent.removeViewInLayout(this); final ViewGroup.LayoutParams layoutParams = getLayoutParams(); if (layoutParams != null) { // 4、将目标布局的根元素添加到parent中，有参数
parent.addView(view, index, layoutParams);
} else { // 5、将目标布局的根元素添加到parent中
parent.addView(view, index);
}
} /**
* 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() { // 1、获取ViewStub的parent view，也是目标布局根元素的parent view
final ViewParent viewParent = getParent(); if (viewParent != null && viewParent instanceof ViewGroup) { if (mLayoutResource != 0) { final ViewGroup parent = (ViewGroup) viewParent; /// 2、加载目标布局 牛逼的方法
final View view = inflateViewNoAdd(parent); // 3、将ViewStub自身从parent中移除
replaceSelfWithView(view, parent);
mInflatedViewRef = new WeakReference<>(view); if (mInflateListener != null) {
mInflateListener.onInflate(this, view);
} return view;
} else { // TODO: 2018/5/23 必须设置布局的文件
throw new IllegalArgumentException("ViewStub must have a valid layoutResource");
}
} else { // TODO: 2018/5/23 iewParent instanceof ViewGroup 不属于的话，就好比在一个TextView创建一个ViewStub直接爆炸
throw new IllegalStateException("ViewStub must have a non-null ViewGroup viewParent");
}
} /**
* Specifies the inflate listener to be notified after this ViewStub successfully
* inflated its layout resource.
*
* @param inflateListener The OnInflateListener to notify of successful inflation.
*
* @see ViewStub.OnInflateListener
*/
public void setOnInflateListener(OnInflateListener inflateListener) {
mInflateListener = inflateListener;
} /**
* Listener used to receive a notification after a ViewStub has successfully
* inflated its layout resource.
*
* @see ViewStub#setOnInflateListener(ViewStub.OnInflateListener)
*/
public static interface OnInflateListener { /**
* Invoked after a ViewStub successfully inflated its layout resource.
* This method is invoked after the inflated view was added to the
* hierarchy but before the layout pass.
*
* @param stub The ViewStub that initiated the inflation.
* @param inflated The inflated View.
*/
void onInflate(ViewStub stub, View inflated);
} /** @hide **/
public class ViewReplaceRunnable implements Runnable { public final View view;
ViewReplaceRunnable(View view) { this.view = view;
} @Override
public void run() {
replaceSelfWithView(view, (ViewGroup) getParent());
}
}
}最后做了一张图
说明一下
ViewStub的原理很简单！好吧，这个有点皮
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