大家好，又见面了，我是你们的朋友全栈君。
源码：
package android.view;
import android.annotation.IdRes;
import android.annotation.LayoutRes;
import android.content.Context;
import android.content.res.TypedArray;
import android.graphics.Canvas;
import android.util.AttributeSet;
import android.widget.RemoteViews.RemoteView;
import com.android.internal.R;
import java.lang.ref.WeakReference;
/** * ViewStub是一个用户不可见的，大小为0的view，用于在app运行时懒加载一些布局。 *当ViewStub设置用户可见(setVisibility)时，或者当调用inflate()方法时，布局会就被初始化。ViewStub会用初始化后的布局文件替换自己放入其父布局中(参考inflate中的replaceSelfWithView方法)。 * * 因此ViewStub会在view布局中存在一直到setVisibility或inflate被调用了。 * * 当被初始化的View添加到ViewStub的父布局时会使用ViewStub的布局属性，因此你可以使用ViewStub的inflatedId属性定义／重写View的id。例如： * * * <ViewStub android:id="@+id/stub" * android:inflatedId="@+id/subTree" * android:layout="@layout/mySubTree" * android:layout_width="120dip" * android:layout_height="40dip" /> * * * 上面例子里边定义了一个id为stub的ViewStub。在初始化布局subTree之后，这个ViewStub就会从它父布局中移除。新创建的mySubTree布局的View的id就会使用布局中定义的subTree，最后这个新创建的view的宽为120dip，高为40dip. * * * 如下方式是最标准的初始化布局View的方式： * * * ViewStub stub = findViewById(R.id.stub); * View inflated = stub.inflate(); * * * 当inflate()方法被调用后，ViewStub会被替换为初始化后的布局View，并且View会被返回。从而让app获取到view的引用而不用在调用一次findViewById(). * * @attr ref android.R.styleable#ViewStub_inflatedId * @attr ref android.R.styleable#ViewStub_layout */
@RemoteView
public final class ViewStub extends View {
private int mInflatedId;
private int mLayoutResource;
private WeakReference<View> mInflatedViewRef;
private LayoutInflater mInflater;
private OnInflateListener mInflateListener;
public ViewStub(Context context) {
this(context, 0);
}
/** * 通过传入布局id创建ViewStub（猜测是用于代码中动态初始化时调用的）。 * * @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);
}
/** * 解析布局文件创建ViewStub，并获取mInflatedId及mLayoutResource等属性。最后设置不可见及不绘制自身。 * */
public ViewStub(Context context, AttributeSet attrs, int defStyleAttr, int defStyleRes) {
super(context);
final TypedArray a = context.obtainStyledAttributes(attrs,
R.styleable.ViewStub, defStyleAttr, defStyleRes);
saveAttributeDataForStyleable(context, R.styleable.ViewStub, attrs, a, defStyleAttr,
defStyleRes);
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();
setVisibility(GONE);
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) {
setMeasuredDimension(0, 0);
}
/** * 不做任何绘制，所以比普通view节省了时间与内存。 */
@Override
public void draw(Canvas canvas) {
}
@Override
protected void dispatchDraw(Canvas canvas) {
}
/** * 当设置visibility为VISIBLE或INVISIBLE时，inflate方法就会被调用 * ，且初始化出来的view对象会替换StubView在其父布局中的位置。 * * @param visibility One of {@link #VISIBLE}, {@link #INVISIBLE}, or {@link #GONE}. * * @see #inflate() */
@Override
@android.view.RemotableViewMethod(asyncImpl = "setVisibilityAsync")
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();
}
}
}
/** @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);
if (mInflatedId != NO_ID) {
view.setId(mInflatedId);
}
return view;
}
private void replaceSelfWithView(View view, ViewGroup parent) {
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);
}
}
/** * 初始化布局view，替换stub在父布局中的位置（注：stub被替换之后getparent会为空，因此会抛出IllegalArgumentException异常）。 * * * @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 View view = inflateViewNoAdd(parent);
replaceSelfWithView(view, parent);
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");
}
}
/** * 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 android.view.ViewStub.OnInflateListener */
public void setOnInflateListener(OnInflateListener inflateListener) {
mInflateListener = inflateListener;
}
/** * 接收布局view被创建的观察者，如果设置了会在布局view被初始化之后回调 listener的onInflate方法。 * * * @see android.view.ViewStub#setOnInflateListener(android.view.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());
}
}
}
使用方法：
1.在布局中定义ViewStub:
<ViewStub android:id="@+id/stub" android:inflatedId="@+id/subTree" android:layout="@layout/mySubTree" android:layout_width="120dip" android:layout_height="40dip" />
2.布局中使用：
ViewStub stub = findViewById(R.id.stub);
//inflated就是初始化后的布局view对象
View inflated = stub.inflate();
//对inflated操作，设置文字，图片等。
抛砖引玉，不当之处敬请之处，万分感谢?。
注：其实也可以也可以用动态添加的方法添加View：在java/kotlin代码中动态初始化View，然后添加到对应的viewgroup中。
1.在需要的时候通过layoutInflater.inflate(mLayoutResource, parent, false);
2.方法初始化要添加的view，然后调用对应viewGroup.addView(View child/View child, int index/View child, LayoutParams params);方法添加到布局中；
这样的话在布局中就不用多了个ViewStub的view了,性能也更好些。
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