什么是ViewStub？
ViewStub是一个不可见,宽高为0的View，能够在运行时延迟加载布局资源,提高加载效率。缺点是不支持<merage>标签。ViewStubCompat是它的补丁包，用于支持4.1以前的设备。
使用方式
<ViewStub android:id="@+id/stub"
android:inflatedId="@+id/subTree"
android:layout="@layout/mySubTree"
android:layout_width="120dip"
android:layout_height="40dip" />
//防止某些手机中出现多次加载的问题
var inflated: Boolean = false
override fun onCreate(savedInstanceState: Bundle?) {
super.onCreate(savedInstanceState)
setContentView(R.layout.activity_main)
var subTree: View? = null
//ViewStub对象inflate之后就没有用，不需要放到属性中
var stub: ViewStub = findViewById(R.id.stub)
stub.setOnInflateListener { _, _ ->
inflated = true
}
if (!inflated) {
subTree = stub.inflate()
}
//检查ViewStub的inflatedId属性是否为替换的布局的根节点的Id
print("${subTree?.id == stub.inflatedId}")
}
为什么说ViewStub可以提高加载效率？
setWillNotDraw(true)被调用，导致onDraw不被执行在
onMeasure中，直接将宽高设置为0dispatchDraw和onDraw为空实现
ViewStub中的inflate()和setVisibility(int visibility)的区别
-
inflate()是ViewStub的核心逻辑，将加载成功的View保存弱引用对象mInflatedViewRef中。 -
setVisibility(int visibility)中将判断是否已经执行inflate()，如果已经执行，则去判断弱引用中缓存的对象是否存在，存在则去执行加载View的setVisibility(visibility)方法；未执行的话首先执行super.setVisibility(visibility),然后判断是否执行inflate()方法
/**
* A ViewStub is an invisible, zero-sized View that can be used to lazily inflate
* layout resources at runtime.
* 当ViewStub被设置Visible或者inflate()被调用，布局资源将被加载。
* 接下来会将ViewStub对象的父布局中的ViewStub对象替换为加载的View或Views。
* 因此，ViewStub 在View树中直到{@link #setVisibility(int)} or
* {@link #inflate()} 被调用之前都是存在的.
*/
@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);
}
/**
* 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);
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);
}
/**
* 返回加载View所携带的Id(前提是在ViewStub中配置,才会对加载的View执行setId方法，将mInflatedId设置给加载的View,否则返回的就是NO_ID,没有对加载的View执行setId方法),如果ID值是{@link View#NO_ID}, 加载的View将保持原来xml中所设定的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);
}
@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) {
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);
}
}
/**
* 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 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;
}
/**
* Listener used to receive a notification after a ViewStub has successfully
* inflated its layout resource.
*
* @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());
}
}
}
