View的requestLayout()流程
View的requestLayout()流程
requestLayout()常用来对View重新进行测量、布局和绘制。当View需要更新当前的布局时，就向父布局申请重新布局。
虽然是ViewParent接口中定义的方法：
public interface ViewParent {
/**
* Called when something has changed which has invalidated the layout of a
* child of this view parent. This will schedule a layout pass of the view
* tree.
*/
public void requestLayout();
...
}但其完整实现却在View中，而不在ViewGroup类里(后面会讲到)：
/**
* Call this when something has changed which has invalidated the
* layout of this view. This will schedule a layout pass of the view
* tree. This should not be called while the view hierarchy is currently in a layout
* pass ({@link #isInLayout()}. If layout is happening, the request may be honored at the
* end of the current layout pass (and then layout will run again) or after the current
* frame is drawn and the next layout occurs.
*
* <p>Subclasses which override this method should call the superclass method to
* handle possible request-during-layout errors correctly.</p>
*/
@CallSuper
public void requestLayout() {
if (mMeasureCache != null) mMeasureCache.clear();
if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == null) {
// 如果请求的是当前View，则仅触发 request-during-layout 逻辑，而不是其父层次结构中的视图
ViewRootImpl viewRoot = getViewRootImpl();
if (viewRoot != null && viewRoot.isInLayout()) {
if (!viewRoot.requestLayoutDuringLayout(this)) {
return;
}
}
mAttachInfo.mViewRequestingLayout = this;
}
//添加两个标志位: PFLAG_FORCE_LAYOUT和PFLAG_INVALIDATED
//后面执行measure()和layout()流程中用到
mPrivateFlags |= PFLAG_FORCE_LAYOUT;
mPrivateFlags |= PFLAG_INVALIDATED;
//通过mParent父View，层层向上传递requestLayout()
if (mParent != null && !mParent.isLayoutRequested()) {
mParent.requestLayout();
}
if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == this) {
mAttachInfo.mViewRequestingLayout = null;
}
}在继续跟随源码剖析requestLayout()流程之前，先搞清楚View中的mParent从哪来？
1、View中mParent的来历
View中的mParent成员变量通常来说指的是当前View的父容器ViewGroup，但是有一个特殊，对于顶层DecorView来说它的mParent是ViewRootImpl。
/**
* The parent this view is attached to.
* {@hide}
*
* @see #getParent()
*/
@UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.P)
protected ViewParent mParent;mParent通过方法assignParent(ViewParent parent)被赋值：
/*
* Caller is responsible for calling requestLayout if necessary.
* (This allows addViewInLayout to not request a new layout.)
*/
@UnsupportedAppUsage
void assignParent(ViewParent parent) {
if (mParent == null) {
mParent = parent;
} else if (parent == null) {
mParent = null;
} else {
throw new RuntimeException("view " + this + " being added, but"
+ " it already has a parent");
}
}①在DecorView添加到Window中的流程一文最后，通过ViewRootImpl的setView()方法设置传递过来的DecorView，在该方法中会给DecorView的mParent指定为当前ViewRootImpl，ViewRootImpl实现了ViewParent接口。
/**
* We have one child
*/
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView,
int userId) {
synchronized (this) {
if (mView == null) {
mView = view;
...
view.assignParent(this);
...
}
}
}②将View添加到ViewGroup中，通常使用addView(View child)等方法，最后会调用内部的addViewInner()方法，将添加进来的子View的mParent设置成当前所在容器：
private void addViewInner(View child, int index, LayoutParams params,
boolean preventRequestLayout) {
...
// tell our children
if (preventRequestLayout) {
child.assignParent(this);
} else {
child.mParent = this;
}
...
}为了方便理解，简化关系图如下：
2、View、ViewGroup与ViewParent
在开头提到requestLayout()方法是定义在ViewParent接口中的方法
public interface ViewParent {
/**
* Called when something has changed which has invalidated the layout of a
* child of this view parent. This will schedule a layout pass of the view
* tree.
*/
public void requestLayout();
...
}在View类中有同样方法签名的requestLayout()，这本和ViewParent接口中的requestLayout()没什么关系。
public class View {
/**
* Called when something has changed which has invalidated the layout of a
* child of this view parent. This will schedule a layout pass of the view
* tree.
*/
public void requestLayout(){
}
}但是，ViewGroup继承子View并且实现ViewParent接口，本该实现接口中的方法，但已经从View通过继承拥有了部分接口方法的实现，所以无需再在ViewGroup中实现。
public abstract class ViewGroup extends View implements ViewParent, ViewManager {
..
}类的部分方法及继承关系图，如下：
3、requestLayout() 流程
前面都搞清楚，回到主流程，继续往下看。
首先，在requestLayout()请求中会给View添加两个标志位: PFLAG_FORCE_LAYOUT和PFLAG_INVALIDATED。
... //添加两个标志位: PFLAG_FORCE_LAYOUT和PFLAG_INVALIDATED mPrivateFlags |= PFLAG_FORCE_LAYOUT; mPrivateFlags |= PFLAG_INVALIDATED; ...
添加的这两个标志在View之后的测量、布局、绘制流程中起到关键作用：
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
...
//提取标志位PFLAG_FORCE_LAYOUT，是否强制布局
final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
...
final boolean needsLayout = specChanged && (sAlwaysRemeasureExactly || !isSpecExactly || !matchesSpecSize);
if (forceLayout || needsLayout) {
...
//走到熟悉的onMeasure()方法
onMeasure(widthMeasureSpec, heightMeasureSpec);
...
//最后再给标志位中添加PFLAG_LAYOUT_REQUIRED
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}
...
}在measure()测量过程中添加的PFLAG_LAYOUT_REQUIRED标志位，也会在后续的layout()布局中起作用：
public void layout(int l, int t, int r, int b) {
...
//判断有PFLAG_LAYOUT_REQUIRED，将执行onLayout()
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
////走到熟悉的onLayout()方法
onLayout(changed, l, t, r, b);
...
//onLayout完成之后，清除PFLAG_LAYOUT_REQUIRED标志
mPrivateFlags &= ~PFLAG_LAYOUT_REQUIRED;
}
}
然后通过父View mParent，类似责任链模式，层层向上传递，requestLayout()请求最终是到ViewRootImpl中去处理。
...
//通过mParent父View，层层向上传递requestLayout()
if (mParent != null && !mParent.isLayoutRequested()) {
mParent.requestLayout();
}
...ViewRootImpl同样实现了ViewParent接口，不像ViewGroup那样“幸运”，ViewParent接口中的方法都需要在ViewRootImpl中实现。requestLayout()方法在ViewRootImpl中的实现如下：
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {
//必须在主线程调用requestLayout()
checkThread();
//requestLayout()中将mLayoutRequested设置为true
mLayoutRequested = true;
scheduleTraversals();
}
}先通过checkThread()判断是否在UI线程，然后将mLayoutRequested设置为true，随后用scheduleTraversals()方法开启异步遍历。重新执行一遍View绘制的三大流程：measure、layout、draw。导致requestLayout()方法通常会触发View的onMeasure、onLayout、onDraw。
另外还有一个forceLayout()方法，并不会像requestLayout()那样层层向上调用请求立即重新布局，而是设置好标志位等待在下一次布局过程中刷新。
/**
* 强制在下一个布局过程中布局此视图。
* 此方法不会像上调用 requestLayout() 或 forceLayout()。
*/
public void forceLayout() {
if (mMeasureCache != null) mMeasureCache.clear();
mPrivateFlags |= PFLAG_FORCE_LAYOUT;
mPrivateFlags |= PFLAG_INVALIDATED;
}看到这里本篇关于requestLayout()请求流程就可以结束，因为之前已经在View的绘制流程一文知道后续的流程。
还有一个开发者熟悉的方法invalidate()，详见View的invalidate()流程一文。
