自定义CustomEmptyView，并在相关方法中打印日志，如下；
public class CustomEmptyView extends View {
public CustomEmptyView(Context context) {
super(context);
}
public CustomEmptyView(Context context, @Nullable AttributeSet attrs) {
super(context, attrs);
}
public CustomEmptyView(Context context, @Nullable AttributeSet attrs, int defStyleAttr) {
super(context, attrs, defStyleAttr);
}
@Override
protected void onMeasure(int widthMeasureSpec, int heightMeasureSpec) {
super.onMeasure(widthMeasureSpec, heightMeasureSpec);
Log.i("CustomEmptyView", "onMeasure");
}
@Override
protected void onLayout(boolean changed, int left, int top, int right, int bottom) {
super.onLayout(changed, left, top, right, bottom);
Log.i("CustomEmptyView", "onLayout");
}
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
Log.i("CustomEmptyView", "onDraw");
}
}调用CustomEmptyView的invalidate()方法，打印日志如下：
2019-03-26 17:32:34.739 8075-8075/com.example.myapplication I/CustomEmptyView: onDraw
调用CustomEmptyView的requestLayout()方法，打印日志如下：
2019-03-26 17:33:13.497 8075-8075/com.example.myapplication I/CustomEmptyView: onMeasure 2019-03-26 17:33:13.501 8075-8075/com.example.myapplication I/CustomEmptyView: onLayout 2019-03-26 17:33:13.503 8075-8075/com.example.myapplication I/CustomEmptyView: onDraw
从打印日志可以看出，调用invalidate方法只会执行View的onDraw方法，而调用requestLayout则会重新走View的绘制流程。
@CallSuper
public void requestLayout() {
if (mMeasureCache != null) mMeasureCache.clear();
// 如果此时View的还处于layout之前的阶段，则直接调用ViewRootImpl的
// requestLayoutDuringLayout方法将当前View进行传递
if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == null) {
ViewRootImpl viewRoot = getViewRootImpl();
if (viewRoot != null && viewRoot.isInLayout()) {
if (!viewRoot.requestLayoutDuringLayout(this)) {
return;
}
}
mAttachInfo.mViewRequestingLayout = this;
}
// 更新 mPrivateFlags 变量
mPrivateFlags |= PFLAG_FORCE_LAYOUT;
mPrivateFlags |= PFLAG_INVALIDATED;
// 向上递归调用 Parent 的 requestLayout方法，
if (mParent != null && !mParent.isLayoutRequested()) {
mParent.requestLayout();
}
if (mAttachInfo != null && mAttachInfo.mViewRequestingLayout == this) {
mAttachInfo.mViewRequestingLayout = null;
}
}mParent.requestLayout()方法会一层一层的向上递归调用，最终调用ViewRootImpl的requestLayout，代码如下：
// ViewRootImpl
@Override
public void requestLayout() {
if (!mHandlingLayoutInLayoutRequest) {\
// 检查线程，如果不是UI线程则直接抛出异常
checkThread();
// 设置mLayoutRequested为true
mLayoutRequested = true;
// 调用这个方法，准备开始遍历View树
scheduleTraversals();
}
}scheduleTraversals代码多次见到了，比较熟悉。
@UnsupportedAppUsage
void scheduleTraversals() {
if (!mTraversalScheduled) {
// mTraversalScheduled 设置为 true
mTraversalScheduled = true;
// 通过Handler 发送同步屏障，阻塞非异步消息
mTraversalBarrier = mHandler.getLooper().getQueue().postSyncBarrier();
// 通过 mChoreographer post一个mTraversalRunnable，等待VSync信号的到来
mChoreographer.postCallback(
Choreographer.CALLBACK_TRAVERSAL, mTraversalRunnable, null);
notifyRendererOfFramePending();
pokeDrawLockIfNeeded();
}
}当收到Vsync信号后则会执行mTraversalRunnable的run方法：
final class TraversalRunnable implements Runnable {
@Override
public void run() {
// 遍历View树
doTraversal();
}
}void doTraversal() {
if (mTraversalScheduled) {
mTraversalScheduled = false;
// 移除同步屏障
mHandler.getLooper().getQueue().removeSyncBarrier(mTraversalBarrier);
// ...
// 开启View的绘制流程
performTraversals();
if (mProfile) {
Debug.stopMethodTracing();
mProfile = false;
}
}
}
在performTraversals方法中会一次调用performMeasure、performLayout、performDraw。进而调用View的measure、layout和draw方法。
在View的measure方法中，有如下代码：
public final void measure(int widthMeasureSpec, int heightMeasureSpec) {
// ...
final boolean forceLayout = (mPrivateFlags & PFLAG_FORCE_LAYOUT) == PFLAG_FORCE_LAYOUT;
// ...
if (forceLayout || needsLayout) {
// first clears the measured dimension flag
mPrivateFlags &= ~PFLAG_MEASURED_DIMENSION_SET;
resolveRtlPropertiesIfNeeded();
int cacheIndex = forceLayout ? -1 : mMeasureCache.indexOfKey(key);
if (cacheIndex < 0 || sIgnoreMeasureCache) {
// measure ourselves, this should set the measured dimension flag back
onMeasure(widthMeasureSpec, heightMeasureSpec);
mPrivateFlags3 &= ~PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
} else {
long value = mMeasureCache.valueAt(cacheIndex);
// Casting a long to int drops the high 32 bits, no mask needed
setMeasuredDimensionRaw((int) (value >> 32), (int) value);
mPrivateFlags3 |= PFLAG3_MEASURE_NEEDED_BEFORE_LAYOUT;
}
// ...
mPrivateFlags |= PFLAG_LAYOUT_REQUIRED;
}上述代码中通过mPrivateFlags & PFLAG_FORCE_LAYOUT来判断是否要强制布局，在调用requestLayout的时候，有这样一句代码：mPrivateFlags |= PFLAG_FORCE_LAYOUT; 因此，此时forceLayout为true。上述代码的最后一行 mPrivateFlags |= PFLAG_LAYOUT_REQUIRED; 标记了需要layout。
看下layout的代码：
public void layout(int l, int t, int r, int b) {
// ...
// 布局是否发生改变
boolean changed = isLayoutModeOptical(mParent) ? setOpticalFrame(l, t, r, b) : setFrame(l, t, r, b);
// 发生改变或者标记了PFLAG_LAYOUT_REQUIRED才会走onLayout
if (changed || (mPrivateFlags & PFLAG_LAYOUT_REQUIRED) == PFLAG_LAYOUT_REQUIRED) {
onLayout(changed, l, t, r, b);
// ...
}
// ...
}invalidate的意思是”作废，使无效“，在这里意味着调用invalidate方法会使之前的绘制的View失效。看下invalidate的源码：
// View
/**
* Invalidate the whole view. If the view is visible,
* {@link #onDraw(android.graphics.Canvas)} will be called at some point in
* the future.
* <p>
* This must be called from a UI thread. To call from a non-UI thread, call
* {@link #postInvalidate()}.
*/
public void invalidate() {
invalidate(true);
}
/**
* This is where the invalidate() work actually happens. A full invalidate()
* causes the drawing cache to be invalidated, but this function can be
* called with invalidateCache set to false to skip that invalidation step
* for cases that do not need it (for example, a component that remains at
* the same dimensions with the same content).
*
* @param invalidateCache Whether the drawing cache for this view should be
* invalidated as well. This is usually true for a full
* invalidate, but may be set to false if the View's contents or
* dimensions have not changed.
* @hide
*/
@UnsupportedAppUsage
public void invalidate(boolean invalidateCache) {
invalidateInternal(0, 0, mRight - mLeft, mBottom - mTop, invalidateCache, true);
}可以看到invalidate方法的注释：如果View是可见的，则使整个视图无效，onDraw方法将会在某一时刻被调用。在这个方法中仅仅是调用了invalidate的有参重载方法，并且传入了一个true。可见，invalidate实际操作都在这个重载方法中。
invalidate(boolean invalidateCache)方法的注释可以看到：这里是invalidate方法的工作实际执行的地方，一个完整的invalidate()方法会使绘图缓存失效，如果调用invalidate(boolean)时传入invalidateCache为false，那么就会跳过失效步骤。
接着看invalidateInternal方法源码：
// View
void invalidateInternal(int l, int t, int r, int b, boolean invalidateCache,
boolean fullInvalidate) {
if (mGhostView != null) {
mGhostView.invalidate(true);
return;
}
// 判断是否要跳过刷新
if (skipInvalidate()) {
return;
}
// Reset content capture caches
mPrivateFlags4 &= ~PFLAG4_CONTENT_CAPTURE_IMPORTANCE_MASK;
mContentCaptureSessionCached = false;
if ((mPrivateFlags & (PFLAG_DRAWN | PFLAG_HAS_BOUNDS)) == (PFLAG_DRAWN | PFLAG_HAS_BOUNDS)
|| (invalidateCache && (mPrivateFlags & PFLAG_DRAWING_CACHE_VALID) == PFLAG_DRAWING_CACHE_VALID)
|| (mPrivateFlags & PFLAG_INVALIDATED) != PFLAG_INVALIDATED
|| (fullInvalidate && isOpaque() != mLastIsOpaque)) {
if (fullInvalidate) {
mLastIsOpaque = isOpaque();
mPrivateFlags &= ~PFLAG_DRAWN;
}
// 根据条件设置mPrivateFlags
mPrivateFlags |= PFLAG_DIRTY;
if (invalidateCache) {
mPrivateFlags |= PFLAG_INVALIDATED;
mPrivateFlags &= ~PFLAG_DRAWING_CACHE_VALID;
}
// Propagate the damage rectangle to the parent view.
final AttachInfo ai = mAttachInfo;
// 拿到mParent
final ViewParent p = mParent;
if (p != null && ai != null && l < r && t < b) {
final Rect damage = ai.mTmpInvalRect;
damage.set(l, t, r, b);
// 调用parent的invalidateChild
p.invalidateChild(this, damage);
}
// Damage the entire projection receiver, if necessary.
if (mBackground != null && mBackground.isProjected()) {
final View receiver = getProjectionReceiver();
if (receiver != null) {
receiver.damageInParent();
}
}
}
}在上述方法中首先判断是否要跳过刷新，skipInvalidate源码如下：
private boolean skipInvalidate() {
//如果ViewGroup没有正在执行动画，或者View不可见，并且当前View的动画是null,并且mParent不是ViewGroup
return (mViewFlags & VISIBILITY_MASK) != VISIBLE && mCurrentAnimation == null &&
(!(mParent instanceof ViewGroup) ||
!((ViewGroup) mParent).isViewTransitioning(this));
}这里需要知道的是子View会通过mParent持有父View，可以一直向上追溯到DecorView，而在DecorView中mParent持有的是ViewRootImpl。
因此上述第一个中View的mParent不是ViewGroup那么这个View一定是DecorView,也就是说如果DecorView没有设置动画，并且不可见那么久跳过刷新。
接着看invalidateInternal方法，下边会为mPrivateFlags设置各种属性，然后调用了mParent的invalidateChild方法，即可以认为调用了ViewGroup的invalidateChild方法。看下ViewGroup中的实现：
// ViewGroup
public final void invalidateChild(View child, final Rect dirty) {
final AttachInfo attachInfo = mAttachInfo;
// ...
ViewParent parent = this;
if (attachInfo != null) {
// ...
do {
// ...
parent = parent.invalidateChildInParent(location, dirty);
// ...
} while (parent != null);
}
}
public ViewParent invalidateChildInParent(final int[] location, final Rect dirty) {
if ((mPrivateFlags & (PFLAG_DRAWN | PFLAG_DRAWING_CACHE_VALID)) != 0) {
// either DRAWN, or DRAWING_CACHE_VALID
if ((mGroupFlags & (FLAG_OPTIMIZE_INVALIDATE | FLAG_ANIMATION_DONE))
!= FLAG_OPTIMIZE_INVALIDATE) {
dirty.offset(location[CHILD_LEFT_INDEX] - mScrollX,
location[CHILD_TOP_INDEX] - mScrollY);
if ((mGroupFlags & FLAG_CLIP_CHILDREN) == 0) {
dirty.union(0, 0, mRight - mLeft, mBottom - mTop);
}
final int left = mLeft;
final int top = mTop;
if ((mGroupFlags & FLAG_CLIP_CHILDREN) == FLAG_CLIP_CHILDREN) {
if (!dirty.intersect(0, 0, mRight - left, mBottom - top)) {
dirty.setEmpty();
}
}
location[CHILD_LEFT_INDEX] = left;
location[CHILD_TOP_INDEX] = top;
} else {
if ((mGroupFlags & FLAG_CLIP_CHILDREN) == FLAG_CLIP_CHILDREN) {
dirty.set(0, 0, mRight - mLeft, mBottom - mTop);
} else {
// in case the dirty rect extends outside the bounds of this container
dirty.union(0, 0, mRight - mLeft, mBottom - mTop);
}
location[CHILD_LEFT_INDEX] = mLeft;
location[CHILD_TOP_INDEX] = mTop;
mPrivateFlags &= ~PFLAG_DRAWN;
}
mPrivateFlags &= ~PFLAG_DRAWING_CACHE_VALID;
if (mLayerType != LAYER_TYPE_NONE) {
mPrivateFlags |= PFLAG_INVALIDATED;
}
return mParent;
}
return null;
}invalidateChild方法中通过do...while调动了自身的invalidateChildInParent方法,而在invalidateChildInParent方法中最终又return了mParent,即通过递归向上查找View，直到mParent为null。
也就是这里的调用链是 ViewGroup.invalidateChildInParent->ViewGroup.invalidateChildInParent->...->DecoreView.invalidateChildInParent->ViewRootImpl.invalidateChildInParent
即最终追溯到了ViewRootImpl的invalidateChildInParent方法中：
@Override
public ViewParent invalidateChildInParent(int[] location, Rect dirty) {
// 检查线程
checkThread();
// ...
invalidateRectOnScreen(dirty);
return null;
}private void invalidateRectOnScreen(Rect dirty) {
final Rect localDirty = mDirty;
// ...
if (!mWillDrawSoon && (intersected || mIsAnimating)) {
// 开启遍历View树
scheduleTraversals();
}
}这里与requestLayout不同的是mLayoutRequested没有被置为true。
参考：https://www.jianshu.com/p/ce30f200209e
https://juejin.cn/post/6844903808594624525
requestLayout()：该方法会递归调用父窗口的requestLayout()方法，直到触发ViewRootImpl的performTraversals()方法，此时mLayoutRequestede为true，会触发onMesaure()与onLayout()方法，不一定会触发onDraw()方法。
invalidate()：该方法递归调用父View的invalidateChildInParent()方法，直到调用ViewRootImpl的invalidateChildInParent()方法，最终触发ViewRootImpl的performTraversals()方法，此时mLayoutRequestede为false，不会触发onMesaure()与onLayout()方法，当时会触发onDraw()方法。
postInvalidate()：该方法功能和invalidate()一样，只是它可以在非UI线程中调用。一般说来需要重新布局就调用requestLayout()方法，需要重新绘制就调用invalidate()方法。