view.post() & handler.post() & runOnUiThread()

view.post()

[View.java]

public boolean post(Runnable action) {
    final AttachInfo attachInfo = mAttachInfo;
    if (attachInfo != null) {
        return attachInfo.mHandler.post(action);
    }
    // Assume that post will succeed later
    ViewRootImpl.getRunQueue().post(action);
    return true;
}

[ViewRootImpl.java]

static final class RunQueue {
    private final ArrayList mActions = new ArrayList();

    void post(Runnable action) {
        postDelayed(action, 0);
    }

    void postDelayed(Runnable action, long delayMillis) {
        HandlerAction handlerAction = new HandlerAction();
        handlerAction.action = action;
        handlerAction.delay = delayMillis;

        synchronized (mActions) {
            mActions.add(handlerAction);
        }
    }

如果 mAttachInfo != null 则 attachInfo.mHandler.post(action) 这个 attachInfo.mHandler 是什么，在哪初始化不细讲，下文会提到 mAttachInfo 的初始化，反正不管怎么说一定是主线程的 Looper。这个条件下，view.post() 和 handler.post() 可以说用起来没区别

如果 view 的 mAttachInfo == null 就把 Runnable action 加入到 ViewRootImpl 的队列 mActions 中，那什么时候执行呢？

[ViewRootImpl.java]

private void performTraversals() {
    final View host = mView;
    ...
    host.dispatchAttachedToWindow(mAttachInfo, 0); // ViewGroup 会递归，设置 view 的 mAttachInfo
    ...
    getRunQueue().executeActions(mAttachInfo.mHandler); // 遍历队列 handler.post(action)，
    ...
    measureHierarchy(...)
    ...
    performLayout(...)
    ...
    boolean cancelDraw = mAttachInfo.mTreeObserver.dispatchOnPreDraw() ||
            viewVisibility != View.VISIBLE;
            
    if (!cancelDraw && !newSurface) { // 这边我也不清楚，待继续学习
        if (!skipDraw || mReportNextDraw) {
            ...
            performDraw();
        }
    } else {
        if (viewVisibility == View.VISIBLE) {
            // Try again
            scheduleTraversals();
        } else if
        ...
    }
    ...
}

从图中可以看到
a) new ViewRootImpl() 是在 Activity onResume() 之后
题外话，这点比较有意思，更新 UI 判断是否主线程其实都是在 ViewRootImpl 中

void checkThread() {
    if (mThread != Thread.currentThread()) {
        throw new CalledFromWrongThreadException(
                "Only the original thread that created a view hierarchy can touch its views.");
    }
}

那没有初始化 ViewRootImpl 之前，在子线程更新 UI 会怎样？答案是没有问题。可以在 Activity 的 onCreate onResume 中写个子线程更新 UI 试试，当然你不能太耗时。

b) ViewRootImpl performTraversals() 是经过 handler 排队的

c) performTraversals() 中 getRunQueue().executeActions(mAttachInfo.mHandler) 又是放入 handler 排队

所以 view.post(runnable) runnable 执行的时机肯定是在 view 的 measure、layout 之后，经过调试得知最终结果是

onCreate→
onResume→
new ViewRootImpl requestLayout→
====经过 handler→performTraversals→
========view measure→view layout→
========经过 handler→view posted runnable→
========经过 handler→onWindowFocusChanged→
========经过 handler→performTraversals→
============view measure→view layout→view draw

所以想要测试某个 view 的绘制时间，还是在 onDraw 中计算比较准确（2层 post 也差不多）

runOnUiThread()

[Activity.java]

public final void runOnUiThread(Runnable action) {
    if (Thread.currentThread() != mUiThread) {
        mHandler.post(action);
    } else {
        action.run();
    }
}

与 handler.post() 的区别就是如果当前线程是主线程则直接执行，不用排队了