2019-11-12：请谈谈invalidate()和postInvalidate()方法的区别和应用场景？

[Moosphan]

[DaveBoy]

多了个post导致可以子线程调用

[hyyaoming]

1. invalidate()用来重绘UI，需要在UI线程调用。

2. postInvalidate()也是用来重新绘制UI,它可以在UI线程调用，也可以在子线程中调用，postInvalidate()方法内部通过Handler发送了一个消息将线程切回到UI线程通知重新绘制，并不是说postInvalidate()可以在子线程更新UI,本质上还是在UI线程发生重绘，只不过我们使用postInvalidate()它内部会帮我们切换线程

 /**
     * <p>Cause an invalidate to happen on a subsequent cycle through the event loop.
     * Use this to invalidate the View from a non-UI thread.</p>
     *
     * <p>This method can be invoked from outside of the UI thread
     * only when this View is attached to a window.</p>
     *
     * @see #invalidate()
     * @see #postInvalidateDelayed(long)
     */
    public void postInvalidate() {
        postInvalidateDelayed(0);
    }

  /**
     * <p>Cause an invalidate to happen on a subsequent cycle through the event
     * loop. Waits for the specified amount of time.</p>
     *
     * <p>This method can be invoked from outside of the UI thread
     * only when this View is attached to a window.</p>
     *
     * @param delayMilliseconds the duration in milliseconds to delay the
     *         invalidation by
     *
     * @see #invalidate()
     * @see #postInvalidate()
     */
    public void postInvalidateDelayed(long delayMilliseconds) {
        // We try only with the AttachInfo because there's no point in invalidating
        // if we are not attached to our window
        final AttachInfo attachInfo = mAttachInfo;
        if (attachInfo != null) {
            attachInfo.mViewRootImpl.dispatchInvalidateDelayed(this, delayMilliseconds);
        }
    }

  public void dispatchInvalidateDelayed(View view, long delayMilliseconds) {
        Message msg = mHandler.obtainMessage(MSG_INVALIDATE, view);
        mHandler.sendMessageDelayed(msg, delayMilliseconds);
    }

[mlinqirong]

invalidate 和postInvalidate都是重新绘制UI
invalidate 只能在UI线程中调用
postInvalidate 可以在子线程中调用