直接用Invalidate()方法会导致错误:只有主线程才能更新UI

取而代之的是可以使用postInvalidate();

原因:

最终会调用ViewRootImpl类的dispatchInvalidateDelayed(View view, long delayMilliseconds) 方法;

代码如下:

public void dispatchInvalidateDelayed(View view, long delayMilliseconds) {

    Message msg = mHandler.obtainMessage(MSG_INVALIDATE, view);

    mHandler.sendMessageDelayed(msg, delayMilliseconds);

	//看到Handler就不用说为什么它能更新ui了吧

}

在postInvalidate()方法:

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 postInvalidateDelayed(long delayMilliseconds, int left, int top,

        int right, int bottom) {

    // 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) {

        final AttachInfo.InvalidateInfo info = AttachInfo.InvalidateInfo.obtain();

        info.target = this;

        info.left = left;

        info.top = top;

        info.right = right;

        info.bottom = bottom;

		//这个最终调用ViewRootImpl类的dispatchInvalidateRectDelayed(info, delayMilliseconds)方法;

        attachInfo.mViewRootImpl.dispatchInvalidateRectDelayed(info, delayMilliseconds);

    }

}