关于灰度层的show、hide

原理图如下：

- (void)show{

UIWindow *win = [[UIApplication sharedApplication] keyWindow];

UIView *topView = [win.subviews objectAtIndex:0];

[topView addSubview:self];

[UIView animateWithDuration:0.1 animations:^{

[self layoutIfNeeded];

}];

}

- (void)hide{

[UIView animateWithDuration:0.1 animations:^{

self.alpha = 0;

[self layoutIfNeeded];

} completion:^(BOOL finished) {

[self removeFromSuperview];

}];

}