ios自带高斯模糊涂层控件，类似一个view。放上去就可以实现背景高斯模糊。特别是对于需要高斯模糊的区域有动态刷新的view简直好用到哭！！！
UIVisualEffectView *effectView = [[UIVisualEffectView alloc] initWithEffect:[UIBlurEffect effectWithStyle:UIBlurEffectStyleLight]];
effectView.frame = [UIScreen mainScreen].bounds;
[self.view addSubview:effectView];
[self.view sendSubviewToBack:effectView];
