iOS 系统自带模糊效果

        UIBlurEffect *effect = [UIBlurEffect effectWithStyle:UIBlurEffectStyleLight];
        UIVisualEffectView *effectView = [[UIVisualEffectView alloc] initWithEffect:effect];
        effectView.alpha = 0.5;
        effectView.frame = self.backImageView.bounds;
        self.backImageView.image = [UIImage imageNamed:@"banner"];
        [self.backImageView insertSubview:effectView atIndex:0];