iOS8自带模糊效果

typedef NS_ENUM(NSInteger, UIBlurEffectStyle) {
    UIBlurEffectStyleExtraLight,
    UIBlurEffectStyleLight,
    UIBlurEffectStyleDark
} NS_ENUM_AVAILABLE_IOS(8_0);

UIBlurEffect *blurEffect = [UIBlurEffect effectWithStyle:UIBlurEffectStyleDark];
UIVisualEffectView *blurEffectView = [[UIVisualEffectView alloc] initWithEffect:blurEffect];
blurEffectView.frame = self.imageView.bounds;
[self.imageView insertSubview:blurEffectView atIndex:0];