你可以使用UIVisualEffectView来实现这个效果。这是一个本地API，经过了性能和电池寿命的微调，而且很容易实现。

迅速:

//only apply the blur if the user hasn't disabled transparency effects
if !UIAccessibility.isReduceTransparencyEnabled {
    view.backgroundColor = .clear

    let blurEffect = UIBlurEffect(style: .dark)
    let blurEffectView = UIVisualEffectView(effect: blurEffect)
    //always fill the view
    blurEffectView.frame = self.view.bounds
    blurEffectView.autoresizingMask = [.flexibleWidth, .flexibleHeight]

    view.addSubview(blurEffectView) //if you have more UIViews, use an insertSubview API to place it where needed
} else {
    view.backgroundColor = .black
}

objective - c:

//only apply the blur if the user hasn't disabled transparency effects
if (!UIAccessibilityIsReduceTransparencyEnabled()) {
    self.view.backgroundColor = [UIColor clearColor];

    UIBlurEffect *blurEffect = [UIBlurEffect effectWithStyle:UIBlurEffectStyleDark];
    UIVisualEffectView *blurEffectView = [[UIVisualEffectView alloc] initWithEffect:blurEffect];
    //always fill the view
    blurEffectView.frame = self.view.bounds;
    blurEffectView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;

    [self.view addSubview:blurEffectView]; //if you have more UIViews, use an insertSubview API to place it where needed
} else {
    self.view.backgroundColor = [UIColor blackColor];
}

如果你以模态方式呈现这个视图控制器来模糊底层内容，你需要将模态呈现风格设置为Over Current Context，并将背景颜色设置为clear，以确保底层视图控制器在顶部呈现时仍然可见。

简单的答案是添加一个子视图并改变它的alpha。

UIView *mainView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 200, 200)];
UIView *subView = [[UIView alloc] initWithFrame:popupView.frame];
UIColor * backImgColor = [UIColor colorWithPatternImage:[UIImage imageNamed:@"blue_Img.png"]];
subView.backgroundColor = backImgColor;
subView.alpha = 0.5;
[mainView addSubview:subView];

为了帮助大家，下面是我根据Jordan h的回答创建的一个快速扩展。它是用swift 5编写的，可以在Objective C中使用。

extension UIView {

    @objc func blurBackground(style: UIBlurEffect.Style, fallbackColor: UIColor) {
        if !UIAccessibility.isReduceTransparencyEnabled {
            self.backgroundColor = .clear

            let blurEffect = UIBlurEffect(style: style)
            let blurEffectView = UIVisualEffectView(effect: blurEffect)
            //always fill the view
            blurEffectView.frame = self.self.bounds
            blurEffectView.autoresizingMask = [.flexibleWidth, .flexibleHeight]

            self.insertSubview(blurEffectView, at: 0)
        } else {
            self.backgroundColor = fallbackColor
        }
    }

}

注意:如果你想在不影响文本的情况下模糊UILabel的背景，你应该创建一个容器UIView，将UILabel作为子视图添加到容器UIView，将UILabel的backgroundColor设置为UIColor。clear，然后调用blurBackground(style: UIBlurEffect。样式，fallbackColor: UIColor)的容器UIView。下面是用Swift 5写的一个简单的例子:

let frame = CGRect(x: 50, y: 200, width: 200, height: 50)
let containerView = UIView(frame: frame)
let label = UILabel(frame: frame)
label.text = "Some Text"
label.backgroundColor = UIColor.clear
containerView.addSubview(label)
containerView.blurBackground(style: .dark, fallbackColor: UIColor.black)

这是偶然发现的，给了我真正伟大的(几乎复制与苹果)的结果，并使用加速框架。——http://pastebin.com/6cs6hsyQ 不是我写的

我不认为我被允许发布代码，但上面提到的WWDC示例代码是正确的。链接如下:https://developer.apple.com/downloads/index.action?name=WWDC%202013

你要找的文件是UIImage上的category，方法是applyLightEffect。

正如我在上面的评论中提到的，除了模糊，苹果模糊还有饱和度和其他东西。简单的模糊是不行的……如果你想模仿他们的风格。

这是乔伊回答的重要补充

这适用于你想用UINavigationController呈现一个模糊背景的UIViewController的情况。

// suppose you've done blur effect with your presented view controller
UINavigationController *nav = [[UINavigationController alloc] initWithRootViewController];

// this is very important, if you don't do this, the blur effect will darken after view did appeared
// the reason is that you actually present navigation controller, not presented controller
// please note it's "OverFullScreen", not "OverCurrentContext"
nav.modalPresentationStyle = UIModalPresentationOverFullScreen;

UIViewController *presentedViewController = [[UIViewController alloc] init]; 
// the presented view controller's modalPresentationStyle is "OverCurrentContext"
presentedViewController.modalPresentationStyle = UIModalPresentationOverCurrentContext;

[presentingViewController presentViewController:nav animated:YES completion:nil];

享受吧!