项目中有时为了安全起见，在双击Home键，退到不活跃状态时，需要添加模糊效果，比如下图：

要实现这种效果，非常简单，只需在APPdelegate里面添加几句代码即可

//AppDelegate.h中添加以下属性

@property (nonatomic,strong) UIVisualEffectView *visualEffectView;//毛玻璃效果

AppDelegate.m中

- (void)applicationWillResignActive:(UIApplication *)application {
    UIBlurEffect *blurEffect = [UIBlurEffect effectWithStyle:UIBlurEffectStyleLight];
    self.visualEffectView = [[UIVisualEffectView alloc] initWithEffect:blurEffect];
    self.visualEffectView.alpha = 0;
    self.visualEffectView.frame = self.window.frame;
    [self.window addSubview:self.visualEffectView];
    [UIView animateWithDuration:0.2 animations:^{
        self.visualEffectView.alpha = 1;
    }];

}


- (void)applicationDidBecomeActive:(UIApplication *)application {
    // Restart any tasks that were paused (or not yet started) while the application was inactive. If the application was previously in the background, optionally refresh the user interface.
    [UIView animateWithDuration:0.2 animations:^{
        self.visualEffectView.alpha = 0;
    } completion:^(BOOL finished) {
        [self.visualEffectView removeFromSuperview];
    }];

}

以上就是实现毛玻璃的全部代码，是不是很简单呢~~~

后续加上：经测试这种写法有点小问题，就是如果项目中有拨打电话的操作（系统弹出拨打电话的提示框）,这时弹出框下面的整个屏幕也是毛玻璃效果，这并不是我们想看到的，那怎么改呢，其实就是方法写的位置不对，按照下面的方法写

注意：将- (void)applicationWillResignActive:(UIApplication *)application；里面的代码放进- (void)applicationDidEnterBackground:(UIApplication *)application；里面，两个方法里面的代码虽然相似，但是属性值有所改变，要注意！！！

- (void)applicationDidEnterBackground:(UIApplication *)application {
    
    UIBlurEffect *blurEffect = [UIBlurEffect effectWithStyle:UIBlurEffectStyleLight];
    self.visualEffectView = [[UIVisualEffectView alloc] initWithEffect:blurEffect];
    self.visualEffectView.alpha = 1;
    self.visualEffectView.frame = self.window.frame;
    [self.window addSubview:self.visualEffectView];
    [UIView animateWithDuration:0.2 animations:^{
        self.visualEffectView.alpha = 1;
    }];

}


- (void)applicationDidBecomeActive:(UIApplication *)application {
    // Restart any tasks that were paused (or not yet started) while the application was inactive. If the application was previously in the background, optionally refresh the user interface.
    [UIView animateWithDuration:0.2 animations:^{
        self.visualEffectView.alpha = 0;
    } completion:^(BOOL finished) {
        [self.visualEffectView removeFromSuperview];
    }];

}

改成下面的方法后，上面出现的问题就解决了，只是可能首次进入后台的时候不能正常显示效果，支付宝也存在这种情况，但是基本上无伤大雅~~~