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OC Block块:5-系统框架中的Block

Apple定义的系统框架的API中,对Block的使用也比较集中,主要在动画、通知等等几个方面,因此,对于普通开发者来说,重点掌握系统框架API中Block的使用也是比较有必要的。

1、系统框架API中的Block

在iOS4以后,越来越多的系统级的API在使用Block。苹果对于Block的使用主要集中在如下几个方面:

  • 完成处理–Completion Handlers

  • 通知处理–Notification Handlers

  • 错误处理–Error Handlers

  • 枚举–Enumeration

  • 动画与形变–View Animation and Transitions

  • 分类–Sorting

  • 线程管理:GCD/NSOperation

接下来,给大家介绍几个常用的。

2、动画与形变

在UIView类的定义中,提供了若干个包含Block参数的方法,用来设置动画,例如修改View的大小、位置、透明度等等。

@interface UIView(UIViewAnimationWithBlocks) 
+ (void)animateWithDuration:(NSTimeInterval)duration delay:(NSTimeInterval)delay options:(UIViewAnimationOptions)options animations:(void (^)(void))animations completion:(void (^ __nullable)(BOOL finished))completion NS_AVAILABLE_IOS(4_0);
+ (void)animateWithDuration:(NSTimeInterval)duration animations:(void (^)(void))animations completion:(void (^ __nullable)(BOOL finished))completion NS_AVAILABLE_IOS(4_0); // delay = 0.0, options = 0
+ (void)animateWithDuration:(NSTimeInterval)duration animations:(void (^)(void))animations NS_AVAILABLE_IOS(4_0); // delay = 0.0, options = 0, completion = NULL
+ (void)animateWithDuration:(NSTimeInterval)duration delay:(NSTimeInterval)delay usingSpringWithDamping:(CGFloat)dampingRatio initialSpringVelocity:(CGFloat)velocity options:(UIViewAnimationOptions)options animations:(void (^)(void))animations completion:(void (^ __nullable)(BOOL finished))completion NS_AVAILABLE_IOS(7_0);
+ (void)transitionWithView:(UIView *)view duration:(NSTimeInterval)duration options:(UIViewAnimationOptions)options animations:(void (^ __nullable)(void))animations completion:(void (^ __nullable)(BOOL finished))completion NS_AVAILABLE_IOS(4_0);
+ (void)transitionFromView:(UIView *)fromView toView:(UIView *)toView duration:(NSTimeInterval)duration options:(UIViewAnimationOptions)options completion:(void (^ __nullable)(BOOL finished))completion NS_AVAILABLE_IOS(4_0); // toView added to fromView.superview, fromView removed from its superview 
+ (void)performSystemAnimation:(UISystemAnimation)animation onViews:(NSArray *)views options:(UIViewAnimationOptions)options animations:(void (^ __nullable)(void))parallelAnimations completion:(void (^ __nullable)(BOOL finished))completion NS_AVAILABLE_IOS(7_0);
@end

示例:

[UIView animateWithDuration:0.2 animations:^{
        view.alpha = 0.0;
    } completion:^(BOOL finished){
        [view removeFromSuperview];
}];

3、完成与错误处理

完成处理Block在某个动作完成后,通过回调的方式进行执行。错误处理Block会在执行某个操作时,假如发生错误而被执行。

- (IBAction)pressBtn:(id)sender {
    
    CGRect cacheFrame = self.imageView.frame;
    [UIView animateWithDuration:1.5 animations:^{//播放动画Block
        CGRect newFrame = self.imageView.frame;
        newFrame.origin.y = newFrame.origin.y + 150.0;
        self.imageView.frame = newFrame;
        self.imageView.alpha = 0.2;
    }
                     completion:^ (BOOL finished) {//结束回调Block
                         if (finished) {
                             // Revert image view to original.
                             self.imageView.frame = cacheFrame;
                             self.imageView.alpha = 1.0;                                                   
                         }
                     }];
}

4、通知

在注册通知观察者中,有如下的方法,可以在添加/注册观察者时,编写收到通知后需要执行的Block代码。使用这个方法来注册通知,可以使代码简单明了。

- (id )addObserverForName:(nullable NSString *)name object:(nullable id)obj queue:(nullable NSOperationQueue *)queue usingBlock:(void (^)(NSNotification *note))block;

示例:

- (void)viewDidLoad {
    [super viewDidLoad]; 
    //注册观察者
    [[NSNotificationCenter defaultCenter] addObserverForName:@"AnimationCompleted" object:nil  queue:[NSOperationQueue mainQueue]                                                      usingBlock:^(NSNotification *notif) { 
    NSLog(@"ViewController动画结束");                                                
     }];
}

5、线程操作(GCD/NSOperation)

在有关线程操作的GCD以及NSOperation中,也会使用到Block。例如,延迟执行方法。

dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1.0 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^{
       //延迟N秒后执行的代码
   });