等待两种异步方法完成

Question

我想初始化一个模型，让模型做一些异步的东西，并提交一个新的viewcontroller一旦完成。但是，我如何等待两个异步方法完成，以及如何设置回调方法？

伪

在我StartViewController.m：

-(void)openArticle 
{ 
    article = [Article initWithObject:someObject]; 
    article.callback = changeView; 
} 

-(void)changeView 
{ 
[self presentViewController:someController]; 
} 

在我ArticleModel.m：

-(void)initWithObject:someObject 
{ 
    [self loadImage] 
    [self geoCode] 
} 

-(void)loadImage 
{ 
    runAsyncMethod: success:^() // This one is actually a AFNetworking setImageWithURLRequest 
} 

-(void)geoCode 
{ 
    runAnotherAsyncMethod: success:^() // This one is actually a geocodeAddressString operation 
} 

Answer 1

可以使用dispatch_group小号

- (void)initWithObject:(id)someObject 
{ 
    self = [super init]; 
    if (self) { 
    self.dispatch_group = dispatch_group_create(); 

    [self loadImage] 
    [self geoCode] 

    dispatch_group_notify(self.dispatch_group, dispatch_get_main_queue(), ^{ 
     NSLog(@"Push new view controller"); 
    }); 
    } 
    return self; 
} 

- (void)loadImage 
{ 
    dispatch_group_enter(self.dispatch_group); 

    __weak __typeof(self) weakSelf = self; 
    runAsyncMethod: success:^{ 
    __typeof(weakSelf) strongSelf = weakSelf; 
    if (strongSelf.dispatch_group) { 
     dispatch_group_leave(strongSelf.dispatch_group); // You need to ensure that this is called in both success and failure 
    } 
    } 

} 

- (void)geoCode 
{ 
    dispatch_group_enter(self.dispatch_group); 

    __weak __typeof(self) weakSelf = self; 
    runAnotherAsyncMethod: success:^{ 
    __typeof(weakSelf) strongSelf = weakSelf; 
    if (strongSelf.dispatch_group) { 
     dispatch_group_leave(strongSelf.dispatch_group); 
    } 
    } 
} 

Answer 2

你做不等待。如果你等待，它不是异步的！如果您要等待，您将会失去整个异步点。

你要做的是，当你的success处理程序被调用时，你走出主线程（以防万一你在后台线程上被回调），现在做你需要做的任何事情。换句话说，你只要让你的success处理程序在被调用的时候被调用。

在你的情况，你可能想链上的事情你想做的事：

• 呼叫loadImage

• 在其回调，调用geoCode

• 在其回调，一步进入主线程并呈现新的视图控制器。

Answer 3

我不知道您的需求，但本地GCD方式等几个非同步的任务是

void dispatch_barrier_async(dispatch_queue_t queue, dispatch_block_t block); 

https://developer.apple.com/library/ios/documentation/Performance/Reference/GCD_libdispatch_Ref/Reference/reference.html#//apple_ref/c/func/dispatch_barrier_async

Answer 4

您可以使用dispatch_group这样，当一个方法结束，它只是叶子群组。我自己使用类似的代码，它的作用就像一个魅力。

- (void)initWithObject:someObject { 

    // Create a dispatch group 
    dispatch_group_t group = dispatch_group_create(); 
    [self loadImageWithDispatchGroup:group]; 
    [self geoCodeWithDispatchGroup:group]; 

    // Here we wait for all the requests to finish 
    dispatch_group_notify(group, dispatch_get_main_queue(), ^{ 
     // Do whatever you need to do when all requests are finished 
    }); 
} 

- (void)loadImageWithDispatchGroup:(dispatch_group_t)group { 
    dispatch_group_enter(group); 
    runAsyncMethod: success:^() // This one is actually a AFNetworking setImageWithURLRequest 
    // In your success or failure AFNetworking method, call this as soon as the request ended 
    dispatch_group_leave(group); 
} 

- (void)geoCodeWithDispatchGroup:(dispatch_group_t)group { 
    dispatch_group_enter(group); 
    runAnotherAsyncMethod: success:^() // This one is actually a geocodeAddressString operation 
    // In your success async geocode callback method, call this as soon as the request ended 
    dispatch_group_leave(group); 
}