ios – 等待异步方法在for循环中完成

概述我有一个包含三个异步方法的for循环,我希望在这3个异步方法完成后进行一些处理. -(void)getAllUsersInformations{dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ for(User *user in users){ 我有一个包含三个异步方法的for循环,我希望在这3个异步方法完成后进行一些处理.

-(voID)getAllUsersinformations{dispatch_async(dispatch_get_global_queue(disPATCH_QUEUE_PRIORITY_DEFAulT,0),^{           for(User *user in users){              [self getUserInfo:user];           }      //Here,I want to reload the table vIEw for example,after finishing the for loop (executing the whole three methods).    });}-(voID)getUserInfo:(User*)user{     [self getinformations:user];     [self getExperIEnces:user];     [self getEducation:user];}

你有任何技术可以得到这个结果吗？
非常感谢你.

解决方法 一种GCD方法是使用dispatch_group.因此,在启动异步任务之前,调用dispatch_group_enter,然后在异步任务完成时调用dispatch_group_leave,然后可以创建dispatch_group_notify,当异步任务完成时将调用它.您可以将此与完成块模式结合(无论如何,这对于异步方法都是一个好主意)：

>如果getinformations,getExperIEnces和getEducation本身就是所有异步方法,那么首先需要的是一些机制来了解它们何时完成.一种常见的解决方案是为每个实现完成块模式.例如：

// added completionHandler parameter which will be called when the retrIEval// of the "informations" is done.- (voID)getinformations:(User*)user completionHandler:(voID (^)(voID))completionHandler {    // do whatever you were before,but in the asynchronous task's completion block,call this    // completionHandler()    //    // for example    NSURLRequest *request;    [NSURLConnection sendAsynchronousRequest:request queue:nil completionHandler:^(NSURLResponse *response,NSData *data,NSError *connectionError) {        // handle the request here        // the important thing is that the completion handler should        // be called _insIDe_ the this block        if (completionHandler) {            completionHandler();        }    }];}

对getExperIEnces和getEducation也重复这个过程.
>然后,您可以使用调度组通知您完成这三个请求中的每个请求的时间,并在发生时调用getUserInfo中的完成块：

// added completion handler that will be called only when `getinformations`,// `getExperIEnces` and `getEducation` are all done.//// this takes advantage of the completion block we added to those three// methods above- (voID)getUserInfo:(User*)user completionHandler:(voID (^)(voID))completionHandler {    dispatch_group_t group = dispatch_group_create();    // start the three requests    dispatch_group_enter(group);    [self getinformations:user completionHandler:^{        dispatch_group_leave(group);    }];    dispatch_group_enter(group);    [self getExperIEnces:user completionHandler:^{        dispatch_group_leave(group);    }];    dispatch_group_enter(group);    [self getEducation:user completionHandler:^{        dispatch_group_leave(group);    }];    // this block will be called asynchronously only when the above three are done    dispatch_group_notify(group,dispatch_get_main_queue(),^{        if (completionHandler) {            completionHandler();        }    });}

>然后你在getAllUsersinformations重复这个过程：

// call new getUserInfo,using dispatch group to keep track of whether// all the requests are done-(voID)getAllUsersinformations {    dispatch_group_t group = dispatch_group_create();    for(User *user in users){        dispatch_group_enter(group);        [self getUserInfo:user completionHandler:^{            dispatch_group_leave(group);        }];    }    dispatch_group_notify(group,^{        [self.tableVIEw reloadData];    });}

两个最后的想法：

>概述了所有这些,我必须承认我可能会将这些请求包装在并发/异步自定义NSOperation子类中,而不是使用调度组.请参阅Concurrency Programming Guide的“配置并发执行 *** 作”部分.这是对代码进行更彻底的重构,因此我不会在此处解决此问题,但它允许您限制将同时运行的这些请求的数量,从而降低潜力超时问题.>我不知道有多少用户请求正在进行,但您可能需要考虑在用户信息进入时更新UI,而不是等待所有内容完成.这又是对代码进行更彻底的重构,但可能会导致更具响应性的内容.

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