如何先执行block中的代码后在执行下面的代码

一.问题

-(CLPlacemark*)getStartplacemark

     {

         __block CLPlacemark* start;

         [ self.geoCoder geocodeAddressString:_SourceText.text completionHandler:^(NSArray*placemarks, NSError*error) {

             CLPlacemark *startPlaceMark=[placemarks firstObject];

             start=startPlaceMark;

                      }];

         NSLog(@"%@",start);//这里打印出的是一个 null,是因为 block 是异步的,所以会先走下面的 return,

         returnstart;

}

二.解决方法

- (CLPlacemark*)getStartplacemark

{

//先创建一个semaphore

dispatch_queue_t queue = dispatch_get_global_queue(0, 0);

dispatch_semaphore_t semaphore = dispatch_semaphore_create(1);

    __block CLPlacemark* start;

    [self.geoCoder geocodeAddressString:_SourceText.text completionHandler:^(NSArray *placemarks, NSError *error) {      

      CLPlacemark *startPlaceMark = [placemarks firstObject];

        start = startPlaceMark;

       //发出已完成的信号

        dispatch_semaphore_signal(semaphore);

    }];

    //等待执行，不会占用资源

    dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

    return start;

}