一.問題

-(CLPlacemark*)getStartplacemark

?????{

?????????__block CLPlacemark* start;

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

?????????????CLPlacemark *startPlaceMark=[placemarks firstObject];

?????????????start=startPlaceMark;

??????????????????????}];

?????????NSLog(@"%@",start);//這里打印出的是一個 null,是因為 block 是異步的,所以會先走下面的 return,

?????????returnstart;

}

二.解決方法

- (CLPlacemark*)getStartplacemark

{

//先創(chuàng)建一個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;

?????? //發(fā)出已完成的信號

????????dispatch_semaphore_signal(semaphore);

? ? }];

????//等待執(zhí)行，不會占用資源

????dispatch_semaphore_wait(semaphore, DISPATCH_TIME_FOREVER);

? ? return start;

}