Objective-C - 等待两个异步方法完成

Question

我正在调用我想以同步顺序执行的四个方法,前两个方法是同步的,后两个方法是异步的(从URL获取数据).

伪代码:

- (void)syncData {

    // Show activity indicator
    [object sync]; // Synchronous method
    [object2 sync]; // Synchronous method
    BOOL object3Synced = [object3 sync]; // Async method
    BOOL object4Synced = [object4 sync]; // Async method
    // Wait for object3 and object4 has finished and then hide activity indicator
}

我怎样才能做到这一点？

Answer 1

使用障碍:

void dispatch_barrier_async(dispatch_queue_t queue, dispatch_block_t block);

提交用于异步执行的屏障块并立即返回.

当屏障块到达专用并发队列的前面时,它不会立即执行.相反,队列等待,直到其当前正在执行的块完成执行.此时,队列自己执行屏障块.在屏障块完成之后,在屏障块之后提交的任何块都不会执行.

这个例子输出1 2 3 4 done虽然是异步的,但它可能是1 2 4 3 done.由于我了解您想要处理活动指标,这应该无关紧要.

#import <Foundation/Foundation.h>
int main(int argc, char *argv[]) {
    @autoreleasepool {
        dispatch_queue_t queue = dispatch_queue_create("com.myqueue", 0);
        dispatch_sync(queue,  ^(){NSLog(@"1");} );
        dispatch_sync(queue,  ^(){NSLog(@"2");});
        dispatch_async(queue, ^(){NSLog(@"3");});
        dispatch_async(queue, ^(){NSLog(@"4");});
        dispatch_barrier_sync(queue, ^(){NSLog(@"done");});
    }
}

有关测试异步代码的其他方法,请参阅:https://stackoverflow.com/a/11179523/412916

Answer 2

假设你实际上有某种方式知道异步方法何时完成,你可能想要的是:

- (void)syncData {

    // Show activity indicator
    [object sync]; // Synchronous method
    [object2 sync]; // Synchronous method

    _object3Synced = _object4Synced = NO;
    [object3 syncWithCompletionHandler:
          ^{
               _object3Synced = YES;
               [self considerHidingActivityIndicator];
          }]; // Async method
    [object4 syncWithCompletionHandler:
          ^{
               _object4Synced = YES;
               [self considerHidingActivityIndicator];
          }]; // Async method
}

- (void)considerHidingActivityIndicator
{
     if(_object3Synced && _object4Synced)
     {
         // hide activity indicator, etc
     }
}