ios - 如何在前一个 dispatch_async 完成时执行第二个 dispatch_async？

Question

我想按顺序添加一个 dispatch_async，但我不希望它们随机启动。我想举个例子:

dispatch_async 1 开始...
dispatch_async 1 结束。

dispatch_async 2 开始...
dispatch_async 2 结束。

dispatch_async 3 开始...
dispatch_async 3 结束。

我需要更新一个 sqlite，第一次调度中的信息对于第二次调度是必需的...

-(void)connectionDidFinishLoading:(NSURLConnection *)connection
{
    NSLog(@"%@",[connection currentRequest]);

    NSLog(@"connectionDidFinishLoading");
    NSError* error;
    NSString *responseKey = [self getResponseKey:connection];

    NSDictionary* response = [NSJSONSerialization JSONObjectWithData:[receivedData objectForKey:responseKey] options:kNilOptions error:&error];
    //NSLog(@"%@", response);

    if (error)
    {
        NSLog(@"Error: %@", error);
        NSLog(@"Error: Response strange format, not an NSArray and not a NSString!\n%@", [[NSString alloc] initWithData:[receivedData objectForKey:responseKey] encoding:NSUTF8StringEncoding]);
    }

    NSLog(@"connection url : %@", connection.currentRequest.URL);

    if ([[NSString stringWithFormat:@"%@", [connection currentRequest]] rangeOfString:@"getSynchroGuest?"].location != NSNotFound) 
    {
        NSLog(@"response success");
        if ([[response valueForKey:@"lignes"] isKindOfClass:[NSArray class]])
        {
            if ([[response valueForKey:@"lignes"] count] > 0)
            {
                dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^(void){
                    //Background Thread

                [self fillDataBaseWithDict:response];

                nbTotal = nbTotal + PACKET_FOR_SYNC;

                [self WebServiceSynchroGuest:self.activityIndicator withSynchroBtn:synchroBtn withNbTotal:nbTotal];
                                 });

            }
        }
...

提前致谢。

SOLUTION:
                dispatch_async(serialDispatchQueue, ^{
                    [self fillDataBaseWithDict:response];
                    nbTotal = nbTotal + PACKET_FOR_SYNC;

                    dispatch_async(dispatch_get_main_queue(), ^(void){
                    [self WebServiceSynchroGuest:self.activityIndicator withSynchroBtn:synchroBtn withNbTotal:nbTotal];
                    });
                });

Answer 1

定义你自己的串行队列

然后添加这样的代码

 dispatch_queue_t yourSerialQueue = dispatch_queue_create("com.testcompany.testproduct.testserialqueue", DISPATCH_QUEUE_SERIAL);
 dispatch_async(yourSerialQueue, ^{ /* first task */ });
 dispatch_async(yourSerialQueue, ^{ /* second task to be executed after first task */ });

串行队列保证任务将按照提交顺序和串行方式(一次一个)执行