当前一个dispatch_async完成后如何执行第二次dispatch_async?

Question

我想按顺序添加一个dispatch_async，但我希望它们不要随机启动。 我想例如：

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 端。

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];
});
}
}
...

提前谢谢。

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

定义自己的串行队列

并添加这样的代码

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提交顺序和串行方式执行(一次一个(

Answer 2

要从字面上理解你的 Q，你必须嵌套调用：

dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), 
^(void)
{ 
// do some work
…
// finished here 
// The next async code
dispatch_async(dispatch_get_global_queue( DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), 
^(void)
{ 
// do some work
…
// finished here 
// and so on
}
}

但是您应该真正考虑使用自定义串行队列或NSOperation等。

使用串行 Q：

dispatch_queue_t stepByStepQueue = dispatch_queue_create("com.you.taks", NULL);
dispatch_async(stepByStepQueue, 
^(void)
{
// Step
}); 
dispatch_async(stepByStepQueue, 
^(void)
{
// By
}); 
dispatch_async(stepByStepQueue, 
^(void)
{
// Step
}); 

Answer 3

您可以将所有dispatch_async作为串行队列并逐个执行

dispatch_group_t组 = dispatch_group_create((;

dispatch_group_async(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {
// block1
NSLog(@"Block1");
[ NSThread sleepForTimeInterval:5.0];
NSLog(@"Block1 End");
});
dispatch_group_async(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {
// block2
NSLog(@"block 2");
[ NSThread sleepForTimeInterval:10.0];
NSLog(@"Block2 End");
});
dispatch_group_async(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {
// block3
NSLog(@"block 3");
[ NSThread sleepForTimeInterval:15.0];
NSLog(@"Block3 End");
});
dispatch_group_notify(group,dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_HIGH, 0), ^ {
// block4
NSLog(@"block 4");
[ NSThread sleepForTimeInterval:20.0];
NSLog(@"Block4 End");
});

Answer 4

最好的方法是使用NSOperations并将它们添加到 queue.so 它将按完成顺序调用

但是如果你想以同样的方式做到这一点，那么定义完成块并在完成块中添加你的dispatch_async2和dispatch_Async3，并在最后调用这些完成块。

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
// do some long running processing here
// Check that there was not a nil handler passed.
if( completionHandler1 )
{
completionHandler1();
}
});
});

Answer 5

添加这样的
代码 您也可以设置 PRIORITY。

dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{

}(;

}(;

Answer 6

你想要的很容易用Operation而不是GCD来实现。Operation类具有addDependency(_ op: Operation)功能。将代码放入BlockOperation实例中，添加依赖项并在OperationQueue中运行它们