我尝试设置一个dispatch_group，这样我就可以进行多次异步调用，将多个返回的数据存储到一个数组中，然后在完成所有调用后处理它们。这是获取JSON的代码：
- (void) getJSONFromArrayOfURLS : (NSArray*) arrURLS {
//create dispatch group
dispatch_group_t jsonGroup = dispatch_group_create();
for(NSURL* thisURL in arrURLS) {
//enter dispatch group
dispatch_group_enter(jsonGroup);
//make nsurlsession query here
NSURLSessionConfiguration* sessionConfig = [NSURLSessionConfiguration defaultSessionConfiguration];
sessionConfig.timeoutIntervalForResource = 10;
NSURLSession* session = [NSURLSession sharedSession];
NSURLSessionDataTask* dataTask = [session dataTaskWithURL:thisURL completionHandler:^(NSData* data, NSURLResponse* response, NSError* error) {
NSHTTPURLResponse* httpResponse = (NSHTTPURLResponse*) response;
if (httpResponse.statusCode == 200) {
//hit the servers but they returned an error
if (error) {
NSString* strErrMsg;
if (error.code == NSURLErrorCancelled) {
strErrMsg = @"The connection to the servers was lost. ";
} else {
strErrMsg = [NSString stringWithFormat:@"The server sent this error message: \n%@", error.localizedDescription];
}
UIAlertController* alertController = [UIAlertController
alertControllerWithTitle:@"There was an error connecting to the servers."
message: strErrMsg
preferredStyle:UIAlertControllerStyleAlert];
UIAlertAction* okAction = [UIAlertAction
actionWithTitle:NSLocalizedString(@"OK", @"OK action")
style:UIAlertActionStyleDefault
handler:^(UIAlertAction *action) {
}
];
[alertController addAction:okAction];
[self presentViewController:alertController animated:YES completion:nil];
//hit servers successfully
} else {
//deseriealize the return
id JSONObject = [NSJSONSerialization JSONObjectWithData:data options:NSJSONReadingAllowFragments error:&error];
[self.arrReturnedData addObject:JSONObject];
}
} else {
//never hit the servers
UIAlertController* alertController = [UIAlertController
alertControllerWithTitle:@"There was an error connecting to the servers."
message: [NSString stringWithFormat:@"The server replied with the following status:\n\n%i", (int)httpResponse.statusCode]
preferredStyle:UIAlertControllerStyleAlert];
UIAlertAction* okAction = [UIAlertAction
actionWithTitle:NSLocalizedString(@"OK", @"OK action")
style:UIAlertActionStyleDefault
handler:^(UIAlertAction *action) {
}
];
[alertController addAction:okAction];
[self presentViewController:alertController animated:YES completion:nil];
}
}];
[dataTask resume];
dispatch_group_leave(jsonGroup);
}
dispatch_group_notify(jsonGroup, dispatch_get_main_queue(), ^(void) {
NSLog(@"%@", self.arrReturnedData);
});
}
我遇到的问题是，如果我在这一行断点，我可以看到JSON数据成功进入：
[self.arrReturnedData addObject:JSONObject];
数组中有一个对象（NSDictionary），带有我期待的数据。但是，更进一步，在这里：
dispatch_group_notify(jsonGroup, dispatch_get_main_queue(), ^(void) {
NSLog(@"%@", self.arrReturnedData);
});
数组为空。我是dispatch_groups的新手，但我的理解是，一旦输入到jsonGroup的所有任务都完成，就会调用group_notify。我没有在任何地方重置数组，所以我的想法是在异步过程中填充数组之前调用它。我接近这个错误吗？