NSURLSession 的 delegateQueue 只能对
等待多个网络请求完成后再执行操作，网上给的方案一般都是GCD的处理。这里给一个NSOperationQueue的做法，使用Operation的依赖来达到同样的效果。
示例代码如下：
NSURL *url = [NSURL URLWithString:@"http://www.cocoachina.com"];
NSOperationQueue *aQueue = [[NSOperationQueue alloc] init];
NSBlockOperation *op1 = [NSBlockOperation blockOperationWithBlock:^{
NSLog(@"run 1");
}];
NSBlockOperation *op2 = [NSBlockOperation blockOperationWithBlock:^{
NSLog(@"run 2");
}];
NSBlockOperation *op3 = [NSBlockOperation blockOperationWithBlock:^{
NSLog(@"run 3");
}];
NSBlockOperation *finalOp = [NSBlockOperation blockOperationWithBlock:^{
NSLog(@"run final");
}];
[finalOp addDependency:op1];
[finalOp addDependency:op2];
[finalOp addDependency:op3];
[aQueue addOperation:finalOp];
NSLog(@"rep 1");
[[NSURLSession.sharedSession dataTaskWithURL:url completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
NSLog(@"res 1");
[aQueue addOperation:op1];
}] resume];
NSLog(@"rep 2");
[[NSURLSession.sharedSession dataTaskWithURL:url completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
NSLog(@"res 2");
[aQueue addOperation:op2];
}] resume];
NSLog(@"rep 3");
[[NSURLSession.sharedSession dataTaskWithURL:url completionHandler:^(NSData * _Nullable data, NSURLResponse * _Nullable response, NSError * _Nullable error) {
NSLog(@"res 3");
[aQueue addOperation:op3];
}] resume];
控制台输出：
rep 1
rep 2
rep 3
res 1
run 1
res 2
run 2
res 3
run 3
run final
