DAN DAN - 3 months ago 15
iOS Question

iOS - Concatenating asynchronous block-based operations

How would you perform N asynchronous operations, such as network calls, working with completion block operations and no delegates/notifications?

Given N methods like this:

- (void)methodNWithCompletion:(void (^)(Result *))completion {
Operation *operation = [Operation new];

// ...
// Asynchronous operation performed here
// ...


A straightforward solution would be to call each operation in the completion block of the previous one:

[self method1WithCompletion:^(Result *result) {
// ...
[self method2WithCompletion:^(Result *result) {
// ...
[self method3WithCompletion:^(Result *result) {
// ...
[self method4WithCompletion:^(Result *result) {
NSLog(@"All done");

but I'm looking for a more elegant and reusable solution, easier to write and maintain (with no many indented blocks).

Many thanks,


It all depends on what you want to do. Many powerful sophisticated tools are at your disposal. You can use such things as:

  • Serial queue (if you want the completion blocks run in order)

  • Concurrent queue (if you don't care whether the completion blocks execute simultaneously or in what order)

  • Dispatch group (if there is something you want to do only after all completion blocks have finished)

  • Operation and OperationQueue (if you want to establish the dependency order in which networking operations must take place - see esp. the fantastic WWDC 2015 video on this topic)