Question

I have a controller class that uses asynchronous calls to perform some network operations. My view hierarchy is navigation based e.g. View1 > View2 > View3.

Now lets suppose my View3 (View3Controller) uses asynchronous calls to perform some network operations, and View2Controller also implements some of the delegate methods as well.

User can use “Back” button to navigate back at anytime, and consequently [View3Controller dealloc] gets called. Now i want the async operation to complete! Is there a (safe) way to make sure that View3Controller doesn’t get dealloc‘ed until the async operation is completed?

Answer 1

One solution is to move the asynchronous call to its own class, preferably owned by View2Controller or someone higher up. When pushing View3Controller you just pass along this new class and if the user navigates back it will continue as you want.

Imagine an object like:

AsyncObject : NSObject {}
@property (nonatomic, assign) id delegate;
- (void)start;

and when pushing View3Controller

View3Controller *view3Controller = ...
view3Controller.asyncObject = self.asyncObject;
[self.navigationController pushView....];

and when starting the async operation from within View3Controller

self.asyncObject.delegate = self;
[self.asyncObject start]; 

if the user navigates back you could potentially do something like this in navigationController:willShowViewController:animated: in View2Controller do get the callbacks from asyncObject in case View3Controller gets popped.

- (void)navigationController:(UINavigationController *)navigationController willShowViewController:(UIViewController *)viewController animated:(BOOL)animated {
  if (viewController == self) {
    self.asyncObject.delegate = self;
  }
}

If you need to be able to receive events from the asyncObject for several objects at the same time you should look at notifications or KVO (Key Value Observing) instead of using a delegate.