On-Demand Resources in iOS Tutorial

Best Practices

There’s several things you can do to improve a user’s experience. You can improve error reporting, set download priorities and purge resources no longer in use.

Error Handling

In the previous example, whenever you encountered an error the app would simply state “There was a problem.” There’s not a whole lot the user can do with this.

You can make this a much better experience. Open GameScene.swift, and inside touchesBegan(_:with:), replace onFailure within the LevelOver with the following:

onFailure: { (error) in
  let controller = UIAlertController(
    title: "Error",
    message: "There was a problem.",
    preferredStyle: .alert)

    switch error.code {
    case NSBundleOnDemandResourceOutOfSpaceError:
      controller.message = "You don't have enough space available to download this resource."
    case NSBundleOnDemandResourceExceededMaximumSizeError:
      controller.message = "The bundle resource was too big."
    case NSBundleOnDemandResourceInvalidTagError:
      controller.message = "The requested tag does not exist."
    default:
      controller.message = error.description
    }

    controller.addAction(UIAlertAction(title: "Dismiss", style: .default, handler: nil))
    guard let rootViewController = self.view?.window?.rootViewController else { return }
          
    rootViewController.present(controller, animated: true)        
})

Take a moment to look over this change. It is a fair amount of code, but the main change is the addition of the switch statement. You’ll see that it’s testing the error code returned by the request object. Depending on which case the switch hits, the app will change the error message. This is much nicer. Take a look at each one of these errors.

1. NSBundleOnDemandResourceOutOfSpaceError is encountered when the user does not have enough space on their device to download the requested resources. This is useful since it gives your user a chance to clear up some space and try again.
2. NSBundleOnDemandResourceExceededMaximumSizeError is returned when this resource would exceed the maximum memory for in-use on-demand resources for this app. This would be a good time to purge some resources.
3. NSBundleOnDemandResourceInvalidTagError is returned when the resource tag being requested cannot be found. This would most likely be a bug on your part, and you may want to make sure you have the correct tag name.

Loading Priorities

The next improvement you can make is setting the loading priority of the request. This only requires a single line.

Open ODRManager.swift and add the following to requestSceneWith(tag:onSuccess:onFailure:) immediately after guard let request = currentRequest else { return }:

request.loadingPriority =
  NSBundleResourceRequestLoadingPriorityUrgent

NSBundleResourceRequestLoadingPriorityUrgent tells the operating system to download the content as soon as possible. In the case of downloading the next level of a game, it’s very urgent. You don’t want your users waiting. Remember, if you want to customize the loading priorities, you can use a Double between 0 and 1.

Purging Resources

You can get rid of unneeded resources by calling the method endAccessingResources on the current NSBundleResourceRequest.

Still in ODRManager.swift, add the following line immediately after guard let request = currentRequest else { return }:

// purge the resources associated with the current request
request.endAccessingResources()

Calling endAccessingResources now cleans up after yourself and purges the any resources you no longer need. You’re now being a courteous iOS citizen and cleaning up after yourself.

Where to Go From Here?

You can find the completed project here.

I hope that knowing how to use on-demand resources helps you reduce the size of your initial app downloads and makes your users a little happier.

For more in-depth coverage of on-demand resources, check out this excellent 2016 WWDC Video on Optimizing On-Demand Resources.

If you have any questions or comments, please join the forum discussion below!