This minimal example shows how to create multiple asynchronous operations using the NSOperation and NSOperationQueue classes and specify dependencies between them. Defining dependencies between operations is a very powerful way of doing various tasks which have some sort of inherent ordering built-in, and could potentially take a large and unknown amount of time. Here are just a few examples:
- Download an image from the web, and then convert it into another format locally.
- Drive to the theater, then buy tickets to a movie, then walk inside the theater, take a seat, and enjoy the show.
The basic idea is that you need to do these things in order. The next step can only proceed if the previous step completes successfully (in most cases). For example, you can't walk inside a theater without buying a ticket. Likewise you can't watch the movie unless you go inside the theater and so on.
So how would you model this in code, you ask?
Simple. For each operation/task that needs to happen, create a subclass of NSOperation, or use one of its existing subclasses like NSBlockOperation or NSInvocationOperation if you don't want fine-grained control.
DownloadImageOperation *downloadImage = [[DownloadImageOperation alloc] init];
ConvertImageOperation *convertImage = [[ConvertImageOperation alloc] init];
Define the dependencies between the operations using the addDependency: method of NSOperation.
[convertImage addDependency:downloadImage];
Then, add all operations to a NSOperationQueue and watch it run.
NSOperationQueue *queue = [[NSOperationQueue alloc] init];
[queue addOperations:[NSArray arrayWithObjects:downloadImage, convertImage, nil] waitUntilFinished:NO];
And that is it!
Here are the files worth checking out in this example. The TimedOperation class fakes an asynchronous operation using a timer. You can specify the amount of time you would want this operation to take to completion. Once that time is up, the operation is marked as completed. Three operations are chained in this example. The second operation can only begin once the first one finished, and the third can begin only when the second one finishes.
- main.m is where everything is setup and run.
- TimedWaitOperation.h is the header for the actual operation. It's a subclass of
NSOperation. - TimedWaitOperation.m is the implementation of this custom operation.