PromiseKit is a wonderful library for dealing for async callbacks.

Other similar libraries are Bolts Framework and ReactiveCocoa, but personally I felt the syntax and design of PromiseKit to be much better.

Initially, I wanted to write a guide on some common usage of PromiseKit, but I find that the common recipes by Phil Mitchell himself is great.

Therefore, I will be emphasizing only on some usage/pitfall.

Throwing NSError

You can return NSError (or @throw a message), which will then have the nearest catch blcok to handle

Return in a promise

In a promise, you can return almost anything.

• If return a promise, next then will receive the fulfilled value
• If return NSError, next catch will handle it
• If return any other type, next then will recive it
• If return nil, that’s fine too
• If no return, next then will run immediately

It’s really flexible with promises.

Pitfall: If returning different types, the return type must be explicitly (^id) (any type), not just (^) (no type).

myURLPromise.then(^id(NSURL *url){ 
     if (url) {
         return url; // Got my url, I'm done, return the value
     }
     else {
         return myOtherURLPromise; // Not done, do some more work
     }
}).then(^(NSURL *url){
    // Do something with URL ...
});