An Objective-C category for BFTask class in Bolts-iOS.

With this category, you can:

• chain tasks with dot-notation as JavaScript Promise-like syntax. (no more counting brackets!)
• write a catch block which will be called in error case only.
• write a finally block which won't change the result value unless the block fails.

[self countUsersAsync].continueWith(^id (BFTask *task) {
    // this block is called regardless of the success or failure
    NSNumber *count = task.result;
    if ([count intValue] <= 0) {
        return [BFTask taskWithError:[NSError errorWithDomain:@"MyDomain"
                                                         code:-1
                                                     userInfo:nil]];
    } else {
        return [self makeTotalUserStringAsync:count];
    }
}).then(^id (NSString *totalUserString) {
    // this block is skipped when the previous task is failed.
    [self showMessage:totalUserString];
    return nil;
}).catch(^id (NSError *error) {
    // this block is called in error case.
    [self showErrorMessage:error];
    return nil;
}).finally(^BFTask * (){
    [self updateList];
    return nil;
});

If you want to specify the executor, use continueOn, thenOn, catchOn and finallyOn. You can also use continueOnMain, thenOnMain, catchOnMain and finallyOnMain, they use mainThreadExecutor.

[User findAllAsync].catchOn([BFExecutor mainThreadExecutor], ^id (NSError *error) {
    [self showAlert:error];
});

// same as above
[User findAllAsync].catchOnMain(^id (NSError *error) {
    [self showAlert:error];
});

If the task holds an exception, it is converted to NSError on passing it as a parameter of the catch-block. In this case, the domain of the converted error is BFPTaskErrorDomain and its code is BFPTaskErrorException. The original exception object can be retrieved from user info dictionary like this:

[self doSomethingAsync].catch( ^id (NSError *error) {
    if ([BFPTaskErrorDomain isEqualToString:error.domain]
        && error.code == BFPTaskErrorException) {
        NSException *ex = [error.userInfo objectForKey:BFPUnderlyingExceptionKey];
    }
});

To be exact, Bolts' task is not a Promise. So this is not a Promise too. The purpose of this project is to make task-chains much readable.

If you don't think dot-notation is preferable but do want catch and/or finally, you can still use catchWithExecutor:withBlock: and finallyWithExecutor:withBlock:. They are normal methods.

To tell the truth, I hate dot-notation for methods basically. I want to use it for only properties. I prefer to use bracket-notation for usual methods. But the task-chaining is a special case for me. It's unusual. I feel the task-chaining is not a nested method call, but a flow control.

Write the following line to your Podfile:

pod 'BFTaskPromise', '~> 2.0'

Add BFTask+PromiseLike.h and BFTask+PromiseLike.m in Classes folder to your project.

The parameter of the block passed to then/catch is changed in 2.0. A then-block takes a result value and a catch-block takes an error value.

In prior to 2.0, the block parameter of both then and catch was a BFTask value. So if you have a code for 1.X like:

[foo loadAsync].then(^id (BFTask *task) {
    bar.title = (NSString *)task.result;
}

you must rewrite it to:

[foo loadAsync].then(^id (NSString *result) {
    bar.title = result;
}

The MIT License.