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Objective-C Yield

Introduces concept of multiple return values from Objective-C methods using yield keyword. Integrates nicely with for-in loop and allows early breaks.

Example

This code shows logging of six letters: 3 are logged directly and 3 from the for-in loop:

- (void)produceLetters {
    NSLog(@"A");
    yield(@"B");
    NSLog(@"C");
    yield(@"D");
    NSLog(@"E");
    yield(@"F");
}

- (void)test {
    for (NSString *letter in Yieldable(self, produceLetters)) {
        NSLog(@"%@", letter);
    }
}

On output, they have proper order (newlines removed):

A B C D E F

The yield macro uses magic to pass value to the enumeration and continues execution when needed. The magic is invoking the method on another queue and interrupting its execution.

Usage

Implement a method with void return value and any arguments. This method will be called from another queue, but mutually exclusive with the caller queue, never consurrently.
Use yield(x) to output any number of values. This macro needs ability to call return, so don’t call it from nested blocks or called functions.
Use Yieldable(…) macro in for-in statement. Arguments to this macro are similar to calling the method directly, here’s the example:
```
Yieldable(self, methodWithString:@"String" boolean:YES)
```
You can call break inside the for-in loop and the rest of your method will be skipped. Macro yield() will internally call return.
You can collect all objects to an array using .allObjects property, since the returned object from Yieldable(…) is an NSEnumerator instance. In the example above, the array would contain: B, D, F.