dispatch_api(3) Library Functions Manual dispatch_api(3)

dispatch_api — Designing API using dispatch

The following is a brief summary of some of the common design patterns to consider when designing and implementing API in terms of dispatch queues and blocks.

A general recommendation is to allow both a callback block and target dispatch queue to be specified. This gives the application the greatest flexibility in handling asynchronous events.

It's also recommended that interfaces take only a single block as the last parameter. This is both for consistency across projects, as well as the visual aesthetics of multiline blocks that are declared inline. The dispatch queue to which the block will be submitted should immediately precede the block argument (second-to-last argument). For example:

read_async(file, callback_queue, ^{
	printf("received callback.\n");
});

When function pointer alternatives to interfaces that take blocks are provided, the argument order of the function signature should be identical to the block variant; with the exception that the block argument is replaced with a context pointer, and a new last parameter is added, which is the function to call.

The function based callback should pass the context pointer as the first argument, and the subsequent arguments should be identical to the block based variant (albeit offset by one in order).

It is also important to use consistent naming. The dispatch API, for example, uses the suffix "_f" for function based variants.

dispatch(3), dispatch_async(3), dispatch_queue_create(3)

May 1, 2009 Darwin