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.\" Copyright (c) 2008-2009 Apple Inc. All rights reserved.
.Dd May 1, 2009
.Dt dispatch_api 3
.Os Darwin
.Nm dispatch_api
.Nd 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:
.Bd -literal -offset indent
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.
.Xr dispatch 3 ,
.Xr dispatch_async 3 ,
.Xr dispatch_queue_create 3