PyObjC support for “blocks”

Introduction

Objective-C has the concept of “blocks”, which are basically anonymous inline functions. The syntax for them is like this:

^{ printf("x is %d\n", 42); }

This is a literal for a block that takes no arguments and prints a value when called.

Calling blocks from Python

The Python representation for a block is a callable object, that is you can call the block just like you call any other function object.

PyObjC manages the memory for blocks, it is not necessary to manage the reference counts of blocks in your code.

Limitations

Ancient versions of the Apple Objective-C compiler did not include information to make blocks runtime introspectable. For those systems all APIs returning blocks need to be annotated with the Objective-C signatures of the returned block.

For all APIs that have a block as its argument the Objective-C signature for the block needs to be specified through the metadata system.

The required annotations are shipped with PyObjC’s bindings for system frameworks, that is block APIs “just work”.

It is possible to retrieve and set the call signature of a block using the __block_signature__ attribute on blocks.

Changed in version 2.5: PyObjC can use runtime introspection to retrieve the Objective-C signature for a block.

Implementing blocks in Python

It is very easy to use Objective-C methods that have a block as one of their arguments: just pass an arbitrary callable. PyObjC will automatically wrap your callable in the right low-level datastructure.

One of the side-effects of this is that the variour storage classes that are defined for block-related variables are not relevant for Python users. Blocks behave just like regular functions.

Metadata for blocks

The current implementation of blocks doesn’t allow for full introspection, which means that PyObjC must be taught about the signatures of blocks. This is done using the metadata system.