Version 6 (modified by chak, 10 years ago) (diff)


Haskell Objective-C FFI: Objective-C Classes

Using classes

We might import ObjC classes like so:

foreign import objc "@class UIView" o'UIView :: Class

Problem: We also need to introduce the class type C'UIView. So far, the FFI has no support for the import of types.

Declaring classes

One option might be something like

foreign export "@class MyUIView : UIView" myUIView :: C'UIView

Option 1: value definition

The class definition value myUIView contains the details of the class definition. However, it is odd, because C'UIView should usually be the type of instances if of UIView.

Option 2: incremental definition

In addition to the foreign export declaring the class, the class is populated with methods in further foreign export declarations.

We have the following problems:

  • The ObjC runtime uses two functions to initiate and finalise the declaration of a new class, namely objc_allocateClassPair and objc_registerClassPair. All ivars must be added between these two calls (except for the 64-bit runtime, I guess, but the runtime ref doesn't say that). Methods and protocols can still be added after registering.
  • objc_allocateClassPair expects the superclass as a value of type Class. The documentation does not say whether the superclass must already be registered (or whether it is sufficient to have it allocated at that point).

Problems arise because Haskell declarations are unordered, so we should make no assumption about the order of foreign declarations. Moreover, we need to have foreign imported all class that we subclass (and which are not locally defined) – or we just call objc_getClass. On the other hand, when you subclass an existing class, you almost certainly need to foreign import the superclass anyway, to implement the methods for the new class (so an implicit objc_getClass is probably not worth the effort).

What about ObjC classes that have been declared in other Haskell modules, do we have to foreign import them again or do we just import the Haskell representation?

We may have to compute the class DAG and make sure we generate class initialisation code that at module load time allocates, registers, and subclasses all Objective-C classes in the right order.

BIG Questions

How do we represent ObjC classes in Haskell land?

It could just be a pointer. The advantage of a pointer is that there is no issue of keeping ObjC land and Haskell land data in sync (BIG PLUS). However, what does it mean to export a class in that case? In fact, we would need to allocate, populate, and register the class in ObjC land and then import it into Haskell land. This clashes with that the Haskell identifier mentioned in a foreign export is at a usage, not a defining occurrence.

How do we populate ObjC classes with ivars, methods & properties in Haskell land?