5.11 Objective-C FFI

package: base

The ffi/unsafe/objc library builds on ffi/unsafe to support interaction with Objective-C.

The library supports Objective-C interaction in two layers. The upper layer provides syntactic forms for sending messages and deriving subclasses. The lower layer is a thin wrapper on the Objective-C runtime library functions. Even the upper layer is unsafe and relatively low-level compared to normal Racket libraries, because argument and result types must be declared in terms of FFI C types (see Type Constructors).

5.11.1 FFI Types and Constants

value
_id : ctype?

The type of an Objective-C object, an opaque pointer.

value
_Class : ctype?

The type of an Objective-C class, which is also an _id.

value
_Protocol : ctype?

The type of an Objective-C protocol, which is also an _id.

value
_SEL : ctype?

The type of an Objective-C selector, an opaque pointer.

value
_BOOL : ctype?

The Objective-C boolean type. Racket values are converted for C in the usual way: #f is false and any other value is true. C values are converted to Racket booleans.

value
YES : boolean?

Synonym for #t

value
NO : boolean?

Synonym for #f

5.11.2 Syntactic Forms and Procedures

syntax
(tell result-type obj-expr method-id)
(tell result-type obj-expr arg ...)

result-type =
| #:type ctype-expr

arg = method-id arg-expr
| method-id #:type ctype-expr arg-expr

Sends a message to the Objective-C object produced by obj-expr. When a type is omitted for either the result or an argument, the type is assumed to be _id, otherwise it must be specified as an FFI C type (see Type Constructors).

If a single method-id is provided with no arguments, then method-id must not end with :; otherwise, each method-id must end with :.

Examples:

> (tell NSString alloc)
#<cpointer:id>
> (tell (tell NSString alloc)
initWithUTF8String: #:type _string "Hello")
#<cpointer:id>

syntax
(tellv obj-expr method-id)
(tellv obj-expr arg ...)

Like tell, but with a result type _void.

syntax
(import-class class-id ...)

Defines each class-id to the class (a value with FFI type _Class) that is registered with the string form of class-id. The registered class is obtained via objc_lookUpClass.

Example:

> (import-class NSString)

A class accessed by import-class is normally declared as a side effect of loading a foreign library. For example, if you want to import the class NSString on Mac OS, the "Foundation" framework must be loaded, first. Beware that if you use import-class in DrRacket or a module that requires racket/gui/base, then "Foundation" will have been loaded into the Racket process already. To avoid relying on other libraries to load "Foundation", explicitly load it with ffi-lib:

> (ffi-lib
"/System/Library/Frameworks/Foundation.framework/Foundation")
> (import-class NSString)

syntax
(import-protocol protocol-id ...)

Defines each protocol-id to the protocol (a value with FFI type _Protocol) that is registered with the string form of protocol-id. The registered class is obtained via objc_getProtocol.

Example:

> (import-protocol NSCoding)

syntax
(define-objc-class class-id superclass-expr
  maybe-mixins
  maybe-protocols
  [field-id ...]
  method ...)

maybe-mixins =
| #:mixins (mixin-expr ...)

maybe-protocols =
| #:protocols (protocol-expr ...)

method = (mode maybe-async result-ctype-expr (method-id) body ...+)
| (mode maybe-async result-ctype-expr (arg ...+) body ...+)

mode = +
| -
| +a
| -a

maybe-async =
| #:async-apply async-apply-expr

arg = method-id [ctype-expr arg-id]

Defines class-id as a new, registered Objective-C class (of FFI type _Class). The superclass-expr should produce an Objective-C class or #f for the superclass. An optional #:mixins clause can specify mixins defined with define-objc-mixin. An optional #:protocols clause can specify Objective-C protocols to be implemented by the class.

Each field-id is an instance field that holds a Racket value and that is initialized to #f when the object is allocated. The field-ids can be referenced and set! directly when the method bodys. Outside the object, they can be referenced and set with get-ivar and set-ivar!.

Each method adds or overrides a method to the class (when mode is - or -a) to be called on instances, or it adds a method to the meta-class (when mode is + or +a) to be called on the class itself. All result and argument types must be declared using FFI C types (see Type Constructors). When mode is +a or -a, the method is called in atomic mode (see _cprocedure). An optional #:async-apply specification determines how the method works when called from a foreign thread in the same way as for _cprocedure.

If a method is declared with a single method-id and no arguments, then method-id must not end with :. Otherwise, each method-id must end with :.

If the special method dealloc is declared for mode -, it must not call the superclass method, because a (super-tell dealloc) is added to the end of the method automatically. In addition, before (super-tell dealloc), space for each field-id within the instance is deallocated.

Example:

> (define-objc-class MyView NSView
    [bm] ; <- one field
    (- _racket (swapBitwmap: [_racket new-bm])
       (begin0 bm (set! bm new-bm)))
    (- _void (drawRect: [_NSRect exposed-rect])
       (super-tell drawRect: exposed-rect)
       (draw-bitmap-region bm exposed-rect))
    (- _void (dealloc)
       (when bm (done-with-bm bm))))

syntax
(define-objc-mixin (class-id superclass-id)
  maybe-mixins
  maybe-protocols
  [field-id ...]
  method ...)

Like define-objc-class, but defines a mixin to be combined with other method definitions through either define-objc-class or define-objc-mixin. The specified field-ids are not added by the mixin, but must be a subset of the field-ids declared for the class to which the methods are added.

syntax
self

When used within the body of a define-objc-class or define-objc-mixin method, refers to the object whose method was called. This form cannot be used outside of a define-objc-class or define-objc-mixin method.

syntax
(super-tell result-type method-id)
(super-tell result-type arg ...)

When used within the body of a define-objc-class or define-objc-mixin method, calls a superclass method. The result-type and arg sub-forms have the same syntax as in tell. This form cannot be used outside of a define-objc-class or define-objc-mixin method.

syntax
(get-ivar obj-expr field-id)

Extracts the Racket value of a field in a class created with define-objc-class.

syntax
(set-ivar! obj-expr field-id value-expr)

Sets the Racket value of a field in a class created with define-objc-class.

syntax
(selector method-id)

Returns a selector (of FFI type _SEL) for the string form of method-id.

Example:

> (tellv button setAction: #:type _SEL (selector terminate:))

procedure
(objc-is-a? obj cls) → boolean?
obj : _id
cls : _Class

Check whether obj is an instance of the Objective-C class cls or a subclass.

Changed in version 6.1.0.5 of package base: Recognize subclasses, instead of requiring an exact class match.

procedure
(objc-subclass? subcls cls) → boolean?
subcls : _Class
cls : _Class

Check whether subcls is cls or a subclass.

Added in version 6.1.0.5 of package base.

procedure
(objc-get-class obj) → _Class
obj : _id

Extract the class of obj.