NSWrap

Create Go language bindings for Objective-C.

Using NSWrap, you can work with MacOS interfaces, subclasses, library functions, protocols and delegates entirely in Go.

Getting Started

Installation

NSWrap runs on MacOS and requires clang (from the XCode command line tools) and the MacOS system header files.

go get git.wow.st/gmp/nswrap

The nswrap command line tool should now be installed in your go/bin path.

Since NSWrap uses clang to generate an AST from Objective-C input files, you will need to install XCode and its associated command line tools. Enter clang --version from your terminal prompt to see if you have it installed. You will also need the Objective-C header files for the various frameworks you want to use. Look for them in /System/Library/Frameworks/*/Headers.

Try Out An Example

NSWrap is designed to be easy to use. To get started with an example, visit your Go source directory in a terminal and enter:

cd git.wow.st/gmp/nswrap/examples/app
go generate
go build
./app

Basic Usage

YAML configuration file

NSWrap takes no command line arguments. All configuration directives are included in a file named nswrap.yaml, which must be found in the directory from which NSWrap is invoked.

# nswrap.yaml example

package: MyWrapper
inputfiles:
  - /System/Library/Frameworks/Foundation.framework/Headers/Foundation.h

classes:
  - NSString
  - NSArray

frameworks: [ Foundation ]
pragma [ clang diagnostic ignored "-Wformat-security" ]

Regular expressions are permitted in the names of classes, functions, protocols and protocol methods, overridden superclass methods, and enums.

When invoked, NSWrap creates a subdirectory with the name of the package as specified in nswrap.yaml or, by default, ns if a package name is not specified. In the output directory, a main.go file and, if required, exports.go, will be created or overwritten.

To automatically invoke NSWrap, put a //go:generate nswrap comment at the top of your go source file and use go generate to create your Objective-C bindings.

NSWrap will look for Objective-C header files where directed under inputfiles in your configuration file. CGo will also automatically compile and link any Objective-C implementation (.m) files found in this output directory, so put them in there if you are going to be hand-crafting any Objective-C implementations that need to go in the same package as your automatically generated bindings.

Class and Instance Methods

NSWrap will create bindings for all classes identified in the classes directive of the configuration file. All of the class and instance methods are bound to Go and all types identified in the process are wrapped in Go types (as described below), except for methods that contain unsupported return types or paramater types such as blocks and function pointers.

s1 := ns.NSStringAlloc()        // allocate an instance of NSString
s2 := ns.NSStringWithSting(s1)  // call a class method of NSString
class := ns.NSStringClass()     // class method returning the class of NSString
fmt.Println(s2.UTF8String())    // call UTF8String, an NSString instance method

As seen above, generated class methods will have the same name as their Objective-C method name, converted to the Go TitleCase convention, prefixed with the class name, and, if necessary, disambiguated for overloaded Objective-C methods. Any redundant initial characters are elided (e.g. the Objective-C [NSString stringWithString:aString] is shortened in Go to ns.NSStringWithString(aString)). Instance methods are converted to TitleCase and disambiguated for method overloading as described below.

Note that while return types and parameter types needed for the binding will be defined and wrapped for you in Go types, you will not get any of their methods unless those types also appear in your NSWrap configuration file. For example, the [NSDictionary WithObjects: forKeys:] constructor takes two NSArray parameters, so if you want to use it from Go you will probably want to have NSArray in your configuration file in addition to NSDictionary.

Overloaded Methods

Because Go does not allow overloaded functions, NSWrap automatically disambiguates overloaded method names as required. This is done by successively adding parameter names onto the end of the Go function name until a unique name is created.

For example, NSString provides the folowing compare methods:

- compare:
- compare:options:
- compare:options:range:
- compare:options:range:locale:

These are translated into Go as:

func (o NSString) Compare(string NSString) NSComparisonResult { }

func (o NSString) CompareOptions(string NSString, mask NSStringCompareOptions) NSComparisonResult { }

func (o NSString) CompareOptionsRange(string NSString, mask NSStringCompareOptions,
	rangeOfReceiverToCompare NSRange) NSComparisonResult { }

func (o NSString) CompareOptionsRangeLocale(string NSString, mask NSStringCompareOptions,
	rangeOfReceiverToCompare NSRange, locale NSObject) NSComparisonResult { }

NSString Helpers

When NSWrap sees a class or instance method ending in ...WithString (taking an Objective-C NSString as a parameter), it will automatically create an additional helper method ending in WithGoString that takes a Go string.

str := ns.NSStringWithGoString("** your string goes here **")
fmt.Printf("%s\n",str)

NSWrap creates a Char Go type that is equivalent to a C char. A pointer to Char in Go code can therefore be used with Objective-C functions and methods that take a char* parameter. NSWrap provides the helper functions CharWithGoString and CharWithBytes that take, respectively, Go strings and Go byte arrays ([]byte) and return *Char in Go. As demonstrated above, NSWrap also provides a String() methods so that the *Char and NSString types implement the Stringer Go interface.

Working With NSObject and its Descendants

Objective-C objects are represented in Go by a type and an interface as follows:

type Id struct {
	ptr unsafe.Pointer
}
func (o Id) Ptr() unsafe.Pointer { return o.ptr }

type NSObject interface {
	Ptr() unsafe.Pointer
}

Other object types in Go are structs that directly or indirectly embed Id and therefore implement NSObject.

• The NSObject Interface

The Id type in Go represents the Objective-C type id, which is a pointer to an Objective-C object. Because cgo does not understand this type, NSWrap will always translate it to a void* on the C side. The NSObject interface in Go allows any type that directly or indirectly embeds Id to be used with generic Objective-C functions. For example:

o1 := ns.NSStringWithGoString("my string")
s1 := ns.NSSetWithOBjects(o1)
a := ns.NSMutableArrayWithObjects(o1,s1)

Since NSString and NSSet in Go both implement the NSObject interface, they can both be used as parameters to the NSMutableArray constructor.

This will help you, too, when working with delegates (see below). Classes that accept delegates will generally accept any NSObject in their initWithDelegate() or setDelegate() methods, and may or may not test at runtime if the provided object actually implements the required delegate protocol.

• Inheritance

Objective-C only provides single inheritance. In Go, this is modeled using embedding. Top level objects that inherit from NSObject in Objective-C embed the Go type Id and therefore implement the NSObject Go interface. Other objects embed their superclass. For example:

type NSArray struct { Id }
func (o NSArray) Ptr() unsafe.Pointer { return o.ptr }
func (o Id) NSArray() NSArray {
	ret := NSArray{}
	ret.ptr = o.ptr
	return ret
}

type NSMutableArray struct { NSArray }
func (o NSMutableArray) Ptr() unsafe.Pointer { return o.ptr }
func (o Id) NSMutableArray() NSMutableArray {...}

Observe:

b := ns.NSButtonAlloc()        // NSButton > NSControl > NSView > NSResponder > NSObject
b.InitWithFrame(ns.NSMakeRect(100,100,200,200)) // Method of NSView
b.SetTitle(nst("PUSH"))                         // Method of NSButton
vw := win.ContentView()
vw.AddSubview(b.NSView)				// Pass the button's embedded NSView

In Go, NSButtonAlloc returns a Go object of type ns.NSButton. However, there is no InitWithFrame method for receivers of this type. This is not necessary because NSButton embeds NSControl which in turn embeds NSView. The InitWithFrame method only needs to be implemented for NSView receivers. Go will automatically find the indirectly embedded NSView and call the right method.

Note that, since InitWithFrame() is defined only for NSView and returns an NSView type, the following will not work. Look out for this if you like to chain your Alloc and Init methods and are getting type errors:

//DO NOT DO THIS -- InitWithFrame returns NSView, not NSButton
b := ns.NSButtonAlloc().InitWithFrame(ns.MakeRect(100,100,200,200))

Go has no trouble finding embedded methods for your NSButton and will happily search up the chain through NSControl, NSView, NSResponder and NSObject and all of their associated protocols and categories. As of this writing, on MacOS 10.13.6, NSWrap binds 90 instance methods for NSObject, so things like Hash(), IsEqualTo(), ClassName(), RespondsToSelector and many many others are available and can be called on any object directly from Go.

Go does not perform the same type magic when you use variables as function or method parameters. If you want to pass your NSButton as a parameter to a method that accepts an NSView type, you need to explicitly pass the embedded NSView (b.NSView in the example above).

NSWrap creates a method for Id allowing objects to be converted at run-time to any other class. You will need this for Enumerators, which always return Id. See below under Enumerators for an example, but make sure you know (or test) what type your objects are before converting them. You can implement a somewhat less convenient version of a Go type switch this way.

Because Id can be converted to any type, and every object in the Foundation classes inherits from Id, it is possible to send any message to any object, if you are feeling lucky. If you are not lucky you will get an exception from the Objective-C runtime. You are going to have to explicitly convert your object to the wrong type before the compiler will let you do this.

a := ns.NSArrayWithObjects(o1,o2)      // NSArray embeds Id
fmt.Println(a.NSString().UTF8String()) // DON'T!
//  |         |          \-method of NSString, returns *Char, a "Stringer"
//  |         \-method of Id returning NSString
//  \-calls "String()" on its parameters

The above code will compile, but you will get an exception at runtime:

*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason:
'-[__NSArrayM UTF8String]: unrecognized selector sent to instance 0x4608940'

Variadic Functions

As seen above with the NSMutableArrayWithObjects() constructor example, NSWrap supports variadic functions. Because of the limitations of cgo, there is a numerical limit to the number of parameters in a variadic function call, which defaults to 16 but can be set with the vaargs configuration directive. NSWrap will automatically include a nil sentinel when calling any Objective-C methods with variadic parameter lists. The direct types va_list and va_list_tag are not currently supported.

Pointers to Pointers

When NSWrap encounters a pointer to a pointer to an Objective-C object, it treats it as an array of objects and translates it into a pointer to a Go slice. If you are passing empty slices into these functions, be sure to pre-allocate them to a sufficient capacity. Ssee below for an example. These Go slices can be used for input and output of methods and functions.

Pointers to pointers are sometimes passed to Objective-C methods or functions as a way of receiving output from those functions, especially because Objective-C does not allow for multiple return values. In those cases, after the CGo call, the method parameter will be treated as a nil-terminated array of object pointers that may have been modified by the Objective-C function or method. NSWrap will copy the object pointers back into the input Go slice, up to its capacity (which will never be changed). The input Go slice is then truncated to the appropriate length.

An example in Core Foundation is the getObjects:andKeys:count method for NSDictionary:

	nst := ns.NSStringWithGoString
        dict := ns.NSDictionaryWithObjectsForKeys(
                ns.NSArrayWithObjects(nst("obj1"),nst("obj2")),
                ns.NSArrayWithObjects(nst("key1"),nst("key2")),
        )
        os,ks := make([]ns.Id,0,5), make([]ns.Id,0,5)  // length 0, capacity 5 slices
        dict.GetObjects(&os,&ks,5)
	// last parameter is the count, must be less than or equal to the input slice capacity
        fmt.Printf("Length of os is now %d\n",len(os)) // os and ks slices are now length = 2
        for i,k := range ks {
                fmt.Printf("-- %s -> %s\n",k.NSString(),os[i].NSString())
        }

NSWrap will never check the "count" parameter, so the user will always need to make sure it is less than or equal to the capacity of the input Go slices.

Using pointers to pointers is necessary in many Core Foundation situations where you need to get an error message out of a function or method. Here is an example using [NSString stringWithContentsOfURL...]:

        err := make([]ns.NSError,1)
        n1 = ns.NSStringWithContentsOfURLEncoding(ns.NSURLWithGoString("htttypo://example.com"),0,&err)
        fmt.Printf("err: %s\n",err[0].LocalizedDescription())
//err: The file couldn’t be opened because URL type htttypo isn’t supported.

Selectors

You can specify selectors using a Go string. The Selector() function returns a Go type SEL which corresponds to a pointer to struct objc_selector in C. Among other things, this lets you set actions on NSControls and NSMenuItems:

appMenu.AddItemWithTitle(
		ns.NSStringWithGoString("Quit"),
		ns.Selector("terminate:"),
		ns.NSStringWithGoString("q"))

Enumerators

NSWrap provides a ForIn method for the NSEnumerator type. Call it with a func(ns.Id) bool parameter that returns true to continue and false to stop the enumeration.

a := ns.NSArrayWithObjects(o1,o2,o3)
a.ObjectEnumerator().ForIn(func (o ns.Id) bool {
	switch {
	case o.IsKindOfClass(ns.NSStringClass()):
		fmt.Println(o.NSString().UTF8String())
		return true  // continue enumeration
	default:
		fmt.Println("Unknown class")
		return false  // terminate enumeration
	}
})

As seen above, you can do the usual Objective-C thing for runtime type identification.

Enum Definitions

NSWrap translates C enum values into Go constants. The enums you need are specified in nswrap.yaml by regular expression, which, in the case of named enums, must match the name of the enum itself, or in the case of anonymous enums, must match the name of the constant(s) you are looking for as declared within the enum. The generated constants receive Go types associated with their underlying C types, which are automatically declared by NSWrap as needed.

The following configuration:

# nswrap.yaml
inputfiles: [/System/Library/Frameworks/AppKit.framework/Headers/AppKit.h]
enums:
    - _CLOCK.*             # match constants in an anonymous enum
    - NSWindowOrdering.*   # match a named enum

results in:

//ns/main.go
...
type NSWindowOrderingMode C.enum_NSWindowOrderingMode
const NSWindowAbove NSWindowOrderingMode = C.NSWindowAbove
const NSWindowBelow NSWindowOrderingMode = C.NSWindowBelow
const NSWindowOut NSWindowOrderingMode = C.NSWindowOut

const _CLOCK_REALTIME  = C._CLOCK_REALTIME
const _CLOCK_MONOTONIC  = C._CLOCK_MONOTONIC
const _CLOCK_MONOTONIC_RAW  = C._CLOCK_MONOTONIC_RAW
...

Memory management

Objective-C objects are always allocated and returned from CGo code, and therefore these pointers are not garbage collected by Go. You can use any of the standard Objective-C memory management techniques for those pointers, which seem to work but have not been extensively tested.

Since everything inherits methods from NSObject, you can call Retain(), Release() and Autorelease() on any object.

If the autorelease configuration directive is set to "true", all allocation functions created by NSWrap (i.e. those ending in Alloc) will call autorelease before they return an object. Alternately, objects can be manually sent an autorelease message. If you are not working in an environment (such as an Application Delegate callback) that provides an autorelease pool, you can create your own:

• Work directly with NSAutoreleasePool objects

swamp := ns.NSAutoreleasePoolAlloc().Init()
del := ns.AppDelegateAlloc()
//del.Autorelease() // if autorelease: true is not set in nswrap.yaml
menu := ns.NSMenuAlloc().InitWithTitle(nst("Main"))
//menu.Autorelease()
str := ns.NSStringWithGoString("these objects will be automatically deallocated when swamp is drained.")
...
swamp.Drain()

• ...or use the AutoreleasePool() helper function

NSWrap provides a helper function that can be passed a func() with no parameters or return value. It is conventient to give it an anonymous function and write your code in line, just like you would if you were using an @autoreleasepool { } block.

ns.AutoreleasePool(func() {
	a := MyObjectAlloc().Init()
	b := MyOtherObjectAlloc().Init()
	...
})

You will need to make sure NSAutoreleasePool is included in the classes directive in your configuration file before working with NSAutoreleasePool objects or the AutoreleasePool helper function.

• Pitfalls

Go concurrency does not play well with Objective-C memory management. In particular, an AutoreleasePool needs to be allocated and drained from the same thread, and only objects allocated within that thread will be drained. Objects allocated and autoreleased from a different goroutine in the same thread are at risk of being prematurely drained. Therefore, you should only work with one AutoreleasePool at a time, and only within a thread that is locked to the OS thread (by calling runtime.LockOSThread()). If you will be allocating Objective-C objects from multiple goroutines, it is best not to use the autorelease: true directive as that will cause all objects to receive an autorelease message even if they are created outside the thread where are using your autorelease pool. See examples/memory for some basic tests and read the comments to larn what to avoid.

Delegates

The delegates directive in nswrap.yaml creates a new Objective-C class and associated Go wrapper functions. For example, the following configuration file creates a class called CBDelegate that implements the CBCentralManagerDelegate and CBPeripheralDelegate protocols from Core Bluetooth, along with the Go code you need to allocate and use instances of the new class.

# nswrap.yaml
inputfiles:
    - /System/Library/Frameworks/CoreBluetooth.framework/Headers/CoreBluetooth.h

classes:
    - CBCentralManager

delegates:
    CBDelegate:                                  # a name for your delegate class
        CBCentralManagerDelegate:                # a protocol to implement
            - centralManagerDidUpdateState       # messages you want to respond to
            - centralManagerDidDiscoverPeripheral
            - centralManagerDidConnectPeripheral
        CBPeripheralDelegate:                    # another protocol to implement
            - peripheralDidDiscoverServices
            - peripheralDidDiscoverCharacteristicsForService
            - peripheralDidUpdateValueForCharacteristic
...

The generated delegate inherits from NSObject and, in its interface declaration, is advertised as implementing the protocols specified in nswrap.yaml.

When a delegate is activated and one of the callback methods named in the configuration file is called, the delegate will call back into a Go function exported by NSWrap. If a user-defined callback function has been registered, it will be called with all of its parameters converted to their Go type equivalents. User-defined callbacks are registered by calling a function with the method name in TitleCase + Callback, so in the example above, you would call ns.CentralManagerDidUpdateStateCallback(...) with the name of your callback function to register to receive notifications when your central manager updates its state.

The example in examples/bluetooth implements a working Bluetooth Low-Energy heart rate monitor entirely in Go.

The following Go code instantiates a CBDelegate object, registers a callback for centralManagerDidUpdateState, allocates a CBCentralManager object, and installs our delegate:

func cb(c ns.CBCentralManager) {
	...
}

func main() {
	...
	del := ns.CBDelegateAlloc()
	del.CentralManagerDidUpdateStateCallback(cb)
	cm := ns.CBCentralManagerAlloc().InitWithDelegateQueue(del,queue)

When you provide user-defined callback functions, you will need to specify them with exactly the right type, matching NSWrap's generated Go wrapper types for the callback function and the Go types for all of its parameters. If go build fails, the error messages will point you in the right direction.

$ go build
./main.go:127:43: cannot use didFinishLaunching (type func(ns.NSNotification, bool)) as type
func(ns.NSNotification) in argument to del.ApplicationDidFinishLaunchingCallback

In the above example, the build failed because an extra bool parameter was included in the callback function. The compiler is telling you that the right type for the callback is func(ns.NSNotification) with no return value.

Working with AppKit

You can wrap the AppKit framework classes and create an NSApplication Delegate. This allows you to build a Cocoa application entirely in Go.

Because AppKit uses thread local storage, you will need to make sure all calls into it are done from the main OS thread. This can be a challenge in Go even though runtime.LockOSThread() is supposed to provide this functionality.

This is actually a full working Cocoa application:

# nswrap.yaml
inputfiles:
    - /System/Library/Frameworks/AppKit.framework/Headers/AppKit.h

classes:
    - NSApplication
    - NSWindow
    - NSString
    - NSMenu

enums:
    - NSApplication.*
    - NSBackingStore.*
    - NSWindowStyleMask.*

functions:
    - NSMakeRect

delegates:
    AppDelegate:
      NSApplicationDelegate:
        - applicationDidFinishLaunching
        - applicationShouldTerminateAfterLastWindowClosed
frameworks: [ Foundation, AppKit, CoreGraphics ]

//go:generate nswrap
package main

import (
	"fmt"
	"runtime"
	"git.wow.st/gmp/nswrap/examples/app/ns" // point to your own NSWrap output directory
)

func didFinishLaunching(n ns.NSNotification) {
	fmt.Println("Go: did finish launching!")
}

func shouldTerminate(s ns.NSApplication) ns.BOOL {
	return 1
}

func main() {
	runtime.LockOSThread()
	a := ns.NSApplicationSharedApplication()
	a.SetActivationPolicy(ns.NSApplicationActivationPolicyRegular)
	del := ns.AppDelegateAlloc()
	del.ApplicationDidFinishLaunchingCallback(didFinishLaunching)
	del.ApplicationShouldTerminateAfterLastWindowClosedCallback(shouldTerminate)
	a.SetDelegate(del)

	win := ns.NSWindowAlloc().InitWithContentRectStyleMask(
		ns.NSMakeRect(200,200,600,600),
		ns.NSWindowStyleMaskTitled | ns.NSWindowStyleMaskClosable,
		ns.NSBackingStoreBuffered,
		0,
	)
	win.SetTitle(ns.NSStringWithGoString("Hi World"))
	win.MakeKeyAndOrderFront(win)
	a.Run()
}

Pretty simple right? Not really, NSWrap just generated almost 15,000 lines of code. See examples/app for a slightly more complex example with working menus, visual format-based auto layout, and a custom button class.

Subclasses

NSWrap includes functionality to generate subclasses as specified in nswrap.yaml.

You can override existing methods or create new methods with any type signature you specify using Objective-C method signature syntax.

# nswrap.yaml
...
subclasses:
  myClass:                      # the name of the new class
    yourClass:                  # the superclass to inherit from
      - init.*                  # what methods to override
      - -(void)hi_there:(int)x  # Objective-C prototype of your new method(s)
#       |--this hyphen indicates that this is an instance method

In the example above, your new class will be named myClass in Objective-C and MyClass in Go. It will override any init methods found in yourClass (which must be defined in one of the header files included in the inputfiles directive of nswrap.yaml). In addition, because the second entry under yourClass starts with a -, it will be treated as a new instance method definition for myClass. The remainder of the line will be parsed as an Objective-C method prototype in order to determine the method name, its return type, and the names and types of its parameters if any.

Since multiple inheritance is not permitted in Objective-C, it is not possible to specify more than one superclass in a subclasses entry.

Go callbacks for overridden methods are passed a special struct as their first parameter. This struct is filled with superclass methods, which allows you to do things like this:

func methodCallback(super ns.MyClassSupermethods, param NSString) {
	...
	super.Method(param)
}

You can use subclasses to define new AppKit controls with configurable callbacks. For example, let's make an NSButton that calls back into Go when you press it:

# nswrap.yaml
...
subclasses:
    GButton:
        NSButton:
            - -(void)pressed
...

func pressed() {
	fmt.Println("Button pressed!")
}
...
func didFinishLaunching(n ns.NSNotification) {
	...
	button := ns.GButtonAlloc()
	button.Init()
	button.PressedCallback(pressed)		# register user-defined callback
	button.SetAction(ns.Selector("pressed"))
	button.SetTarget(button)
	button.SetTitle(ns.NSStringWithGoString("PUSH"))
	...
}

Later on you can add your new button to a view and tell Cocoa where to lay it out. It's all a little verbose, but that's because for some reason you decided to write Objective-C code in Go.

Limitations

Blocks

NSWrap does not support methods or functions that take C functions or blocks as parameters or return values.

Why?

Um, I was trying to make a nice modern Go binding for CoreBluetooth on MacOS and got carried away.

Acknowledgements

This work was inspired by Maxim's excellent c-for-go. Much of the infrastructure was lifted from Elliot Chance's equally excellent c2go. Kiyoshi Murata's post on coderwall.com was an essential piece of inspiration.

The combinatorial Objective-C type parsers are mine as are the Objective-C and Go code generators, so this is where you will find all of the bugs.