# 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. ```sh 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: ```sh 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. ```yaml # 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. Since the `NSObject` class is necessary for memory management, NSWrap will automatically include it if it is encountered in an input header file. 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. ```go 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: ```objective-c - compare: - compare:options: - compare:options:range: - compare:options:range:locale: ``` These are translated into Go as: ```go 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. ```go 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. When NSWrap binds a method that returns `*Char` (and is in garbage collected mode, the default), it first calls `strdup` on the output of the underlying Objective-C method. Therefore, the returned pointer is manually allocated and will need to be freed later from Go. NSWrap creates a `(*Char).Free()` method for use when these pointers are no longer needed. This copying is necessary because the Objective-C runtime will sometimes return pointers to internal objects that are impossible to manage from the Go side. NSWrap aims to cause any internal objects to be deallocated as soon as possible so they do not cause memory leaks. This means that any returned C strings need to be copied and memory managed manually from the Go side. 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 `String()` methods so that the `*Char` and `*NSString` types implement the `Stringer` Go interface and therefore can be sent directly to functions like `fmt.Printf`. The `String()` method on `*NSString` creates a temporary `*Char` internally but frees it for you before returning. Since methods returning `*Char` return a pointer that needs to be manually freed, it is important to use these properly in order to avoid leaks: ```go nst := ns.NSStringWithGoString("one string") // NO: the next line leaks a *Char (UTF8String) //mygostring := nst.UTF8String().String() // OK: NSWrap creates a temporary *Char and frees it for you: mygostring := nst.String() // ALSO OK: manually free your own temporary *Char: mytmpchar := nst.UTF8String() mygostring = mytmpchar.String() mytmpchar.Free() ``` In most cases it will be more convenient to convert directly to Go strings instead of `*Char`. ## Working With NSObject and its Descendants Objective-C objects are represented in Go by a type and an interface as follows: ```go type Id struct { ptr unsafe.Pointer } func (o *Id) Ptr() unsafe.Pointer { if o == nil { return nil }; return o.ptr } type NSObject interface { Ptr() unsafe.Pointer } ``` Other object types in Go are structs that directly or indirectly embed `Id` and therefore contain an `unsafe.Pointer` to an Objective-C object, and implement `NSObject` by inheriting the `Ptr()` method. Because of this implementation, you will note that every Objective-C object is represented by at least two pointers -- an underlying pointer to the Objective-C object in CGo memory (allocated by the Objective-C runtime), as well as a pointer allocated by the Go runtime to an `Id` type, or to another type that directly or indirectly embeds `Id`. This "dual pointer" approach is necessary to ensure that memory management can be made to work correctly (see below for details). * 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: ```go 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 allows single inheritance. NSWrap automatically adds inherited methods to classes that are includled in your binding. Types created by NSWrap also "embed" their parent class. For example, 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 direct superclass. For example: ```go type NSArray struct { Id } func (o *NSArray) Ptr() unsafe.Pointer { if o == nil { return nil }; return o.ptr } func (o *Id) NSArray() *NSArray { return (*NSArray)(unsafe.Pointer(o)) } type NSMutableArray struct { NSArray } func (o *NSMutableArray) Ptr() unsafe.Pointer { if o == nil { return nil }; return o.ptr } func (o *Id) NSMutableArray() *NSMutableArray { return (*NSMutableArray)(unsafe.Pointer(o)) } ``` Observe: ```go b := ns.NSButtonAlloc() // NSButton > NSControl > NSView > NSResponder > NSObject b.InitWithFrame(ns.NSMakeRect(100,100,200,200)) b.SetTitle(nst("PUSH")) 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, the `initWithFrame` method is defined in AppKit for `NSView`. NSWrap will find this method and add it to the Go `NSButton` type when creating your wrapper because `NSButton` inherits from `NSControl` which inherits from `NSView`. As of this writing, on MacOS 10.13.6, NSWrap binds 115 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. All objects implement the `NSObject` interface, but from time to time you will encounter a method that takes a parameter of a different type that may not exactly match the type you have. For example, if you want to pass your `NSButton` as a parameter to a method that accepts an `NSView` type, you need to explicitly pass its embedded `NSView` (`&b.NSView` in the example above). This approach is safer than "converting" the button to an `NSView` (see below) because it will only work on objects that directly or indirectly embed an `NSView` Go type. NSWrap creates a method for `Id` allowing objects to be converted at run-time to any other class. You will need this for Enumerators and functions like `NSArray`'s `GetObjects`, for example, which always return `*Id`. Make sure you know (or test) what type your objects are before converting them. You can implement a version of a Go type switch this way: ```go switch { case o.IsKindOfClass(ns.NSStringClass()): // do something with o.NSString() case o.IsKindOfClass(ns.NSSetClass()): // do something with o.NSSet() default: ... } ``` 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. ```go 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: ```sh *** 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 an 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. If there is no output, the length will be set to 0. An example in Core Foundation is the `getObjects:andKeys:count` method for `NSDictionary`: ```go nst := ns.NSStringWithGoString dict := ns.NSDictionaryWithObjectsForKeys( ns.NSArrayWithObjects(nst("obj1"),nst("obj2")), ns.NSArrayWithObjects(nst("key1"),nst("key2")), ) va,ka := make([]*ns.Id,0,5), make([]*ns.Id,0,5) // length 0, capacity 5 slices dict.GetObjectsAndKeysCount(&va,&ka,5) // last parameter is the count, must be less than or equal to the input slice capacity fmt.Printf("Length of va is now %d\n",len(va)) // va and ka slices are now length = 2 for i,k := range ka { fmt.Printf("-- %s -> %s\n",k.NSString(),va[i].NSString()) } ``` NSWrap will not 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, or in other cases where an Objective-C method wants to provide multiple return values. Here is an example using `[NSString stringWithContentsOfURL...]`: ```go err := make([]*ns.NSError,1) n1 = ns.NSStringWithContentsOfURLEncoding(ns.NSURLWithGoString("htttypo://example.com"), 0, &err) if len(err) > 0 { 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`: ```go 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. ```go a := ns.NSArrayWithObjects(o1,o2,o3) i := 0 a.ObjectEnumerator().ForIn(func (o *ns.Id) bool { switch { case o.IsKindOfClass(ns.NSStringClass()): fmt.Printf("%d: %s\n", i, o.NSString()) i++ 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 want 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: ```yaml # 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: ```go //ns/main.go ... const NSWindowAbove NSInteger = C.NSWindowAbove const NSWindowBelow NSInteger = C.NSWindowBelow const NSWindowOut NSInteger = C.NSWindowOut const _CLOCK_REALTIME = C._CLOCK_REALTIME const _CLOCK_MONOTONIC = C._CLOCK_MONOTONIC const _CLOCK_MONOTONIC_RAW = C._CLOCK_MONOTONIC_RAW ... ``` ## 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. ```yaml # 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, if your delegate was named `del`, you would call `del.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: ```go func cb(self ns.CBDelegate, c *ns.CBCentralManager) { ... } var ( del *ns.CBDelegate // use global variables so these don't get garbage collected cm *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.CBDelegate, *ns.NSNotification, bool)) as type func(ns.CBDelegate, *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 and you will want to make use of `runtime.LockOSThread()`. This is actually a full working Cocoa application: ```yaml # 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 //go:generate nswrap package main //go:generate nswrap package main import ( "fmt" "runtime" "ns" // point to your own NSWrap output directory ) func didFinishLaunching(self ns.AppDelegate, n *ns.NSNotification) { fmt.Println("Go: did finish launching!") } func shouldTerminate(self ns.AppDelegate, s *ns.NSApplication) ns.BOOL { return true } var ( a *ns.NSApplication // global vars so these are not garbage collected del *ns.AppDelegate win *ns.NSWindow ) func main() { runtime.LockOSThread() a = ns.NSApplicationSharedApplication() a.SetActivationPolicy(ns.NSApplicationActivationPolicy(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.NSWindowStyleMask(ns.NSWindowStyleMaskTitled | ns.NSWindowStyleMaskClosable), ns.NSBackingStoreType(ns.NSBackingStoreBuffered), false, ) win.SetTitle(ns.NSStringWithGoString("Hi World")) win.MakeKeyAndOrderFront(win) a.Run() } ``` Pretty simple right? Not really, NSWrap just generated 114,000 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 ### NOTE: SUBCLASS FUNCTIONALITY IS CURRENTLY PARTIALLY BROKEN 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. ```yaml # 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) # \--the initial 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 filled with superclass methods, which allows you to do things like this: ```go func methodCallback(self ns.MyClass, 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: ```yaml # nswrap.yaml ... subclasses: GButton: NSButton: - -(void)pressed ... ``` ```go func pressed(self ns.GButton, super ns.GButtonSupermethods) { 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. ## Memory management As mentioned above, NSWrap is designed for there to be at least one Go pointer associated with each underlying Objective-C object pointer. Since Objective-C memory is always allocated by the Objective-C runtime, it is not possible for the Go runtime to have visibility into these memory regions or to directly manage memory used by the CGo code. However, Go will keep track of the associated Go pointer that was created the first time the corresponding Objective-C object was passed over to the Go side and an `Id` or other NSWrap struct type was allocated. Because of this, it is possible to hook into the Go garbage collection system in an attempt to manage Objective-C memory strictly from the Go side. When there are no remaining Go pointers to an NSWrap `Id` struct, it will be deallocated by the Go garbage collector and a finalizer will be called that `release`es the corresponding Objective-C object. The memory management rules work as follows: * Objects in Go are represented by pointers to types that implement the `NSObject` interface * NSObject has one method, `Ptr()`, which returns an `unsafe.Pointer` to an Objective-C object. * All methods that return objects to Go call `retain` except for `new`, `init`, `alloc`, `copy` and `mutableCopy`, which already return retained objects from the Objective-C runtime. * Go wrappers for Objective-C methods call `runtime.SetFinalizer()`, which calls `release` when the associated Go struct is garbage collected. * All Objective-C methods are run inside an `@autoreleasepool {}` block to prevent internal memory leaks within the Objective-C libraries and frameworks. * Objects sent to you in callback functions are not memory managed by Go and must be manually managed using `Retain()` and `Release()` methods if you need to take ownership of them. A rule of thumb is that if you assign such an object to a persistent Go variable for use outside of the callback, call `Retain()`. Because of the linkage with the Go garbage collector described above, there should be no need for any memory management code to be written from the Go side, except in the case mentioned above where your Go delegate receives objects that need to be kept around outside of the callback. Since everything in Objective C inherits methods from `NSObject`, you can call `Retain()`, `Release()` and `Autorelease()` on any object. You can technically bind the `NSAutoreleasePool` class and create and drain instances of it from the Go side, but this is not recommended in the default, garbage collected mode and can run into problems because the Go runtime is inherently multithreaded. See `examples/memory` for an example of manual memory management, which should be possible to do reliably but I'm not sure why you would go through the trouble. NSWrap is doing a number of things behind the scenes to make garbage collection work. As mentioned, all Objective-C methods are called within an `@autorelease {}` block. This is necessary because some foundation classes (notably `NSString`) create internal objects that are `autoreleased` but never returned to the caller. These objects can never be deallocated unless the method in question was called within an autorelease pool. NSWrap assumes you are going to take ownership of every Objective-C object returned by a method, either directly as a return value or through a pointer to a pointer given as a parameter. Therefore, NSWrap calls `retain` on all of these objects before going back to the Go side, unless the object is either `nil` or equivalent to the input object. NSWrap also will not call `retain` on the return values of `init`, `new`, `copy`, `mutableCopy` or `alloc` methods. If you do not want ownership of the object, simply assign it to a local varable and the garbage collector will take care of releasing it. In order for this to work on a pointer to a pointer parameter, NSWrap treats the input parameter as an array with a length specified by either a `range` parameter (of type `NSRange`) or a `count` parameter of an integer type. If there is neither a `range` or `count` parameter, NSWrap assumes the array is length 1. As an example, in Objective-C, if you were to take an object out of an `NSArray` and the array was later deallocated, there is no guarantee that the object you obtained is still around unless you called `retain` on it. This is not necessary with NSWrap, which automatically retains objects returned by methods like `objectAtIndex:` and `getObjects:range` and manages them with the Go garbage collector. The methods described above work for methods that return Objective-C objects, which can be `retain`ed, but not with methods that return other types of pointers such as C strings. NSWrap has a special case for C strings (`*Char` in Go), calling `strdup` on the return value within the `@autoreleasepool` block. This ensures that the string is preserved even if it points to a termporary autoreleased object. Since this behavior results in newly allocated memory, these pointers will need to be freed from Go later on. Since these are pointers to C memory, it is not possible to set a finalizer on these pointers for garbage collection by Go. Note that the Go garbage collector is lazy and will not activate unless your application is running low on heap space. That means in practice that Objective-C objects are going to stick around a lot longer than they might in a pure Objective-C application. If this is an issue, simply run the Go GC manually with `runtime.GC()`. # Limitations ## Blocks and Function Pointers 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 Kupriianov's excellent [c-for-go](https://github.com/xlab/c-for-go). Much of the infrastructure was lifted from Elliot Chance's equally excellent [c2go](https://github.com/elliotchance/c2go). Kiyoshi Murata's post on [coderwall.com](https://coderwall.com/p/l9jr5a/accessing-cocoa-objective-c-from-go-with-cgo) 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.