I'm receiving the error.
Cannot convert value of type '() -> ()' to type 'MIDICompletionProc'
(aka '#convention(c) (UnsafeMutablePointer) ->
()') in coercion.
Everything else looks OK but for the life of me I can't seem to compose the CompletionProc function call.
var mySysExSendRequest : MIDISysexSendRequest
let myCompProc = sysexCompletionProc (a separate static function)
mySysExSendRequest = MIDISysexSendRequest(destination: Dest,
data: dataptr,
bytesToSend: 16,
complete: complete,
reserved: res,
completionProc: sysexCompletionProc as MIDICompletionProc,
completionRefCon: nil)
MIDISendSysex(&mySysExSendRequest)
You're currently supplying a function that takes no arguments, i.e. () -> Void.
Your completion proc must have this signature:
(UnsafeMutablePointer<MIDISysexSendRequest>) -> Void
Also note that since the proc has to be compatible with #convention(c) semantics it cannot be a class method or a closure.
It may be possible to work around that restriction by passing a pointer to self in the completionRefCon field and then casting that back (using "unsafe" methods) within the callback.
Related
After reading:
https://github.com/rodionovd/SWRoute/wiki/Function-hooking-in-Swift
https://github.com/rodionovd/SWRoute/blob/master/SWRoute/rd_get_func_impl.c
I understood that Swift function pointer is wrapped by swift_func_wrapper and swift_func_object (according to the article in 2014).
I guess this still works in Swift 3, but I couldn't find which file in https://github.com/apple/swift best describes these structs.
Can anyone help me?
I believe these details are mainly part of the implementation of Swift's IRGen – I don't think you'll find any friendly structs in the source showing you the full structure of various Swift function values. Therefore if you want to do some digging into this, I would recommend examining the IR emitted by the compiler.
You can do this by running the command:
xcrun swiftc -emit-ir main.swift | xcrun swift-demangle > main.irgen
which will emit the IR (with demangled symbols) for a -Onone build. You can find the documentation for LLVM IR here.
The following is some interesting stuff that I've been able to learn from going through the IR myself in a Swift 3.1 build. Note that this is all subject to change in future Swift versions (at least until Swift is ABI stable). It goes without saying that the code examples given below are only for demonstration purposes; and shouldn't ever be used in actual production code.
Thick function values
At a very basic level, function values in Swift are simple things – they're defined in the IR as:
%swift.function = type { i8*, %swift.refcounted* }
which is the raw function pointer i8*, along with a pointer to its context %swift.refcounted*, where %swift.refcounted is defined as:
%swift.refcounted = type { %swift.type*, i32, i32 }
which is the structure of a simple reference-counted object, containing a pointer to the object's metadata, along with two 32 bit values.
These two 32 bit values are used for the reference count of the object. Together , they can either represent (as of Swift 4):
The strong and unowned reference count of the object + some flags, including whether the object uses native Swift reference counting (as opposed to Obj-C reference counting), and whether the object has a side table.
or
A pointer to a side table, which contains the above, plus the weak reference count of the object (on forming a weak reference to an object, if it doesn't already have a side table, one will be created).
For further reading on the internals of Swift reference counting, Mike Ash has a great blog post on the subject.
The context of a function usually adds extra values onto the end of this %swift.refcounted structure. These values are dynamic things that the function needs upon being called (such as any values that it has captured, or any parameters that it has been partially applied with). In quite a few cases, function values won't need a context, so the pointer to the context will simply be nil.
When the function comes to be called, Swift will simply pass in the context as the last parameter. If the function doesn't have a context parameter, the calling convention appears to allow it to be safely passed anyway.
The storing of the function pointer along with the context pointer is called a thick function value, and is how Swift usually stores function values of known type (as opposed to a thin function value which is just the function pointer).
So, this explains why MemoryLayout<(Int) -> Int>.size returns 16 bytes – because it's made up of two pointers (each being a word in length, i.e 8 bytes on a 64 bit platform).
When thick function values are passed into function parameters (where those parameters are of non-generic type), Swift appears to pass the raw function pointer and context as separate parameters.
Capturing values
When a closure captures a value, this value will be put into a heap-allocated box (although the value itself can get stack-promoted in the case of a non-escaping closure – see later section). This box will be available to the function through the context object (the relevant IR).
For a closure that just captures a single value, Swift just makes the box itself the context of the function (no need for extra indirection). So you'll have a function value which looks like a ThickFunction<Box<T>> from the following structures:
// The structure of a %swift.function.
struct ThickFunction<Context> {
// the raw function pointer
var ptr: UnsafeRawPointer
// the context of the function value – can be nil to indicate
// that the function has no context.
var context: UnsafePointer<Context>?
}
// The structure of a %swift.refcounted.
struct RefCounted {
// pointer to the metadata of the object
var type: UnsafeRawPointer
// the reference counting bits.
var refCountingA: UInt32
var refCountingB: UInt32
}
// The structure of a %swift.refcounted, with a value tacked onto the end.
// This is what captured values get wrapped in (on the heap).
struct Box<T> {
var ref: RefCounted
var value: T
}
In fact, we can actually verify this for ourselves by running the following:
// this wrapper is necessary so that the function doesn't get put through a reabstraction
// thunk when getting typed as a generic type T (such as with .initialize(to:))
struct VoidVoidFunction {
var f: () -> Void
}
func makeClosure() -> () -> Void {
var i = 5
return { i += 2 }
}
let f = VoidVoidFunction(f: makeClosure())
let ptr = UnsafeMutablePointer<VoidVoidFunction>.allocate(capacity: 1)
ptr.initialize(to: f)
let ctx = ptr.withMemoryRebound(to: ThickFunction<Box<Int>>.self, capacity: 1) {
$0.pointee.context! // force unwrap as we know the function has a context object.
}
print(ctx.pointee)
// Box<Int>(ref:
// RefCounted(type: 0x00000001002b86d0, refCountingA: 2, refCountingB: 2),
// value: 5
// )
f.f() // call the closure – increment the captured value.
print(ctx.pointee)
// Box<Int>(ref:
// RefCounted(type: 0x00000001002b86d0, refCountingA: 2, refCountingB: 2),
// value: 7
// )
ptr.deinitialize()
ptr.deallocate(capacity: 1)
We can see that by calling the function between printing out the value of the context object, we can observe the changing in value of the captured variable i.
For multiple captured values, we need extra indirection, as the boxes cannot be stored directly as the given function's context, and may be captured by other closures. This is done by adding pointers to the boxes to the end of a %swift.refcounted.
For example:
struct TwoCaptureContext<T, U> {
// reference counting header
var ref: RefCounted
// pointers to boxes with captured values...
var first: UnsafePointer<Box<T>>
var second: UnsafePointer<Box<U>>
}
func makeClosure() -> () -> Void {
var i = 5
var j = "foo"
return { i += 2; j += "b" }
}
let f = VoidVoidFunction(f: makeClosure())
let ptr = UnsafeMutablePointer<VoidVoidFunction>.allocate(capacity: 1)
ptr.initialize(to: f)
let ctx = ptr.withMemoryRebound(to:
ThickFunction<TwoCaptureContext<Int, String>>.self, capacity: 1) {
$0.pointee.context!.pointee
}
print(ctx.first.pointee.value, ctx.second.pointee.value) // 5 foo
f.f() // call the closure – mutate the captured values.
print(ctx.first.pointee.value, ctx.second.pointee.value) // 7 foob
ptr.deinitialize()
ptr.deallocate(capacity: 1)
Passing functions into parameters of generic type
You'll note that in the previous examples, we used a VoidVoidFunction wrapper for our function values. This is because otherwise, when being passed into a parameter of generic type (such as UnsafeMutablePointer's initialize(to:) method), Swift will put a function value through some reabstraction thunks in order to unify its calling convention to one where the arguments and return are passed by reference, rather than value (the relevant IR).
But now our function value has a pointer to the thunk, rather than the actual function we want to call. So how does the thunk know which function to call? The answer is simple – Swift puts the function that we want to the thunk to call in the context itself, which will therefore look like this:
// the context object for a reabstraction thunk – contains an actual function to call.
struct ReabstractionThunkContext<Context> {
// the standard reference counting header
var ref: RefCounted
// the thick function value for the thunk to call
var function: ThickFunction<Context>
}
The first thunk that we go through has 3 parameters:
A pointer to where the return value should be stored
A pointer to where the arguments for the function are located
The context object which contains the actual thick function value to call (such as shown above)
This first thunk just extracts the function value from the context, and then calls a second thunk, with 4 parameters:
A pointer to where the return value should be stored
A pointer to where the arguments for the function are located
The raw function pointer to call
The pointer to the context of the function to call
This thunk now retrieves the arguments (if any) from the argument pointer, then calls the given function pointer with these arguments, along with its context. It then stores the return value (if any) at the address of the return pointer.
Like in the previous examples, we can test this like so:
func makeClosure() -> () -> Void {
var i = 5
return { i += 2 }
}
func printSingleCapturedValue<T>(t: T) {
let ptr = UnsafeMutablePointer<T>.allocate(capacity: 1)
ptr.initialize(to: t)
let ctx = ptr.withMemoryRebound(to:
ThickFunction<ReabstractionThunkContext<Box<Int>>>.self, capacity: 1) {
// get the context from the thunk function value, which we can
// then get the actual function value from, and therefore the actual
// context object.
$0.pointee.context!.pointee.function.context!
}
// print out captured value in the context object
print(ctx.pointee.value)
ptr.deinitialize()
ptr.deallocate(capacity: 1)
}
let closure = makeClosure()
printSingleCapturedValue(t: closure) // 5
closure()
printSingleCapturedValue(t: closure) // 7
Escaping vs. non-escaping capture
When the compiler can determine that the capture of a given local variable doesn't escape the lifetime of the function it's declared in, it can optimise by promoting the value of that variable from the heap-allocated box to the stack (this is a guaranteed optimisation, and occurs in even -Onone). Then, the function's context object need only store a pointer to the given captured value on the stack, as it is guaranteed not to be needed after the function exits.
This can therefore be done when the closure(s) capturing the variable are known not to escape the lifetime of the function.
Generally, an escaping closure is one that either:
Is stored in a non-local variable (including being returned from the function).
Is captured by another escaping closure.
Is passed as an argument to a function where that parameter is either marked as #escaping, or is not of function type (note this includes composite types, such as optional function types).
So, the following are examples where the capture of a given variable can be considered not to escape the lifetime of the function:
// the parameter is non-escaping, as is of function type and is not marked #escaping.
func nonEscaping(_ f: () -> Void) {
f()
}
func bar() -> String {
var str = ""
// c doesn't escape the lifetime of bar().
let c = {
str += "c called; "
}
c();
// immediately-evaluated closure obviously doesn't escape.
{ str += "immediately-evaluated closure called; " }()
// closure passed to non-escaping function parameter, so doesn't escape.
nonEscaping {
str += "closure passed to non-escaping parameter called."
}
return str
}
In this example, because str is only ever captured by closures that are known not to escape the lifetime of the function bar(), the compiler can optimise by storing the value of str on the stack, with the context objects storing only a pointer to it (the relevant IR).
So, the context objects for each of the closures1 will look like Box<UnsafePointer<String>>, with pointers to the string value on the stack. Although unfortunately, in a Schrödinger-like manner, attempting to observe this by allocating and re-binding a pointer (like before) triggers the compiler to treat the given closure as escaping – so we're once again looking at a Box<String> for the context.
In order to deal with the disparity between context objects that hold pointer(s) to the captured values rather than holding the values in their own heap-allocated boxes – Swift creates specialised implementations of the closures that take pointers to the captured values as arguments.
Then, a thunk is created for each closure that simply takes in a given context object, extracts the pointer(s) to the captured values from it, and passes this onto the specialised implementation of the closure. Now, we can just have a pointer to this thunk along with our context object as the thick function value.
For multiple captured values that don't escape, the additional pointers are simply added onto the end of the box, i.e
struct TwoNonEscapingCaptureContext<T, U> {
// reference counting header
var ref: RefCounted
// pointers to captured values (on the stack)...
var first: UnsafePointer<T>
var second: UnsafePointer<U>
}
This optimisation of promoting the captured values from the heap to the stack can be especially beneficial in this case, as we're no longer having to allocate separate boxes for each value – such as was the case previously.
Furthermore it's worth noting that lots of cases with non-escaping closure capture can be optimised much more aggressively in -O builds with inlining, which can result in context objects being optimised away entirely.
1. Immediately-evaluated closures actually don't use a context object, the pointer(s) to the captured values are just passed directly to it upon calling.
I have the following function in Swift 3
func fetchOrders(_ completionHandler: (_ orders: [Order]) -> Void)
{
ordersStore.fetchOrders { (orders: () throws -> [Order]) -> Void in
do {
let orders = try orders()
completionHandler(orders)
} catch {
completionHandler([])
}
}
}
What does _ completionHandler argument in fetchOrders mean?
What does (orders: () throws -> [Order]) mean?
PS : I am new to iOS and Swift
There's quite a lot in here, so we'll break it down one piece at a time:
func fetchOrders(_ completionHandler: (_ orders: [Order]) -> Void)
This is a function called fetchOrders.
It has one parameter (completionHandler) and returns nothing.
The first _ indicates that there is no "external name" of the first parameter. That is, you do not have to label it (in fact, you cannot). (For subtle reasons that don't really matter here, I believe the author made a mistake using _ there, and I would not have done that.)
The completionHandler is the "internal name," what the parameter is called inside the function.
The type of completionHandler is (_ orders: [Order]) -> Void. We'll break that down now.
This value is a closure that takes an [Order] (array of Order) and returns Void. Informally this means "returns nothing" but literally means it returns the empty tuple ().
The _ orders: syntax is in practice a comment. In principle the _ is an external name (but that's the only legal external name for a closure), and orders is an internal name, but in reality, closures parameters do not have names in any meaningful way, so this is purely informational.
I believe this is a poor use of the closure parameter commenting system. Since orders tells us nothing more than [Order], I would have omitted it, and made the type just ([Order]) -> Void.
Now we'll turn to the next line:
ordersStore.fetchOrders { (orders: () throws -> [Order]) -> Void in
This calls the fetchOrders method on ordersStore. We can tell from this code that fetchOrders takes a closure parameter. This is called "trailing closure" syntax in Swift, and is why I would not have used the _ for our closure. With trailing closure syntax, the external name of the parameter is not needed.
The author has provided type information here that probably wasn't necessary, but we can explore it anyway. This could likely have been written as just { orders in, but then the reader would probably have been surprised by this somewhat unusual code.
We have been passed a closure called orders that takes nothing and returns [Order] or throws an error. Basically this is a way to say that fetchOrders might fail.
The author is working around an awkwardness in Swift's throws system, which does not have a natural way to express an asynchronous action that might fail. This is one way to fix it; you pass a throwing (i.e. a possibly failing) function. I don't favor this approach, I favor using a Result enum for this case because I think it scales better and avoids possible unintended side effects, but that's a debatable point (and the Swift community hasn't really decided how to deal with this common problem).
This all leads us to:
do {
let orders = try orders()
completionHandler(orders)
} catch {
completionHandler([])
}
This is where the orders closure is evaluated. (This is very important; if orders has side effects, this is when they occur, which may be on a different queue than was intended. That's one reason I don't favor this pattern.) If the closure succeeds, we return its result, otherwise we return [] in the catch below.
In this particular case, the throws approach is slightly silly, because it's silently flattened into [] without even a log message. If we don't care about the errors, then failure should have just returned [] to start with and not messed with throws. But it's possible that other callers do check the errors.
In either case, we call the completionHandler closure with our result, chaining this back to our original caller.
This do/catch block could have been more simply written as:
let completedOrders = try? orders() ?? []
completionHandler(completedOrders)
This makes it clearer that we're ignoring errors by turning it into an optional, and avoids code duplication of the call to completionHandler.
(I just add the extra let binding to make the code a little easier to read; it isn't needed.)
The completionHandler argument means that the expected parameter (named completionHandler) must be a function that takes a list of Order objects and does not return any value.
completionHandler is the a variable name. In this specific example, this variable is a callback. You know is a callback function because (orders: [Order]) -> Void is it's data type; in this particular case, said data type is a function that receives an array of Order objects in a variable _orders and doesn't have a return value (the Void part).
TL;DR:
it's the variable name, of type:
function which receives an array of Order as a parameter and acts as a callback.
I am building an app using Swift for the first time and using the AlamoFire library. I've built apps using Obj-C and AFNetworking before, but I'm having a hard time groking this Swift response method:
Alamofire.request(.GET, "http://someapi.com/thing.json")
.responseJSON { _, _, JSON, _ in
println(JSON)
}
The actual method definition is:
public func responseJSON(options: NSJSONReadingOptions = .AllowFragments, completionHandler: (NSURLRequest, NSHTTPURLResponse?, AnyObject?, NSError?) -> Void) -> Self {
return response(serializer: Request.JSONResponseSerializer(options: options), completionHandler: { request, response, JSON, error in
completionHandler(request, response, JSON, error)
})
}
I don't really understand what's going on here when I use this response method.
Why am I not using parens in the method call?
Am I just passing a block or anonymous function into this method?
What is the significance of passing underscores (_)?
What is the in keyword doing?
It's all here:
https://developer.apple.com/library/ios/documentation/Swift/Conceptual/Swift_Programming_Language/Closures.html#//apple_ref/doc/uid/TP40014097-CH11-ID102
No parens: that's trailing closure syntax. Idea is that closures can get long, hard to remember/determine long-distance paren pair.
Yes, block = anonymous function = (anonymous) closure. Yes, you're passing that as the only parameter. Since it's omitted, 'options' gets its default value of '.AllowFragments'. Trailing closure { ... } gets bound to completionHandler parameter.
The '_' is the 'don't care about this parameter' syntax ... e.g. if the function doesn't use the parameter, no point to giving it a local name.
in is part of closure syntax: indicates start of function body. Definitely read the whole closure syntax chapter above. It's designed to be extremely terse, nothing intuitive about it.
I just have a quick conceptual question about this closure here:
func getRandomUser(onCompletion: (JSON) -> Void) {
let route = baseURL
makeHTTPGetRequest(route, onCompletion: { json, err in
onCompletion(json)
})
}
What does the line onCompletion(json) do exactly? Is this a recursive call to onCompletion?
Let's look at your function line-by-line:
1. func getRandomUser(onCompletion: (JSON) -> Void) {
2. let route = baseURL
3. makeHTTPGetRequest(route, onCompletion: { json, err in
4. onCompletion(json)
5. })
6. }
Line 1: this gives the signature of your function; it takes one argument, and that argument is of type (JSON) -> Void, which means the argument it accepts is a closure that itself takes one argument of type JSON and does not have a return value, i.e. "returns Void", -> Void or -> (); note that the function definition also includes a local parameter name for that argument: within the function body, that closure is assigned to the constant onCompletion
Line 2: constant assignment...
Line 3: this calls the function makeHTTPGetRequest(_:onCompletion:), which takes two arguments: a route (which does not use an external label in the call, hence the _ in the function name as given previously), and a closure - this closure is of type (JSON, NSError?) -> Void; note that on this line where onCompletion occurs, this is the external label required for the second argument when calling the makeHTTPGetRequest(_:onCompletion:) function, there is not any assignment taking place (as occurred on Line 1)
Line 4: the closure assigned to the constant onCompletion is called with one argument...
So while it may be confusing that the text onCompletion occurs twice in this section of code (before the closure is actually invoked on Line 4), there is only one constant that goes by that name - the other occurrence is simply an external label for an argument.
If it helps at all, we can actually eliminate the use of the onCompletion label altogether by taking advantage of Swift's "trailing closure syntax" and rewrite it like so:
func getRandomUser(onCompletion: (JSON) -> Void) {
let route = baseURL
makeHTTPGetRequest(route) { json, err in onCompletion(json) }
}
...but that's a topic for another post :)
The onCompletion parameter of makeHTTPGetRequest is not available in its own scope. It's basically declaring what onCompletion is. It would then be calling onCompletion on getRandomUser.
According to the author of this page this is declaring a dictionary of dictionaries in swift:
var routeMap : Dictionary<String, Dictionary<String, () -> ()>> = [:]
can you explain what are this doing, specially the part with these hieroglyphs () -> ()>> = [:] at the end?
It appears that the guy is joining a series of commands together. If this is the case, if you can unfold that code into several lines I appreciate.
thanks.
Let's start at the way end.
[:]
This is simply initializing an empty Dictionary. It is very similar to calling "" to initialize an empty String, or [] to initialize an empty Array.
Now, let's jump to the type declaration.
Dictionary<String, Dictionary<String, () -> ()>>
This is a dictionary that maps Strings to Dictionaries. Let's look closer at the type of those inner dictionaries.
Dictionary<String, () -> ()>
This maps a String to a closure. A closure is pretty much just a block from Objective C. That's what the () -> () means. Let's dive deeper.
() -> ()
This is the syntax for declaring a closure. The left value are the parameters. The right are the return types. In this case, we have one parameter, and one return type.
()
This means void in Swift. In fact, in Swift.h, we can see this on line 3953:
typealias Void = ()
So ultimately, we have a closure that gives no (void) parameters, and has no (void) return value.
Some more examples of closures might help understand the syntax. Let's imagine a closure that takes a String and converts it to an int. The type would look like this:
let stringToInt: (String) -> (Int) = ...
Now, one with a void input. Let's have a random number generator:
let randomDouble: () -> (Double) = ...
This takes no inputs, and returns a Double.
Finally, let's have a void to void.
let printHelloWorld: () -> () = ...
You can see this takes no arguments and returns nothing. It's more of a method than a function, but it can still do stuff, like modify properties, or in this case, print to the console.