I am porting some AppDelegate code for a plugin Objective-C to Swift.
My Objective-C is pretty good but I am stumped at understanding whats going on here. This is the code I am stuck on porting:
void (^safeHandler)(UIBackgroundFetchResult) = ^(UIBackgroundFetchResult result){
dispatch_async(dispatch_get_main_queue(), ^{
completionHandler(result);
});
};
I understand that the the ^ is for code blocks, but looking at this as a whole I am lost as to what is going on and how to port this to Swift.
I don't understand how there are two bracketed portions here:
void (^safeHandler)(UIBackgroundFetchResult)
If you can advise what that syntax is and how to port to Swift I would greatly appreciate it!
It's a block, which is a closure in Swift. This block is named safeHandler and takes a UIBackgroundFetchResult parameter and returns void.
This is the equivalent type:
let safeHandler: (UIBackgroundFetchResult) -> ()
And the whole thing would be
let safeHandler: (UIBackgroundFetchResult) -> () = { result in
dispatch_async(dispatch_get_main_queue()) {
completionHandler(result)
}
}
Note: block syntax is notoriously wonky. You can use this site to see the various block syntax forms: http://goshdarnblocksyntax.com/
Related
I have a completion handler in a framework written in Objective C...
This is a typedef for a block type. It takes an array of PHErrors.
typedef void (^PHBridgeSendErrorArrayCompletionHandler)(NSArray *errors);
When I try to use this in Swift, I'm doing....
anObject.aMethod(completionHandler: { (errors: [ AnyObject?]) -> () in
...rest of code
}
But I keep getting this error:
Cannot convert value of type '([AnyObject?]) -> ()' to expected argument type 'PHBridgeSendErrorArrayCompletionHandler!'
Can anyone help, I'm baffled, it looks like it should work to me.
Or better yet, you can still use your typedef as typealias.
DEFINE
typealias PHBridgeSendErrorArrayCompletionHandler = (_ errors: [Error]?) -> Void
IMPLEMENTATION
func myFunctionWithErrorCompletion(completion: PHBridgeSendErrorArrayCompletionHandler) {
// Define empty array to add errors to
var errors:[Error]?
// Do Your Logic that may store errors to array
// Completion and pass errors
completion(errors)
}
USAGE
func anotherOfMyFunctions() {
// Call the function
myFunctionWithErrorCompletion { (errors) in
if let completionErrors = errors {
// React to errors
}
}
}
anObject.aMethod(completionHandler: { (errors: [ AnyObject?]) -> () in
}
should be
anObject.aMethod() { errors in
}
In order to dig any deeper, I have to know what PHBridgeSendErrorArrayCompletionHandler is
So my friend solved this problem by simply changing AnyObject to Any
(errors: [Any]?) in
Which baffles me because all objects in an NSArray are objects! So didn't think to try Any.
Im pretty new to Swift mind
Try this..
anObject.aMethod(completionHandler: { (errors:NSArray?) -> () in
...rest of code
}
I'm using Swift to make a game in SpriteKit.
In Objective-C I could use the following method:
(void)enumerateChildNodesWithName:(NSString *)name usingBlock:(void (^)(SKNode *node, BOOL *stop))block
to perform actions on that *node, but I can't get this function working in Swift. Basically, I don't know how to reference that node in Swift.
This is the code I'm using, but I'm having trouble with the "usingBlock:" part. I've tried many things for many hours, but have not succeeded. Help please!
func spawnEnemy() -> () {
let enemy = SKSpriteNode(imageNamed: "enemy")
enemy.name = "enemy"
enemy.position = CGPointMake(100, 100)
self.addChild(enemy)
}
func checkCollisions() -> () {
self.enumerateChildNodesWithName("enemy", usingBlock: ((SKNode!, CMutablePointer<ObjCBool>) -> Void)?)
}
For now, don't trust autocomplete to insert the code you need — it drops in signatures from the "header", but a block signature is not the same as the declaration you need when inserting your own closure for a block parameter.
The formal way to write a closure would be to replicate the signature inside braces, adding local parameter names and using the in keyword to mark the start of the closure body:
self.enumerateChildNodesWithName("enemy", usingBlock: {
(node: SKNode!, stop: UnsafeMutablePointer <ObjCBool>) -> Void in
// do something with node or stop
})
But Swift's type inference means you don't have to write that much. Instead, you can just name the parameters, because their type (as well as the closure's return type) is known:
self.enumerateChildNodesWithName("enemy", usingBlock: {
node, stop in
// do something with node or stop
})
You can also use trailing closure syntax:
self.enumerateChildNodesWithName("enemy") {
node, stop in
// do something with node or stop
}
(You can even drop the local parameter names and refer to parameters by position — e.g. $0 for node — but here isn't a great place to do that because it makes your code far less readable. It's best to reserve $0 and friends for closures where it's blindingly obvious what the parameters are, like the closures you use with map and sort.)
See Closures in The Swift Programming Language for further explanation.
Also, because stop is an UnsafeMutablePointer, the syntax for using it is a bit different than in ObjC: set stop.memory = true to break out of enumeration.
Does anybody know how handlers (blocks) work in swift? I am trying to get this code running but i can't find any documentation of the right syntax for the completionHandler.
let url:NSURL = NSURL(string:"some url")
let request:NSURLRequest = NSURLRequest(URL:url)
let queue:NSOperationQueue = NSOperationQueue()
NSURLConnection.sendAsynchronousRequest(request:request, queue:queue, completionHandler handler:((NSURLResponse!, NSData!, NSError!) -> Void)!)
Like this:
NSURLConnection.sendAsynchronousRequest(request, queue: queue, completionHandler:{ response, data, error in /* Your code */ })
Or more verbose variant.
NSURLConnection.sendAsynchronousRequest(request, queue: queue, completionHandler:{ (response: NSURLResponse!, data: NSData!, error: NSError!) -> Void in
/* Your code */
})
You need to use this code:
NSURLConnection.sendAsynchronousRequest(request,queue:queue,completionHandler:{response,data,error in /* code goes here */ })
For more info, you can refer to this tutorial, or or check the answers to How to parse a JSON file in swift?.
sendAsynchronousRequest has been deprecated in newer versions of Swift. Move to dataTaskWithRequest, luckily it is used pretty much the same way
let request:NSURLRequest = NSURLRequest(URL:NSURL(string:"http://YOUR_DESIRED_URL.com")!)
let config = NSURLSessionConfiguration.defaultSessionConfiguration()
let session = NSURLSession(configuration: config)
let task = session.dataTaskWithRequest(request, completionHandler: {(data, response, error) in
});
task.resume()
The right term you are looking for here is Closure. Closures in Swift are similar to blocks in C and Objective-C. In addition to Tomáš's answer there is another short version to use the completion handler here:
NSURLConnection.sendAsynchronousRequest(request, queue: queue, completionHandler: {$0; $1; $2})
Here I have used Shorthand Argument Names. I am accessing response as $0, data as $1 and error as $3. I find this syntax more easy to read and write unless the parameters are large in number otherwise the code will become unreadable.
I've run into a situation while using a library called TransitionKit (helps you write state machines) where I am want to supply entry and exit actions in the form of a callback.
Sadly, the callbacks include two completely useless parameters. A typical block has to look like this:
^void (TKState *state, TKStateMachine *stateMachine) {
// I TOTALLY don't want parameters `state` or `stateMachine` used here
};
(this is an anonymous code block. Read up on blocks here if you're unclear)
As I've noted in the comment, I really don't want those parameters even mentioned in the body there. I've tried simply removing the parameter names like suggested in this question like so:
^void (TKState *, TKStateMachine *) {
// I foobar all I like here
};
but sadly the code won't compile then :(.
How can I enforce this non-usage of parameters in code?
This is what I could come up with. Quite a hack and relies on the GCC poison pragma, which is not standard but a GNU extension - although, given that you are probably compiling this with clang anyway, it should not be a problem.
#define _state state
#define _stateMachine stateMachine
#pragma GCC poison state stateMachine
Then this compiles:
^(TKState *_state, TKStateMachine *_stateMachine) {
do_something();
}
But this doesn't:
^(TKState *_state, TKStateMachine *_stateMachine) {
do_something(state, stateMachine);
}
You could just have a function that took one kind of block, and returned another, like this:
#class TKState, TKStateMachine; // here so this will compile
typedef void (^LongStateBlock)(TKState *state, TKStateMachine *stateMachine);
static inline LongStateBlock Adapter(void(^block)()) {
void(^heapBlock)() = [block copy]; // forces block to be on heap rather than stack, a one-time expense
LongStateBlock longBlock = ^(TKState *s __unused, TKStateMachine *sm __unused) {
heapBlock();
};
// this is the non-ARC, MRR version; I'll leave ARC for the interested observer
[heapBlock release];
return [[longBlock copy] autorelease];
}
And in practice:
// this represents a library method
- (void)takesLongStateBlock:(LongStateBlock)longBlock
{
// which hopefully wouldn't look exactly like this
if (longBlock) longBlock(nil, nil);
}
- (void)yourRandomMethod
{
[self takesLongStateBlock:^(TKState *state, TKStateMachine *stateMachine) {
NSLog(#"Gratuitous parameters, AAAAHHHH!");
}];
[self takesLongStateBlock:Adapter(^{
NSLog(#"So, so clean.");
})];
}
The whole thing is gisted, and should compile inside any class. It does what you expect when you call -yourRandomMethod.
AFAIK there is no way to do what you want when you are creating a block, you can only miss the parameter names when you are declaring a block variable(a reference to a block, to avoid misunderstandings)
So here you can miss the param names:
void (^myBlock)(SomeClass *);
But not when you create a block:
myBlock = ^(SomeClass *o)
{
};
I'd write
^void (TKState *unused_state, TKStateMachine *unused_stateMachine) {
// Anyone using unused_state or unused_stateMachine gets what they deserve.
};
Of course someone can use the parameters. But then whatever you do, they can change the code. If someone is intent on shooting themselves in the foot, there is no stopping them.
I'm putting together a Mac OS X Application and I'm trying to register to receive Display Reconfiguration notices, but I'm very lost right now. I've been reading Apple's documentation and some forums posts, etc., but everything seems to assume a better knowledge of things than I apparently possess. I understand that I have to request the callback inside a run loop for it to work properly. I don't know how to set up a basic run loop for it, though. I also feel like the example Apple has in their documentation is missing stuff they are expecting me to already know. To display my ignorance here is what I feel like things should look like.
NSRunLoop *rLoop = [NSRunLoop currentRunLoop];
codeToStartRunLoop
void MyDisplayReconfigurationCallBack (
CGDirectDisplayID display,
CGDisplayChangeSummaryFlags flags,
void *userInfo);
{
if (flags & kCGDisplayAddFlag) {
NSLog (#"Display Added");
}
else if (kCGDisplayRemoveFlag) {
NSLog (#"Display Removed");
}
}
CGDisplayRegisterReconfigurationCallback(MyDisplayReconfigurationCallBack, NULL);
The actual code I got was from Apple's Example, but it tells me that flags is an undeclared identifier at this point and won't compile. Not that it would work right since I don't have it in a run loop. I was hoping to find a tutorial somewhere that explains registering for system callback in a run loop but have not been successful. If anyone could point me in the right direction I'd super appreciate it.
(I'm sure that you'll be able to tell from my question that I'm very green. I taught myself Objective-C out of a book as my first programming language. I skipped C, so every once in a while I hit a snag somewhere that I can't figure out.)
If you're writing a Mac OS X application, the AppKit has already set up a run loop for you, so you don't need to worry about that part. You really only need to create your own run loop in Cocoa when you are also creating your own thread.
For the "undeclared identifier" part, it looks like it's due to a typo/syntax mistake:
void MyDisplayReconfigurationCallBack (CGDirectDisplayID display,
CGDisplayChangeSummaryFlags flags,
void *userInfo);
// Semicolon makes this an invalid function definition^^
{
// This is an anonymous block,* and flags wasn't declared in it
if (flags & kCGDisplayAddFlag) {
// etc.
}
Also, unlike some other languages, you can't declare or define functions inside of other functions, methods, or blocks* -- they have to be at the top level of the file. You can't put this in the same place where you call CGDisplayRegisterReconfigurationCallback.
Just as an sample (I have no idea what the rest of your code really looks like):
// MyClassThatIsInterestedInDisplayConfiguration.m
#import "MyClassThatIsInterestedInDisplayConfiguration.h"
// Define callback function at top level of file
void MyDisplayReconfigurationCallBack (
CGDirectDisplayID display,
CGDisplayChangeSummaryFlags flags,
void *userInfo)
{
if (flags & kCGDisplayAddFlag) {
NSLog (#"Display Added");
}
else if (kCGDisplayRemoveFlag) {
NSLog (#"Display Removed");
}
}
#implementation MyClassThatIsInterestedInDisplayConfiguration
- (void) comeOnBabyAndDoTheRegistrationWithMe {
// Register callback function inside a method
CGDisplayRegisterReconfigurationCallback(MyDisplayReconfigurationCallBack,
NULL);
}
#end
*The basic C curly-brace-delimited thing, not the new cool Obj-C ad hoc function thing.