Completion callback using closures in Swift [duplicate] - ios

Question

In Objective-C, you can define a block's input and output, store one of those blocks that's passed in to a method, then use that block later:
// in .h
typedef void (^APLCalibrationProgressHandler)(float percentComplete);
typedef void (^APLCalibrationCompletionHandler)(NSInteger measuredPower, NSError *error);
// in .m
#property (strong) APLCalibrationProgressHandler progressHandler;
#property (strong) APLCalibrationCompletionHandler completionHandler;
- (id)initWithRegion:(CLBeaconRegion *)region completionHandler:(APLCalibrationCompletionHandler)handler
{
self = [super init];
if(self)
{
...
_completionHandler = [handler copy];
..
}
return self;
}
- (void)performCalibrationWithProgressHandler:(APLCalibrationProgressHandler)handler
{
...
self.progressHandler = [handler copy];
...
dispatch_async(dispatch_get_main_queue(), ^{
_completionHandler(0, error);
});
...
}
So I'm trying to do the equivilant in Swift:
var completionHandler:(Float)->Void={}
init() {
locationManager = CLLocationManager()
region = CLBeaconRegion()
timer = NSTimer()
}
convenience init(region: CLBeaconRegion, handler:((Float)->Void)) {
self.init()
locationManager.delegate = self
self.region = region
completionHandler = handler
rangedBeacons = NSMutableArray()
}
The compiler doesn't like that declaration of completionHandler. Not that I blame it, but, how do I define a closure that can be set and used later in Swift?

Answer 1

The compiler complains on
var completionHandler: (Float)->Void = {}
because the right-hand side is not a closure of the appropriate signature, i.e. a closure taking
a float argument. The following would assign a "do nothing" closure to the
completion handler:
var completionHandler: (Float)->Void = {
(arg: Float) -> Void in
}
and this can be shortened to
var completionHandler: (Float)->Void = { arg in }
due to the automatic type inference.
But what you probably want is that the completion handler is initialized to nil
in the same way that an Objective-C instance variable is inititialized to nil. In Swift
this can be realized with an optional:
var completionHandler: ((Float)->Void)?
Now the property is automatically initialized to nil ("no value").
In Swift you would use optional binding to check of a the
completion handler has a value
if let handler = completionHandler {
handler(result)
}
or optional chaining:
completionHandler?(result)

Answer 2

Objective-C
#interface PopupView : UIView
#property (nonatomic, copy) void (^onHideComplete)();
#end
#interface PopupView ()
...
- (IBAction)hideButtonDidTouch:(id sender) {
// Do something
...
// Callback
if (onHideComplete) onHideComplete ();
}
#end
PopupView * popupView = [[PopupView alloc] init]
popupView.onHideComplete = ^() {
...
}
Swift
class PopupView: UIView {
var onHideComplete: (() -> Void)?
#IBAction func hideButtonDidTouch(sender: AnyObject) {
// Do something
....
// Callback
if let callback = self.onHideComplete {
callback ()
}
}
}
var popupView = PopupView ()
popupView.onHideComplete = {
() -> Void in
...
}

Answer 3

I've provide an example not sure if this is what you're after.
var completionHandler: (_ value: Float) -> ()
func printFloat(value: Float) {
print(value)
}
completionHandler = printFloat
completionHandler(5)
It simply prints 5 using the completionHandler variable declared.

Answer 4

Closures can be declared as typealias as below
typealias Completion = (Bool, Any, Error) -> Void
If you want to use in your function anywhere in code; you can write like normal variable
func xyz(with param1: String, completion: Completion) {
}

Answer 5

In Swift 4 and 5. I created a closure variable containing two parameter dictionary and bool.
var completionHandler:([String:Any], Bool)->Void = { dict, success in
if success {
print(dict)
}
}
Calling the closure variable
self.completionHandler(["name":"Gurjinder singh"],true)

Answer 6

This works too:
var exeBlk = {
() -> Void in
}
exeBlk = {
//do something
}
//instead of nil:
exeBlk = {}

Answer 7

Depends on your needs there is an addition to accepted answer. You may also implement it like this:
var parseCompletion: (() ->Void)!
and later in some func assign to it
func someHavyFunc(completion: #escaping () -> Void){
self.parseCompletion = completion
}
and in some second function use it
func someSecondFunc(){
if let completion = self.parseCompletion {
completion()
}
}
note that #escaping parameter is a mandatory here

Answer 8

For me following was working:
var completionHandler:((Float)->Void)!