Closure

Closures

Intro

Closures are just a short hand version of functions in a function as a parameter passing around.

It is mostly used to run asynchronous methods and execute the closure function code only after receiving certain type of input from the parent function or service. You could say it is a synchronous method calling or promises type of behavior. In swift it is referred as “Completion handlers”

Syntax

Creation

Basic closure syntax defined in swift

typealias Handler<OpenWeatherModel> = (Result<OpenWeatherModel, Error>) -> Void

func getWeatherDataByLatLon(latitudeWeather : Double,
							longitudeWeather : Double,
							completionHandler : @escaping Handler<OpenWeatherModel>) {
	guard error == nil else { return completionHandler(.failure(error!)) }
	
	guard let data = data else { return }
	do {
		let response = try JSONDecoder()
		.decode(OpenWeatherModel.self, from: data)
		completionHandler(.success(response))
	} catch let error {
		print("Error in retrieving data" + error.localizedDescription)
	}
}

Usage

Calling closure

NetworkAPI
.sharedInstance()
.getWeatherDataByLatLon(latitudeWeather : currLat,
						longitudeWeather: currLong,
						completionHandler : { [weak self] result in        
	switch result {
	case .success(let data):
		print("In success block")
		self?.openWeatherResponse?.append(data)
		DispatchQueue.main.async {
			self?.resultsTable.reloadData()
		}
	case .failure(let error): break
		// Check for internal error enum cases
		print("Some error occured")
	} 
})

Super in closure

Using 'super' in a closure where 'self' is explicitly captured is not yet supported

class Parent {
   func foo() {
   }
}

class Child: Parent {
   override func foo() {
      UIView.animateWithDuration(1) { [weak self] in // or any closure-producing context
         self?.super_foo()
      }
   }

   private func super_foo() {
      super.foo()
   }
}

Completion Handler

SO | Completion handler closure

Unit Tests closure

callback example
Open: unit tests _closure _ swift.png

Reference

open radar mirror

SO | store a closure as a variable

Closures Vs. Combine Vs. Async Await