Swift debounce function based GCD

debounce.swift

import Foundation

/**
    Creates and returns a new debounced version of the passed block which will postpone its execution until after wait seconds have elapsed since the last time it was invoked.
    It is like a bouncer at a discotheque. He will act only after you shut up for some time.
    This technique is important if you have action wich should fire on update, however the updates are to frequent.
    
    Inspired by debounce function from underscore.js ( http://underscorejs.org/#debounce )
*/
public func dispatch_debounce_block(wait : NSTimeInterval, queue : dispatch_queue_t = dispatch_get_main_queue(), block : dispatch_block_t) -> dispatch_block_t {
    var cancelable : dispatch_block_t!
    return {
        cancelable?()
        cancelable = dispatch_after_cancellable(dispatch_time(DISPATCH_TIME_NOW, Int64(wait * Double(NSEC_PER_SEC))), queue, block)
    }
}

// Big thanks to Claus Höfele for this function
// https://gist.github.com/choefele/5e5a981ed731472b80d9
func dispatch_after_cancellable(when: dispatch_time_t, queue: dispatch_queue_t, block: dispatch_block_t) -> () -> Void {
    var isCancelled = false
    dispatch_after(when, queue) {
        if !isCancelled {
            block()
        }
    }
    
    return {
        isCancelled = true
    }
}

debounceTest.swift

import XCTest

class debounceTest: XCTestCase {

    func test_dispatch_debounce_block_create_with_no_execute() {
        // given
        let expectation = expectationWithDescription("expect block to be executed");
        
        var counter = 0
        
        let bouncedBlock = dispatch_debounce_block(0.1){
            counter++
        }
        
        // when
        // no executed
        
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(0.5 * Double(NSEC_PER_SEC))), dispatch_get_main_queue()){
            // fulfill  expectation after 0.5 seconds
            expectation.fulfill()
        }
        
        // then
        waitForExpectationsWithTimeout(1) {
            error in
            XCTAssertEqual(counter, 0, "never executed")
        }
        
    }
    
    func test_dispatch_debounce_block_executing_only_once() {
        // given
        let expectation = expectationWithDescription("expect block to be executed");
        
        var counter = 0
        
        let bouncedBlock = dispatch_debounce_block(0.1){
            counter++
        }
        
        // when
        // execute bounce block 10 time in a row
        for _ in 1 ... 10 {
            bouncedBlock()
        }
        
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(0.5 * Double(NSEC_PER_SEC))), dispatch_get_main_queue()){
            // fulfill  expectation after 0.5 seconds
            expectation.fulfill()
        }
        
        // then
        waitForExpectationsWithTimeout(1) {
            error in
            // only one execution should be performed
            XCTAssert(counter == 1, "called only once")
        }
        
    }
    
    func test_dispatch_debounce_block_executing_twice() {
        // given
        let expectation = expectationWithDescription("expect block to be executed");
        
        var counter = 0
        
        let bouncedBlock = dispatch_debounce_block(0.1){
            counter++
        }
        
        // when
        // execute bounce block 10 time in a row
        for _ in 1 ... 10 {
            bouncedBlock()
        }
        
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(0.5 * Double(NSEC_PER_SEC))), dispatch_get_main_queue()){
            // execute bounce block after 0.5 seconds
            bouncedBlock()
        }
        
        
        dispatch_after(dispatch_time(DISPATCH_TIME_NOW, Int64(2 * Double(NSEC_PER_SEC))), dispatch_get_main_queue()){
            // fulfill  expectation after 2 seconds
            expectation.fulfill()
        }
        
        // then
        waitForExpectationsWithTimeout(3) {
            error in
            XCTAssertEqual(counter, 2, "called twice")
        }
        
    }

}