add Function.debounce

Author: SergioCrisostomo

Docs/Types/Function.md

@@ -368,6 +368,34 @@ Executes a function in the specified intervals of time. Periodic execution can b
 
 - [MDN setInterval][], [MDN clearInterval][]
 
+Function: Function.debounce {#Function:Function-debounce}
+---------------------------------------------------------
+
+This method will return a new function that will be called only once per group of close calls. After a defined delay it will be able to be called again.
+
+### Syntax:
+
+	var debounceFn = myFn.debounce(delay, leading);
+
+### Arguments:
+
+1. delay - (*number*, optional, defaults to 250ms) The delay to wait before a call to the debounced function can happen again.
+2. leading - (*boolean*, optional, defaults to false) If the call to the debounced function should happen in leading phase of group of calls or after.
+
+### Returns:
+
+* (*function*) A debounce function that will be called only once per group of close function calls.
+
+### Examples:
+
+	// get scroll position after scroll has stopped
+	var getNewScrollPosition = function () {
+		var scroll = window.getScroll();
+		alert(scroll.y);
+	}
+	window.addEvent('scroll', getNewScrollPosition.debounce(500));
+
+
 
 Deprecated Functions {#Deprecated-Functions}
 ============================================

Source/Types/Function.js

@@ -72,6 +72,35 @@ Function.implement({
 
 	periodical: function(periodical, bind, args){
 		return setInterval(this.pass((args == null ? [] : args), bind), periodical);
+	},
+
+	debounce: function(delay, leading){
+
+		// in case delay is omitted and `leading` is first argument
+		if (typeof delay == 'boolean'){
+			leading = delay;
+			delay = false;
+		}
+
+		var timeout, args, self,
+			fn = this,
+			callNow = leading;
+
+		var later = function(){
+			if (leading) callNow = true;
+			else fn.apply(self, args);
+			timeout = null;
+		};
+
+		return function(){
+			self = this;
+			args = arguments;
+
+			clearTimeout(timeout);
+			timeout = setTimeout(later, delay || 250);
+			if (callNow) fn.apply(self, args);
+			callNow = false;
+		};
 	}
 
 });

Specs/Types/Function.js

@@ -460,3 +460,136 @@ describe('Function.periodical', function(){
 	});
 
 });
+
+describe('Debounce', function(){
+	var periodical,
+		counter = 0,
+		debounceCalls = 0;
+
+	// spy, to count when original fn was called
+	function targetFn(){
+		debounceCalls++;
+	}
+
+	// call function every 10ms
+	function caller(debounceFn, cb){
+		periodical = setInterval(function(){
+			counter++;
+			debounceFn();
+		}, 10);
+	}
+
+	beforeEach(function(){
+		expect(counter).to.equal(0);
+		expect(debounceCalls).to.equal(0);
+	});
+
+	afterEach(function(){
+		counter = debounceCalls = 0;
+		clearInterval(periodical);
+	});
+
+	it('should debounce with default values', function(done){
+		var debounceFn = targetFn.debounce();
+		caller(debounceFn);
+
+		var firstCheck = false;
+		var wait = setInterval(function(){
+			// keep calling for 400ms, no call should be done
+			if (!firstCheck && counter > 40){
+				clearInterval(periodical);
+				expect(debounceCalls).to.equal(0);
+				firstCheck = true;
+			}
+			// wait for debouced call to come
+			if (firstCheck && debounceCalls > 0){
+				clearInterval(wait);
+				done();
+			}
+		}, 10);
+	});
+
+	it('should debounce early', function(done){
+		var debounceFn = targetFn.debounce(100, true),
+			time = 0,
+			firstCheck;
+		caller(debounceFn);
+		var wait = setInterval(function(){
+			time++;
+			// there should already be a function called
+			if (counter > 5 && !firstCheck){
+				clearInterval(periodical);
+				expect(debounceCalls).to.equal(1);
+				firstCheck = true;
+			}
+			// no more debounced call should be done
+			if (time > 40){
+				expect(debounceCalls).to.equal(1);
+				clearInterval(wait);
+				done();
+			}
+		}, 10);
+	});
+
+	it('should debounce early when `leading` is passed alone', function(done){
+		var debounceFn = targetFn.debounce(true),
+			time = 0,
+			firstCheck;
+		caller(debounceFn);
+		var wait = setInterval(function(){
+			time++;
+			// there should already be a function called
+			if (counter > 5 && !firstCheck){
+				clearInterval(periodical);
+				expect(debounceCalls).to.equal(1);
+				firstCheck = true;
+			}
+			// no more debounced call should be done
+			if (time > 40){
+				expect(debounceCalls).to.equal(1);
+				clearInterval(wait);
+				done();
+			}
+		}, 10);
+	});
+
+	it('should debounce late', function(done){
+		var debounceFn = targetFn.debounce(150);
+		caller(debounceFn);
+		var time = 0;
+		var firstCheck;
+		var wait = setInterval(function(){
+			time++;
+			// no early calls
+			if (counter > 5 && !firstCheck){
+				clearInterval(periodical);
+				expect(debounceCalls).to.equal(0);
+				firstCheck = true;
+			}
+			// should have been called after
+			if (time > 30){
+				expect(debounceCalls).to.equal(1);
+				clearInterval(wait);
+				done();
+			}
+		}, 10);
+	});
+
+	it('should have the right context', function(done){
+		var context = {};
+		var _context;
+		var fn = function(){
+			_context = this;
+		}.bind(context);
+		var debounceFn = fn.debounce();
+		debounceFn(); // trigger the function, once is enough
+		var wait = setInterval(function(){
+			if (_context){
+				clearInterval(wait);
+				expect(_context).to.equal(context);
+				done();
+			}
+		}, 10);
+	});
+
+});