In angular.js, what does an ampersand in the directive mean?

Question

In this code:

app.directive( 'myCustomDirective', function() {   return {     restrict: 'EA',     scope: { value: '=myCustomDirective', clickable: '&', editing: '=' },     template: '<span ng-click="handleClick()" ng-bind="value"></span>',     ... 

What does clickable: '&' denote?

When I do find Angular documentation, I cannot understand it. It seems to be written in a catch-22 code that can be understood only by people who already understand what is being explained.

For example, I found this video, which apparently explains the & thing, but he may as well have been speaking Mandarin.

Answer 1

Update: The new directive API is here

There is some more explanations on the Understanding Scopes doc, and they provide this useful link.

When I wanted to understand this stuff, I made a Fiddle.

angular.module('scopePropertiesModule', [])   .directive('scopeProps', function(){     return {       restrict: 'C',       scope: {parameterTitle:'@',                bidirecTitle:'=',               delegateDisplay:'&'},       template: '<div>' +                   'Parameter title :<br>' +                    '<input ng-model="parameterTitle"> => {{parameterTitle}}<br>'+                   '<br>' +                   'Biderectional title :<br>' +                   '<input ng-model="bidirecTitle"> => {{bidirecTitle}}<br>' +                    '<br>' +                   'Delegate display :<br>' +                   '{{delegateDisplay()}}<br>' +                    '</div>'       } }); 

and the markup:

<div class="scopeProps"      parameter-title="{{title}}"       bidirec-title="title"      delegate-display="displayTitle()"></div> 

Feel free to play with it.