Recursion in Angular directives

Question

There are a couple of popular recursive angular directive Q&A's out there, which all come down to one of the following solutions:

• manually incrementally 'compile' HTML based on runtime scope state
  • example 1 [stackoverflow]
  • example 2 [angular jsfiddles page]
• don't use a directive at all, but a <script> template which refers to itself
  • example 1 [google groups]

The first one has the problem that you can't remove previously compiled code unless you comprehensibly manage the manual compile process. The second approach has the problem of... not being a directive and missing out on its powerful capabilities, but more urgently, it can't be parameterised the same way a directive can be; it's simply bound to a new controller instance.

I've been playing with manually doing an angular.bootstrap or @compile() in the link function, but that leaves me with the problem of manually keeping track of elements to remove and add.

Is there a good way to have a parameterized recursive pattern that manages adding/removing elements to reflect runtime state? That is to say, a tree with a add/delete node button and some input field whose value is passed down a node's child nodes. Perhaps a combination of the second approach with chained scopes (but I have no idea how to do this)?

Answer 1

Inspired by the solutions described in the thread mentioned by @dnc253, I abstracted the recursion functionality into a service.

module.factory('RecursionHelper', ['$compile', function($compile){
    return {
        /**
         * Manually compiles the element, fixing the recursion loop.
         * @param element
         * @param [link] A post-link function, or an object with function(s) registered via pre and post properties.
         * @returns An object containing the linking functions.
         */
        compile: function(element, link){
            // Normalize the link parameter
            if(angular.isFunction(link)){
                link = { post: link };
            }

            // Break the recursion loop by removing the contents
            var contents = element.contents().remove();
            var compiledContents;
            return {
                pre: (link && link.pre) ? link.pre : null,
                /**
                 * Compiles and re-adds the contents
                 */
                post: function(scope, element){
                    // Compile the contents
                    if(!compiledContents){
                        compiledContents = $compile(contents);
                    }
                    // Re-add the compiled contents to the element
                    compiledContents(scope, function(clone){
                        element.append(clone);
                    });

                    // Call the post-linking function, if any
                    if(link && link.post){
                        link.post.apply(null, arguments);
                    }
                }
            };
        }
    };
}]);

Which is used as follows:

module.directive("tree", ["RecursionHelper", function(RecursionHelper) {
    return {
        restrict: "E",
        scope: {family: '='},
        template: 
            '<p>{{ family.name }}</p>'+
            '<ul>' + 
                '<li ng-repeat="child in family.children">' + 
                    '<tree family="child"></tree>' +
                '</li>' +
            '</ul>',
        compile: function(element) {
            // Use the compile function from the RecursionHelper,
            // And return the linking function(s) which it returns
            return RecursionHelper.compile(element);
        }
    };
}]);

See this Plunker for a demo. I like this solution best because:

1. You don't need an special directive which makes your html less clean.
2. The recursion logic is abstracted away into the RecursionHelper service, so you keep your directives clean.

Update: As of Angular 1.5.x, no more tricks are required, but works only with template, not with templateUrl

Answer 2

Manually adding elements and compiling them is definitely a perfect approach. If you use ng-repeat then you will not have to manually remove elements.

Demo: http://jsfiddle.net/KNM4q/113/

.directive('tree', function ($compile) {
return {
    restrict: 'E',
    terminal: true,
    scope: { val: '=', parentData:'=' },
    link: function (scope, element, attrs) {
        var template = '<span>{{val.text}}</span>';
        template += '<button ng-click="deleteMe()" ng-show="val.text">delete</button>';

        if (angular.isArray(scope.val.items)) {
            template += '<ul class="indent"><li ng-repeat="item in val.items"><tree val="item" parent-data="val.items"></tree></li></ul>';
        }
        scope.deleteMe = function(index) {
            if(scope.parentData) {
                var itemIndex = scope.parentData.indexOf(scope.val);
                scope.parentData.splice(itemIndex,1);
            }
            scope.val = {};
        };
        var newElement = angular.element(template);
        $compile(newElement)(scope);
        element.replaceWith(newElement);
    }
}
});

Answer 3

I don't know for sure if this solution is found in one of the examples you linked or the same basic concept, but I had a need of a recursive directive, and I found a great, easy solution.

module.directive("recursive", function($compile) {
    return {
        restrict: "EACM",
        priority: 100000,
        compile: function(tElement, tAttr) {
            var contents = tElement.contents().remove();
            var compiledContents;
            return function(scope, iElement, iAttr) {
                if(!compiledContents) {
                    compiledContents = $compile(contents);
                }
                iElement.append(
                    compiledContents(scope, 
                                     function(clone) {
                                         return clone; }));
            };
        }
    };
});

module.directive("tree", function() {
    return {
        scope: {tree: '='},
        template: '<p>{{ tree.text }}</p><ul><li ng-repeat="child in tree.children"><recursive><span tree="child"></span></recursive></li></ul>',
        compile: function() {
            return  function() {
            }
        }
    };
});​

You should create the recursive directive and then wrap it around the element that makes the recursive call.

Answer 4

As of Angular 1.5.x, no more tricks are required, the following has been made possible. No more need for dirty work arounds!

This discovery was a by product of my hunt for a better/cleaner solution for a recursive directive. You can find it here https://jsfiddle.net/cattails27/5j5au76c/. It supports as far is 1.3.x.

angular.element(document).ready(function() {
  angular.module('mainApp', [])
    .controller('mainCtrl', mainCtrl)
    .directive('recurv', recurveDirective);

  angular.bootstrap(document, ['mainApp']);

  function recurveDirective() {
    return {
      template: '<ul><li ng-repeat="t in tree">{{t.sub}}<recurv tree="t.children"></recurv></li></ul>',
      scope: {
        tree: '='
      },
    }
  }

});

  function mainCtrl() {
    this.tree = [{
      title: '1',
      sub: 'coffee',
      children: [{
        title: '2.1',
        sub: 'mocha'
      }, {
        title: '2.2',
        sub: 'latte',
        children: [{
          title: '2.2.1',
          sub: 'iced latte'
        }]
      }, {
        title: '2.3',
        sub: 'expresso'
      }, ]
    }, {
      title: '2',
      sub: 'milk'
    }, {
      title: '3',
      sub: 'tea',
      children: [{
        title: '3.1',
        sub: 'green tea',
        children: [{
          title: '3.1.1',
          sub: 'green coffee',
          children: [{
            title: '3.1.1.1',
            sub: 'green milk',
            children: [{
              title: '3.1.1.1.1',
              sub: 'black tea'
            }]
          }]
        }]
      }]
    }];
  }

<script src="https://cdnjs.cloudflare.com/ajax/libs/angular.js/1.5.8/angular.min.js"></script>
<div>
  <div ng-controller="mainCtrl as vm">
    <recurv tree="vm.tree"></recurv>
  </div>
</div>

Answer 5

After using several workarounds for a while, I've repeatedly come back to this issue.

I'm not satisfied by the service solution since it works for directives that can inject the service but does not work for anonymous template fragments.

Similarly, solutions which depend upon specific template structure by doing DOM manipulation in the directive are too specific and brittle.

I have what I believe is a generic solution that encapsulates the recursion as a directive of its own that interferes minimally with any other directives and can be used anonymously.

Below is a demonstration that you can also play around with at plnkr: http://plnkr.co/edit/MSiwnDFD81HAOXWvQWIM

var hCollapseDirective = function () {
  return {
    link: function (scope, elem, attrs, ctrl) {
      scope.collapsed = false;
      scope.$watch('collapse', function (collapsed) {
        elem.toggleClass('collapse', !!collapsed);
      });
    },
    scope: {},
    templateUrl: 'collapse.html',
    transclude: true
  }
}

var hRecursiveDirective = function ($compile) {
  return {
    link: function (scope, elem, attrs, ctrl) {
      ctrl.transclude(scope, function (content) {
        elem.after(content);
      });
    },
    controller: function ($element, $transclude) {
      var parent = $element.parent().controller('hRecursive');
      this.transclude = angular.isObject(parent)
        ? parent.transclude
        : $transclude;
    },
    priority: 500,  // ngInclude < hRecursive < ngIf < ngRepeat < ngSwitch
    require: 'hRecursive',
    terminal: true,
    transclude: 'element',
    $$tlb: true  // Hack: allow multiple transclusion (ngRepeat and ngIf)
  }
}

angular.module('h', [])
.directive('hCollapse', hCollapseDirective)
.directive('hRecursive', hRecursiveDirective)

/* Demo CSS */
* { box-sizing: border-box }

html { line-height: 1.4em }

.task h4, .task h5 { margin: 0 }

.task { background-color: white }

.task.collapse {
  max-height: 1.4em;
  overflow: hidden;
}

.task.collapse h4::after {
  content: '...';
}

.task-list {
  padding: 0;
  list-style: none;
}


/* Collapse directive */
.h-collapse-expander {
  background: inherit;
  position: absolute;
  left: .5px;
  padding: 0 .2em;
}

.h-collapse-expander::before {
  content: '•';
}

.h-collapse-item {
  border-left: 1px dotted black;
  padding-left: .5em;
}

.h-collapse-wrapper {
  background: inherit;
  padding-left: .5em;
  position: relative;
}

<!DOCTYPE html>
<html>

  <head>
    <link href="collapse.css" rel="stylesheet" />
    <link href="style.css" rel="stylesheet" />
    <script data-require="angular.js@1.3.15" data-semver="1.3.15" src="https://code.angularjs.org/1.3.15/angular.js"></script>
    <script src="//cdnjs.cloudflare.com/ajax/libs/jquery/2.1.1/jquery.min.js" data-semver="2.1.1" data-require="jquery@*"></script>
    <script src="script.js"></script>
    <script>
      function AppController($scope) {
        $scope.toggleCollapsed = function ($event) {
          $event.preventDefault();
          $event.stopPropagation();
          this.collapsed = !this.collapsed;
        }
        
        $scope.task = {
          name: 'All tasks',
          assignees: ['Citizens'],
          children: [
            {
              name: 'Gardening',
              assignees: ['Gardeners', 'Horticulture Students'],
              children: [
                {
                  name: 'Pull weeds',
                  assignees: ['Weeding Sub-committee']
                }
              ],
            },
            {
              name: 'Cleaning',
              assignees: ['Cleaners', 'Guests']
            }
          ]
        }
      }
      
      angular.module('app', ['h'])
      .controller('AppController', AppController)
    </script>
  </head>

  <body ng-app="app" ng-controller="AppController">
    <h1>Task Application</h1>
    
    <p>This is an AngularJS application that demonstrates a generalized
    recursive templating directive. Use it to quickly produce recursive
    structures in templates.</p>
    
    <p>The recursive directive was developed in order to avoid the need for
    recursive structures to be given their own templates and be explicitly
    self-referential, as would be required with ngInclude. Owing to its high
    priority, it should also be possible to use it for recursive directives
    (directives that have templates which include the directive) that would
    otherwise send the compiler into infinite recursion.</p>
    
    <p>The directive can be used alongside ng-if
    and ng-repeat to create recursive structures without the need for
    additional container elements.</p>
    
    <p>Since the directive does not request a scope (either isolated or not)
    it should not impair reasoning about scope visibility, which continues to
    behave as the template suggests.</p>
    
    <p>Try playing around with the demonstration, below, where the input at
    the top provides a way to modify a scope attribute. Observe how the value
    is visible at all levels.</p>
    
    <p>The collapse directive is included to further demonstrate that the
    recursion can co-exist with other transclusions (not just ngIf, et al)
    and that sibling directives are included on the recursive due to the
    recursion using whole 'element' transclusion.</p>
    
    <label for="volunteer">Citizen name:</label>
    <input id="volunteer" ng-model="you" placeholder="your name">
    <h2>Tasks</h2>
    <ul class="task-list">
      <li class="task" h-collapse h-recursive>
        <h4>{{task.name}}</h4>
        <h5>Volunteers</h5>
        <ul>
          <li ng-repeat="who in task.assignees">{{who}}</li>
          <li>{{you}} (you)</li>
        </ul>
        <ul class="task-list">
          <li h-recursive ng-repeat="task in task.children"></li>
        </ul>
      <li>
    </ul>
    
    <script type="text/ng-template" id="collapse.html">
      <div class="h-collapse-wrapper">
        <a class="h-collapse-expander" href="#" ng-click="collapse = !collapse"></a>
        <div class="h-collapse-item" ng-transclude></div>
      </div>
    </script>
  </body>

</html>

Answer 6

Now that Angular 2.0 is out in preview I think it's ok to add an Angular 2.0 alternative into the mix. At least it will benefit people later:

The key concept is to build a recursive template with a self reference:

<ul>
    <li *for="#dir of directories">

        <span><input type="checkbox" [checked]="dir.checked" (click)="dir.check()"    /></span> 
        <span (click)="dir.toggle()">{{ dir.name }}</span>

        <div *if="dir.expanded">
            <ul *for="#file of dir.files">
                {{file}}
            </ul>
            <tree-view [directories]="dir.directories"></tree-view>
        </div>
    </li>
</ul>

You then bind a tree object to the template and watch the recursion take care of the rest. Here is a full example: http://www.syntaxsuccess.com/viewarticle/recursive-treeview-in-angular-2.0

Answer 7

There is a really really simple workaround for this that does not require directives at all.

Well, in that sense, maybe it is not even a solution of the original problem if you assume you need directives, but it IS a solution if you want a recursive GUI structure with parametrized sub-structures of the GUI. Which is probably what you want.

The solution is based on just using ng-controller, ng-init and ng-include. Just do it as follows, assume that your controller is called "MyController", your template is located in myTemplate.html and that you have an initialization function on your controller called init that takes argument A, B, and C, making it possible to parametrize your controller. Then the solution is as follows:

myTemplate.htlm:

<div> 
    <div>Hello</div>
    <div ng-if="some-condition" ng-controller="Controller" ng-init="init(A, B, C)">
       <div ng-include="'myTemplate.html'"></div>
    </div>
</div>

I found by plain conincidence that this kind of structure can be made recursive as you like in plain vanilla angular. Just follow this design pattern and you can use recursive UI-structures without any advanced compilation tinkering etc.

Inside your controller:

$scope.init = function(A, B, C) {
   // Do something with A, B, C
   $scope.D = A + B; // D can be passed on to other controllers in myTemplate.html
} 

The only downside I can see is the clunky syntax you have to put up with.

Answer 8

I ended up creating a set of basic directives for recursion.

IMO It is far more basic than the solution found here, and just as flexible if not more, so we are not bound to using UL/LI structures etc... But obviously those make sense to use, however the directives are unaware of this fact...

A Super simple example would be:

<ul dx-start-with="rootNode">
  <li ng-repeat="node in $dxPrior.nodes">
    {{ node.name }}
    <ul dx-connect="node"/>
  </li>
</ul>

The implementation of 'dx-start-with' an 'dx-connect' is found at: https://github.com/dotJEM/angular-tree

This means you don't have to create 8 directives if you need 8 different layouts.

To create a tree-view on top of that where you can add or delete nodes would then be rather simple. As in: http://codepen.io/anon/pen/BjXGbY?editors=1010

angular
  .module('demo', ['dotjem.angular.tree'])
  .controller('AppController', function($window) {

this.rootNode = {
  name: 'root node',
  children: [{
    name: 'child'
  }]
};

this.addNode = function(parent) {
  var name = $window.prompt("Node name: ", "node name here");
  parent.children = parent.children || [];
  parent.children.push({
    name: name
  });
}

this.removeNode = function(parent, child) {
  var index = parent.children.indexOf(child);
  if (index > -1) {
    parent.children.splice(index, 1);
  }
}

  });

<div ng-app="demo" ng-controller="AppController as app">
  HELLO TREE
  <ul dx-start-with="app.rootNode">
<li><button ng-click="app.addNode($dxPrior)">Add</button></li>
<li ng-repeat="node in $dxPrior.children">
  {{ node.name }} 
  <button ng-click="app.removeNode($dxPrior, node)">Remove</button>
  <ul dx-connect="node" />
</li>
  </ul>

  <script src="https://ajax.googleapis.com/ajax/libs/angularjs/1.5.0/angular.min.js"></script>
  <script src="https://rawgit.com/dotJEM/angular-tree-bower/master/dotjem-angular-tree.min.js"></script>

</div>

From this point on, the controller and template could be wrapped in it's own directive if one would wish for it.

Answer 9

You can use angular-recursion-injector for that: https://github.com/knyga/angular-recursion-injector

Allows you to do unlimited depth nesting with conditioning. Does recompilation only if needed and compiles only right elements. No magic in code.

<div class="node">
  <span>{{name}}</span>

  <node--recursion recursion-if="subNode" ng-model="subNode"></node--recursion>
</div>

One of the things that allows it to work faster and simpler then the other solutions is "--recursion" suffix.