A Practical Guide to AngularJS Directives (Part Two)

Sandeep Panda
This entry is part 2 of 2 in the series A Practical Guide to AngularJS Directives

A Practical Guide to AngularJS Directives

The first part of this tutorial provided a basic overview of AngularJS directives. In the end of the tutorial we also learned how to isolate the scope of a directive. This article will pick up exactly where part one ended. First, we will see how you can access parent scope’s properties inside your directive while maintaining an isolated scope. Next, we will discuss how to choose the correct scope for a directive by exploring concepts such as controller functions and transclusions. The article wraps up with a walkthrough of a note taking application.

Binding Between Isolated and Parent Scope Properties

Often, it’s convenient to isolate a directive’s scope, especially if you are manipulating many scope models. But, you may also need to access some parent scope properties inside the directive in order for the code to work. The good news is that Angular gives you enough flexibility to selectively pass parent scope properties to the directive through bindings. Let’s revisit our hello world directive, which changes its background color automatically when somebody types a color name into the text field. Recall that we isolated the scope of the directive and the code stopped working? Well, let’s make it work now!

Assume that the variable app is initialized and refers to the Angular module. The directive is shown below.

app.directive('helloWorld', function() {
  return {
    scope: {},
    restrict: 'AE',
    replace: true,
    template: '<p style="background-color:{{color}}">Hello World</p>',
    link: function(scope, elem, attrs) {
      elem.bind('click', function() {
        elem.css('background-color','white');
        scope.$apply(function() {
          scope.color = "white";
        });
      });
      elem.bind('mouseover', function() {
        elem.css('cursor', 'pointer');
      });
    }
  };
});

The markup, with utilizes the directive is shown in the following code sample.

<body ng-controller="MainCtrl">
  <input type="text" ng-model="color" placeholder="Enter a color"/>
  <hello-world/>
</body>

This code is not currently functional. Since we have an isolated scope, the expression {{color}} inside the directive template evaluates against this scope (not parent’s). But the ng-model directive on the input element refers to the parent scope property color. So, we need a way to bind these two isolated and parent scope properties. In Angular, this binding can be achieved by setting attributes on the directive element in HTML and configuring the scope property in the directive definition object. Let’s explore a few ways of setting up the binding.

Option 1: Use @ for One Way Text Binding

In the directive definition, shown below, we have specified that the isolated scope property color should be bound to the attribute colorAttr, which is applied to the directive in the HTML. If you look at the markup, you can see the expression {{color}} is assigned to color-attr. When the value of the expression changes, the attribute color-attr also changes. This in turn changes the isolated scope property, color.

app.directive('helloWorld', function() {
  return {
    scope: {
      color: '@colorAttr'
    },
    ....
    // the rest of the configurations
  };
});

The updated markup is shown below.

<body ng-controller="MainCtrl">
  <input type="text" ng-model="color" placeholder="Enter a color"/>
  <hello-world color-attr="{{color}}"/>
</body>

We call this one way binding because with this technique you can only pass strings to the attribute (using expressions, {{}}). When the parent scope property changes, your isolated scope model also changes. You can even watch this scope property inside the directive and trigger tasks when a change occurs. However, the reverse is not true! You can’t change the parent scope model by manipulating the isolated scope.

Note:
If the isolated scope property and the attribute name is same you can write the directive definition like this:

app.directive('helloWorld', function() {
  return {
    scope: {
      color: '@'
    },
    ....
    // the rest of the configurations
  };
});

The directive is invoked in HTML like this:

<hello-world color="{{color}}"/>

Option 2: Use = for Two Way Binding

Let’s change the directive definition as shown below.

app.directive('helloWorld', function() {
  return {
    scope: {
      color: '='
    },
    ....
    // the rest of the configurations
  };
});

And change the HTML like this:

<body ng-controller="MainCtrl">
  <input type="text" ng-model="color" placeholder="Enter a color"/>
  <hello-world color="color"/>
</body>

Unlike @, this technique lets you assign an actual scope model to the attribute rather than just plain strings. As a result you can pass values ranging from simple strings and arrays to complex objects to the isolated scope. Also, a two way binding exists. Whenever the parent scope property changes, the corresponding isolated scope property also changes, and vice versa. As usual, you can watch this scope property for changes.

Option 3: Use & to Execute Functions in the Parent Scope

It’s sometimes necessary to call functions defined in the parent scope from a directive with isolated scope. To refer to functions defined in outer scope we use &. Let’s say we want to call a function sayHello() from the directive. The following code explains how it is achieved.

app.directive('sayHello', function() {
  return {
    scope: {
      sayHelloIsolated: '&amp;' 
    },
    ....
    // the rest of the configurations
  };
});

The directive is used in HTML like this:

<body ng-controller="MainCtrl">
  <input type="text" ng-model="color" placeholder="Enter a color"/>
  <say-hello sayHelloIsolated="sayHello()"/>
</body>

This Plunker example demonstrates this concepts.

Parent Scope vs. Child Scope vs. Isolated Scope

As an Angular beginner one might get confused while choosing the right scope for a directive. By default a directive does not create a new scope and uses the parent’s scope. But in many cases this is not what we want. If your directive manipulates the parent scope properties heavily and creates new ones, it might pollute the scope. Letting all the directives use the same parent scope is not a good idea because anybody can modify our scope properties. So, the following guidelines may help you choose the right scope for your directive.

  1. Parent Scope (scope: false) – This is the default case. If your directive does not manipulate the parent scope properties you might not need a new scope. In this case, using the parent scope is okay.
  2. Child Scope (scope:true) – This creates a new child scope for a directive which prototypically inherits from the parent scope. If the properties and functions you set on the scope are not relevant to other directives and the parent, you should probably create a new child scope. With this you also have all the scope properties and functions defined by the parent.
  3. Isolated Scope (scope:{}) – This is like a sandbox! You need this if the directive you are going to build is self contained and reusable. Your directive might be creating many scope properties and functions which are meant for internal use, and should never be seen by the outside world. If this is the case, it’s better to have an isolated scope. The isolated scope, as expected, does not inherit the parent scope.

Transclusion

Transclusion is a feature which lets us wrap a directive around arbitrary content. We can later extract and compile it against the correct scope, and finally place it at the specified position in the directive template. If you set transclude:true in the directive definition, a new transcluded scope will be created which prototypically inherits from the parent scope. If you want your directive with isolated scope to contain an arbitrary piece of content and execute it against the parent scope, transclusion can be used.

Let’s say we have a directive registered like this:

app.directive('outputText', function() {
  return {
    transclude: true,
    scope: {},
    template: '<div ng-transclude></div>'
  };
});

And it is used like this:

<div output-text>
  <p>Hello {{name}}</p>
</div>

ng-transclude says where to put the transcluded content. In this case the DOM content <p>Hello {{name}}</p> is extracted and put inside <div ng-transclude></div>. The important point to remember is that the expression {{name}} interpolates against the property defined in the parent scope rather than the isolated scope. A Plunker to experiment with is located here. If you want to learn more about scopes go though this document.

Differences Between transclude:'element' and transclude:true

Sometimes we need to transclude the element on which the directive is applied rather than just the contents. In those cases transclude:'element' is used. This, unlike transclude:true, includes the element itself in the directive template marked with ng-transclude. As a result of transclusion your link function gets a transclude linking function prebound to the correct directive scope. This linking function is also passed another function with a clone of the DOM element which is to be transcluded. You can perform tasks like modifying the clone and adding it to the DOM. Directives like ng-repeat use this technique to repeat the DOM elements. Have a look at the following Plunker which repeats a DOM element using this technique and changes the background color of the second instance.

Also note that by using transclude:'element', the element on which the directive is applied is converted into an HTML comment. So, if you combine transclude:'element' with replace:false, the directive template essentially gets innerHTMLed to the comment – which means nothing really happens! Instead, if you choose replace:true the directive template will replace the HTML comment and things will work as expected. Using replace:false with transclude:'element' is good for cases where you want to repeat the DOM element and don’t want to keep the first instance of the element (which is converted to a comment).

The controller Function and require

The controller function of a directive is used if you want to allow other directives to communicate with yours. In some cases you may need to create a particular UI component by combining two directives. For example you can attach a controller function to a directive as shown below.

app.directive('outerDirective', function() {
  return {
    scope: {},
    restrict: 'AE',
    controller: function($scope, $compile, $http) {
      // $scope is the appropriate scope for the directive
      this.addChild = function(nestedDirective) { // this refers to the controller
        console.log('Got the message from nested directive:' + nestedDirective.message);
      };
    }
  };
});

This code attaches a controller named outerDirective to the directive. When another directive wants to communicate, it needs to declare that it requires your directive’s controller instance. This is done as shown below.

app.directive('innerDirective', function() {
  return {
    scope: {},
    restrict: 'AE',
    require: '^outerDirective',
    link: function(scope, elem, attrs, controllerInstance) {
      //the fourth argument is the controller instance you require
      scope.message = "Hi, Parent directive";
      controllerInstance.addChild(scope);
    }
  };
});

The markup would look something like this:

<outer-directive>
  <inner-directive></inner-directive>
</outer-directive>

require: '^outerDirective' tells Angular to search for the controller on the element and its parent. In this case the found controller instance is passed as the fourth argument to the link function. In our case we are sending the scope of the nested directive to the parent. To try things out, open this Plunker with your browser console opened. The last section of the this Angular resource gives an excellent example of inter directive communication. It’s definitely a must read!

A Note Taking App

In this section we are going to build a simple note taking app using directives. We will make use of HTML5 localStorage to store the notes. The end product is going to look like this. We will create a directive that will render a notepad. A user can view the list of notes he/she has made. When he clicks the button add new the notepad becomes editable and allows a note to be created. The note is automatically saved when the back button is clicked. The notes are saved using a factory called notesFactory, with help from localStorage. The factory code is pretty straightforward and self explanatory. So, let’s concentrate on the directive code only.

Step 1

We start by registering the directive notepad.

app.directive('notepad', function(notesFactory) {
  return {
    restrict: 'AE',
    scope: {},
    link: function(scope, elem, attrs) {
    },
    templateUrl: 'templateurl.html'
  };
});

Please note a few things about the directive:

  • The scope is isolated, as we want the directive to be reusable. The directive will have many properties and functions that are not relevant outside.
  • The directive can be used as an attribute or element as specified by the restrict property.
  • The link function is empty initially.
  • The directive gets its template from templateurl.html.

Step 2

The following HTML forms the template for the directive.

<div class="note-area" ng-show="!editMode">
  <ul>
    <li ng-repeat="note in notes|orderBy:'id'">
      <a href="#" ng-click="openEditor(note.id)">{{note.title}}</a>
    </li>
  </ul>
</div>
<div id="editor" ng-show="editMode" class="note-area" contenteditable="true" ng-bind="noteText"></div>
<span><a href="#" ng-click="save()" ng-show="editMode">Back</a></span>
<span><a href="#" ng-click="openEditor()" ng-show="!editMode">Add Note</a></span>

The important points to note are:

  • The note object encapsulates title, id, and content.
  • ng-repeat is used to loop through the notes and sort them by ascending order of an autogenerated id.
  • We will have a property editMode which wil indicate the mode we are in. In edit mode this property will be true and the editable div will be visible. The user writes the note here.
  • If editMode is false we are in viewing mode and display the notes.
  • The two buttons are also shown/hidden based on editMode.
  • The ng-click directive is used to react to button clicks. These methods, along with the properties like editMode, will be added to scope.
  • The editable div is bound to noteText, which holds the user entered text. If you want to edit an existing note, this model initializes this div with that note content.

Step 3

Let’s create a new function in our scope called restore() that will initialize various controls for our app. This will be called when the link function runs and each time the save button is clicked.

scope.restore = function() {
  scope.editMode = false;
  scope.index = -1;
  scope.noteText = '';
};

We create this function inside the link function. editMode and noteText have already been explained. index is used to track which note is being edited. If we are creating a new note, index is -1. If we are editing an existing note it contains that note object’s id.

Step 4

Now we need to create two scope functions that handle the edit and save actions.

scope.openEditor = function(index) {
  scope.editMode = true;

  if (index !== undefined) {
    scope.noteText = notesFactory.get(index).content;
    scope.index = index;
  } else {
    scope.noteText = undefined;
  }
};

scope.save = function() {
  if (scope.noteText !== '') {
    var note = {};

    note.title = scope.noteText.length > 10 ? scope.noteText.substring(0, 10) + '. . .' : scope.noteText;
    note.content = scope.noteText;
    note.id = scope.index != -1 ? scope.index : localStorage.length;
    scope.notes = notesFactory.put(note);
  }

  scope.restore();
};

The important points about these functions are:

  • openEditor prepares the editor. If we are editing a note, it gets the content of that note and updates the editable div thanks to ng-bind.
  • If we are creating a new note we need to set noteText to undefined in order for watchers to fire when we save the note.
  • If the function argument index is undefined, it means the user is going to create a new note.
  • The save function takes help from the notesFactory to save the note. After saving, it refreshes the notes array so that the watchers can detect a change and the list of notes can be updated.
  • The save function calls restore() at the end to reset the controls so that we can get back to viewing mode from edit mode.

Step 5

When the link function runs we initialize the notes array and bind a keydown event to the editable div so that our noteText model stays in sync with the div content. We use this noteText to save note content.

var editor = elem.find('#editor');

scope.restore();  // initialize our app controls
scope.notes = notesFactory.getAll(); // load notes

editor.bind('keyup keydown', function() {
  scope.noteText = editor.text().trim();
});

Step 6

Finally, use the directive just like any other HTML element and start taking notes!

<h1 class="title">The Note Making App</h1>
<notepad/>

Conclusion

An important point to note is that whatever we do with jQuery can be done with Angular directives with much less code. So, before using jQuery try to figure out if the same thing can be done in a better way without any DOM manipulation. Try to minimize the use of jQuery with Angular.

With regards to the note taking demo, the delete note feature has been intentionally left out. The reader is encouraged to experiment and implement this feature. The source code for the demo is available for download from GitHub.

A Practical Guide to AngularJS Directives

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  • http://gadgeticworld.com/ Sandeep Panda

    Hey Alex, thanks. :) Glad that you liked the tutorial.