Todd Motto

Todd Motto

Owner, Ultimate Angular

Killing it with Angular Directives; Structure and MVVM
Apr 17, 2015
10 mins read

In this post I’m going to outline my approach on writing Directives for Angular 1.x releases. There’s a lot of confusion around how and why and where to do things with Directives, but they are actually very simple once you grasp the concepts and separation ideas. This post isn’t going to cover nesting Directives/data flow into them from parent scopes etc, but will cover my ideal way of creating/structuring and separating all concerns in the Directive, and how to use controller and link properties correctly.

We’re going to cover a basic Directive, the structure, and how things should be structured to make the best use of Angular. Let’s create a pseudo “file upload” Directive to demonstrate how I’d approach.

Note, this code will not actually work, nor do I intend it to as this is purely for demonstration purposes on structuring Directives properly.

Structure

Following my AngularJS style-guide, naturally, we’ll create a basic Directive definition and pass it into Angular’s .directive() method:

function fileUpload () {

return {};

}
angular
.module('app')
.directive('fileUpload', fileUpload);

Now the definition is in place, let’s add some basic properties to the file upload component:

function fileUpload () {

return {
restrict: 'E',
scope: {},
template: [
'<div>',
'</div>'
].join(''),
controllerAs: 'vm',
controller: function () {},
link: function () {}
};

}
angular
.module('app')
.directive('fileUpload', fileUpload);

This essentially completes my “Directive boilerplate”, the basics I need to get something up and running.

Controller (presentational layer)

Instead of binding the Controller (I do this with link as well) to the Object (controller: fn), I move the controller property’s binding “out” into the main fileUpload function definition and assign it back to the Object. This allows me to have a bit more space, as writing functions directly to Objects I find a little less pleasing, and I can comment the functions better. It just looks like normal JavaScript rather than sticking to some “strict looking API”.

Let’s have a look at adding some comments and moving the functions up top.

/**
* @name fileUpload
* @desc <file-upload> Directive
*/

function fileUpload () {

/**
* @name fileUploadCtrl
* @desc File Upload Controller
* @type {Function}
*/

function fileUploadCtrl() {

}

/**
* @name link
* @desc File Upload Link
* @type {Function}
*/

function link() {

}

return {
restrict: 'E',
scope: {},
template: [
'<div>',
'</div>'
].join(''),
controllerAs: 'vm',
controller: fileUploadCtrl
link: link
};

}
angular
.module('app')
.directive('fileUpload', fileUpload);

Sexy? Of course it is. By now you should’ve spotted controllerAs: 'vm' which I alias the Controller under vm (standing for ViewModel). This treats a Controller as a ViewModel for the “Presentation Model” design pattern. Read up on my ControllerAs article if you’re not familiar. Essentially instead of injecting $scope, the Controller binds itself to the $scope under our vm alias, essentially creating $scope.vm. I treat this as a Controller “Class” and we can dive into using the this keyword instead of $scope.

/**
* @name fileUploadCtrl
* @desc File Upload Controller
* @type {Function}
*/

function fileUploadCtrl() {
this.files = [];
this.uploadFiles = function () {

};
}

Using this is great and far better than $scope in my eyes, we only use $scope for things like $on events or $watch, treating it a little differently than our Controller Class - the “ViewModel”.

We’ve got our function setup now, however I prefer using an “exports” style and bind all my functions and variables that way, adding any appropriate comments. Let’s see what that looks like altogether:

/**
* @name fileUpload
* @desc <file-upload> Directive
*/

function fileUpload () {

/**
* @name fileUploadCtrl
* @desc File Upload Controller
* @type {Function}
*/

function fileUploadCtrl() {

/**
* @name files
* @desc Contains all files passed in by the user
* @type {Array}
*/

var files = [];

/**
* @name uploadFiles
* @desc Uploads our files
* @type {Array}
*/

function uploadFiles() {

}

// exports
this.files = files;
this.uploadFiles = uploadFiles;

}

/**
* @name link
* @desc File Upload Link
* @type {Function}
*/

function link() {

}

return {
restrict: 'E',
scope: {},
template: [
'<div>',
'</div>'
].join(''),
controllerAs: 'vm',
controller: fileUploadCtrl
link: link
};

}
angular
.module('app')
.directive('fileUpload', fileUpload);

Template integration

The next phase would be setting up some kind of file upload element with an <input type=file> with a model bound to it. Let’s add that to our Directive with ng-model attributes and values, and also an “upload” button with ng-change (yes we could make this a form with ng-submit but let’s keep it simple).

// ...
template: [
'<div>',
'<input type="file" ng-model="vm.files">',
'<button type="button" ng-change="vm.uploadFiles(vm.files);">',
'</div>'
].join('')
// ...

Let’s add some comments to our pseudo Controller to see how we’d handle the upload. Note I’ve added UploadService (great name) to the fileUploadCtrl arguments to be dependency injected.

/**
* @name fileUploadCtrl
* @desc File Upload Controller
* @type {Function}
*/

function fileUploadCtrl(UploadService) {

/**
* @name files
* @desc Contains all files passed in by the user
* @type {Array}
*/

var files = [];

/**
* @name uploadFiles
* @desc Uploads our files
* @type {Array}
*/

function uploadFiles(files) {
// hand off our files to a Service
UploadService
.uploadFiles(files)
.then(function (response) {
// success, we could get our file Object back
// and render it in the View for the user
// maybe some ng-repeat with a list of files inside
}, function (reason) {
// error stuff if not handled globally
})
}

// exports
this.files = files;
this.uploadFiles = uploadFiles;

}

Wait, not a lot has changed, why not? Let’s look at why.

Services (business logic layer)

Anything that communicates with an API, such as posting files off to a backend should never be done in a Controller, I repeat NEVER! Why? Separation of concerns. Of course we could do it, but that makes our lives harder as Controllers are to be treated as ViewModels, not ViewModelBusinessLogicThings.

I’m not going to write some pseudo code for our Service, but understanding why it’s passed into our Controller to abstract business logic is highly important to getting your Directive structures and dependencies manageable and scalable from the beginning.

All a Service should do is provide us with necessary Model data for our Controller (ViewModel) to make a copy of, and present to the user how we see fit.

Directives are fantastic as they offer us a door into our Directives that shouldn’t be handled in the presentational logic layer (Controller) or a business logic layer (Service) - that thing we call the DOM (Document Object Model). We need the DOM sometimes, and Angular gives it to us on a plate.

Our file upload Directive wouldn’t be complete without some drag and drop funk, so we’ll use the DOM events provided to us, dragover, drop etc. First let’s add a <div class="drop-zone"> to our Directive, which will serve as our “drag and drop” area.

// ...
template: [
'<div>',
'<input type="file" ng-model="vm.files">',
'<button type="button" ng-change="vm.uploadFiles(vm.files);">',
'<div class="drop-zone">Drop your files here!</div>',
'</div>'
].join('')
// ...

So, now we need to tie into our Directive. Our link function comes in handy here, I’ve also injected $scope, $element, $attrs (I like dollar-prefixing, sorry, iAttrs just makes me cry).

We’ll need to bind our special event listeners to our .drop-zone element. Remember, we want to keep our link functions as light as possible, I rarely augment $scope in them and neither should you.

Adding in some event listeners on our element:

/**
* @name link
* @desc File Upload Link
* @type {Function}
*/

function link($scope, $element, $attrs) {
var drop = $element.find('.drop-zone')[0];
drop.addEventListener('dragenter', function(e) {
// do something on "dragenter"
}, false);
drop.addEventListener("dragleave", function(e) {
// do something on "dragleave"
}, false);
drop.addEventListener("dragover", function(e) {
// do something on "dragover"
}, false);
drop.addEventListener('drop', function(e) {
// do something on "drop"
}, false);
}

Again, I’d comment and abstract these into a better, cleaner view, I don’t need to provide dragenter, dragleave or dragover support for this demo, so we’ll drop those:

/**
* @name link
* @desc File Upload Link
* @type {Function}
*/

function link($scope, $element, $attrs) {

/**
* @name drop
* @desc Drop zone element
* @type {Element}
*/

var drop = $element.find('.drop-zone')[0];

/**
* @name onDrop
* @desc Callback on "drop" event
* @type {Function}
* @param {Event} e Event passed in to grab files from
*/

function onDrop(e) {

}

// events
drop.addEventListener('drop', onDrop, false);

}

So what now, we’ve got our event listener setup, we can grab our files from e.dataTransfer.files and pass them off to our upload API, but we want to use the same function as in our Controller, the uploadFiles method.

We can pass our Controller into the Directive itself, I use a $ctrl alias just to keep it short and sweet, but this gives us access to our functions.

/**
* @name link
* @desc File Upload Link
* @type {Function}
*/

function link($scope, $element, $attrs, $ctrl) {

/**
* @name drop
* @desc Drop zone element
* @type {Element}
*/

var drop = $element.find('.drop-zone')[0];

/**
* @name onDrop
* @desc Callback on "drop" event
* @type {Function}
* @param {Event} e Event passed in to grab files from
*/

function onDrop(e) {
if (e.dataTransfer && e.dataTransfer.files) {
$ctrl.uploadFiles(e.dataTransfer.files);
}
}

// events
drop.addEventListener('drop', onDrop, false);

}

Smashing! Code reuse, using our Controller’s uploadFiles method to pass it back into our API, this will then make any presentational logic changes alongside it, as previously mentioned we might show the uploaded file(s) to the user, so this would all be reused and handled in the Controller.

But wait, it won’t work just yet… We forgot the magic line $scope.$apply():

/**
* @name onDrop
* @desc Callback on "drop" event
* @type {Function}
* @param {Event} e Event passed in to grab files from
*/

function onDrop(e) {
if (e.dataTransfer && e.dataTransfer.files) {
$ctrl.uploadFiles(e.dataTransfer.files);
// force a $digest cycle
$scope.$apply();
}
}

The $digest cycle will then be kicked off so Angular can run our code and update our application with any data that’s changed. We need to do this as the drop event listener is outside of Angular’s ecosystem and it never knew the event took place, so we have to tell it something’s happened.

All together now:

/**
* @name fileUpload
* @desc <file-upload> Directive
*/

function fileUpload () {

/**
* @name fileUploadCtrl
* @desc File Upload Controller
* @type {Function}
*/

function fileUploadCtrl(UploadService) {

/**
* @name files
* @desc Contains all files passed in by the user
* @type {Array}
*/

var files = [];

/**
* @name uploadFiles
* @desc Uploads our files
* @type {Array}
*/

function uploadFiles(files) {
// hand off our files to a Service
UploadService
.uploadFiles(files)
.then(function (response) {
// success, we could get our file Object back
// and render it in the View for the user
// maybe some ng-repeat with a list of files inside
}, function (reason) {
// error stuff if not handled globally
})
}

// exports
this.files = files;
this.uploadFiles = uploadFiles;

}

/**
* @name link
* @desc File Upload Link
* @type {Function}
*/

function link($scope, $element, $attrs, $ctrl) {

/**
* @name drop
* @desc Drop zone element
* @type {Element}
*/

var drop = $element.find('.drop-zone')[0];

/**
* @name onDrop
* @desc Callback on "drop" event
* @type {Function}
* @param {Event} e Event passed in to grab files from
*/

function onDrop(e) {
if (e.dataTransfer && e.dataTransfer.files) {
$ctrl.uploadFiles(e.dataTransfer.files);
// force a $digest cycle
$scope.$apply();
}
}

// events
drop.addEventListener('drop', onDrop, false);

}

return {
restrict: 'E',
scope: {},
template: [
'<div>',
'<input type="file" ng-model="vm.files">',
'<button type="button" ng-change="vm.uploadFiles(vm.files);">',
'<div class="drop-zone">Drop your files here!</div>',
'</div>'
].join(''),
controllerAs: 'vm',
controller: fileUploadCtrl
link: link
};

}
angular
.module('app')
.directive('fileUpload', fileUpload);

Recapping and MVVM (Model-View-ViewModel)

This approach allows the Controller to be used as a ViewModel, and to use the link function to properly deal with DOM manipulation whilst making easy work of communicating back to our Controller. The approach also promotes better separation of concerns, as well as separation of the code itself, such as breaking the functions out and assigning them, rather than nesting under another layer of code (such as inside an Object).

Any suggestions/improvements feel free to make changes/create issues on GitHub. Enjoy!

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