Friday, July 22, 2016

Component Lifecycle: $doCheck (angular 1.5.x)

Since angular 1.5 components got introduced together with a well defined lifecycle. Currently their are 4 hooks you can use in angular 1 components:

  • $onInit
  • $onChanges
  • $postLink
  • $onDestroy


In version 1.5.8 a new hook is introduced: $doCheck. And this is the equivalent of the angular 2 ngDoCheck implementation. It also serves the same purpose as the $onChanges, allow to act on changes made to the bindable fields of a component. As $onChanges uses the built-in change detection of angular, the $doCheck implementation is totally up to you. The hook is called for every digest cycle of the component and just let’s you know you should check your bindings on changes so you can act on it.


One of the case this could be useful is when you make use of the one-way (<) binding for passing objects. In this case the $onChanges hook will be called if the reference of the object changes and not when fields on the object it self change. So currently you had 2 possibilities to solve this:

  1. Always make sure you are passing a new object. This way $onChanges hook will be called for every change because the reference of the object will change from time to time.
  2. Add a watch on the object to keep track of the changes. This also means you need to destroy and recreate the the watch every the reference of the object changes and you have an (unwanted) dependency on $scope inside your component.
   1: module.component("component",{
   2:     template: "<div>{{$ctrl.item}}</div>",
   3:     bindings: {
   4:         inputItem: "<item"
   5:     },
   6:     controller: ["$scope", function($scope){
   7:         var $ctrl = this;
   8:         var destroyWatch;
   9:         this.$onChanges = function(changeObj){
  10:             if(changeObj.inputItem){
  11:                 this.item = 
  12:                   angular.copy(changeObj.inputItem.currentValue);
  13:                 if(destroyWatch) destroyWatch();
  14:                 destroyWatch = $ (){ 
  15:                     return changeObj.inputItem.currentValue 
  16:                 }, function (){ /* handle Changes */ })
  17:             }
  18:         }
  19:     }
  20: }]);

The $doCheck hook now adds a third possibility to solve this issue. By checking manually if the object has changed you can act on it. This can be done by storing the passed value into a local variable, so it can be used in the next call as previous value for comparison.

   1: module.component("component",{
   2:     template: "<div>{{$ctrl.item}}</div>",
   3:     bindings: {
   4:         inputItem: "<item"
   5:     },
   6:     controller: function(){
   7:         var $ctrl = this;
   8:         var previousInputItem;
   9:         this.$doCheck = function(){
  10:             if(!angular.equals(previousInputItem, this.inputItem)){
  11:                 previousInputItem = this.inputItem;
  12:                 this.item = angular.copy(this.inputItem);
  13:             }
  14:         }
  15:     }
  16: });


Change detection in angular 1.x is done using digest cycles and for every cycle the $doCheck hook will be called. This means this will be called a lot. This is why you have the be careful using this hook so it doesn’t cause any performance issues. Also keep in mind that any change made to the model inside the $doCheck hook will trigger a new digest cycle. If implemented wrong this can result into a loop of digest cycles.

In angular 2 the change is implemented on a different (more performant) way and this will result in less calls of the ngDoCheck. It will also throw an error if you trigger changes outside of the component in prod mode.


The $doCheck hook gives you the ability to take the change detection of the bindable fields into your own hands. This makes it possible to detect changes inside objects and arrays without having to change the reference. Ditto for detecting changes in date objects.

The $onChanges and $doCheck hook can easily live side-by-side. The $doCheck hook doesn’t effect the $onChanges hook at all. Of course fields checked in the $doCheck hook no longer need to be handled in the $onChanges hook. In angular 2 it’s even recommended to not use these 2 hooks together.

Sunday, May 1, 2016

Components in angular 1.5.x

In my previous article I described my view on a component based architecture. In this post I want to focus on how I applied this in a real life application with angular. Since the 1.5 version was released, components have become first class citizens just like in Angular 2, Ember 2, React,… On the other hand components aren’t so revolutionary. We have already known them for a long time as directives, but we never used them like this. Generally, we would only use directives for manipulating the DOM directly or in some cases to build reusable parts.
The components in angular 1.5 are a special kind of directive with a bit more restrictions and a simpler configuration. The main differences between directives and components are:
  • Components are restricted to elements. You can’t write a component as an attribute.
  • Components always have an isolated scope. This enforces a clearer dataflow. No longer will we have code that will change data on shared scopes.
  • No more link functions, but only a well-defined API on the controller.
  • A component is defined by an object and no longer by a function call.

Well-defined Lifecycle

Components have a well-defined lifecycle:
  • $onInit: This callback is called on each controller after all controllers for the element are constructed and their bindings initialized. In this callback you are also sure that all the controllers you require on are initialized and can be used. (since angular 1.5.0)
  • $onChanges: This callback is called each time the one-way bindings are updated.  The callback provides an object containing the changed bindings with their current- and previous value. Initially this callback is called before the $onInit with the original values of the bindings at initialization time. This is why this is the ideal place for cloning your objects passed through the bindings to ensure modifications will only affect the inner state.
    Please be aware that the changes callback on the one-way bindings for objects will only be triggered if the object reference changes. If a property inside the object changes, the changes callback won’t be called. This avoids adding a watch to monitor the changes made on the parent scope (works correctly since angular 1.5.5)
  • $postLink: This is called once all child elements are linked. This is similar to the (post) link function inside directives. Setting up DOM handlers or direct DOM manipulation can be done here. (since angular 1.5.3)
  • $onDestroy: This is the equivalent of the $destroy event emitted by the scope of a directive/controller. The ideal place to clean up external references and avoid memory leaks. (since angular 1.5.3)

Well-defined structure

Components also have a clean structure. They exist out of 3 parts:
  • A view, this can be a template or an external file.
  • A controller which describes the behaviour of the component.
  • Bindings, the in- and outputs of the component. The inputs will receive the data from a parent component and by using callbacks, will inform the parent component of any changes made to the component.


Because components should only be allowed to modify their internal state and should never modify any data directly outside its component and scope. We should no longer make use of the commonly used two-way binding (bindings: { twoWay: “=” } ). Instead since angular 1.5 we now have a one-way binding for expressions (bindings: { oneWay: “<” } ).
The main difference with the two-way binding is the fact that the bound properties won’t be watched inside the component. So if you assign a new value to the property, it won’t affect the object on the parent. But be careful, this doesn’t apply on the fields of the property, that is why you always should clone the objects passed through the bindings if you want to change them. A good way to do this is working with named bindings, this way you can reuse the name of the bound property inside the component without affecting the object in the parent scope.
   1: module.component("component",{
   2:     template: "<div>{{$ctrl.item}}</div>"
   3:     bindings: {
   4:         inputItem: "<item"
   5:     }
   6:     controller: function(){
   7:         var $ctrl = this;
   8:         this.$onChanges = function(changeObj){
   9:             if(changeObj.inputItem){
  10:                 this.item = 
  11:                     angular.clone(changeObj.inputItem.currentValue);
  12:             }
  13:         }
  14:     }
  15: });
Another way to pass data is by using the “@” binding. This can be used if the value you are passing is a string value. The last way is using a “&” binding or a callback to retrieve data through a function call on the parent. This can be useful if you want to provide an auto complete component with search results data.
For exposing the changes inside the component we can only make use of the “&” binding. Calling a callback in the parent scope is the only way that we can and should pass data to the parent scope/component.
Let’s rephrase a little:
  • “=” binding (two-way) should no longer be used to avoid unwanted changes on the parent’s scope.
  • “<” binding (one-way) should be used to retrieve data from the parent scope passed as an expression.
  • “@” binding (string) should be used to retrieve string values from the parent scope.
  • “&” binding (callback) can be used to either retrieve data from the parent scope or be used as the only way to pass data to the parent scope.


The components way of working in angular 1.5 closes the gap for migrating your code to an angular 2 application a bit more. By building your apps in the angular 2 style you are already thinking on the same level. This will ease the migration path by shifting away from the controller way of working.
Directives will still have an existence beside the components. You will still have some cases where you will want to use attributes instead of elements or have the need of a shared scope for the UI state. But in most cases components will do the trick.

Saturday, April 23, 2016

Component based architecture

Once angular 1.5 got released and components seems to be the future of Frond-end development, I started looking to Component Based Architecture. A good well defined component is easy to reuse and the flow of the data is very clear. That is way it is also easily testable. Next to these dumb/presentational components we also have some smart components who are aware of services/routing. These smart components control the dumb/presentational components. They are responsible to act on the changes made in the dumb/presentational components and delegate these changes back.
Further on in the post I will only speak of dumb components, but the same applies on presentational components.
The schema gives you an overflow of how every thing works together. The redlines show the flow of the data. The blue line are the change events emitted by the dumb components. The smart component is just responsible for retrieving and storing the data and keep everything consistent. This schema I found on a very good blog post about Refactoring angular apps to components.

Smart Components

We call the Smart Components smart, because they have a notion where they can retrieve and store the data. They are able to communicate with services and provide the dumb components of their data. They also handle the actions/events raised by the dumb components and act upon them. This can be storing the data to a service or updating a local data context, but also providing dumb components which have auto complete fields with their items.
In most cases smart components won’t have a lot of markup. Mostly the mark-up will be present in the dumb components, but it’s not wrong to have some in the smart component. (In most cases this will be markup to structure your markup or parts that aren’t or won’t be reusable.)
By making your smart component the single source of truth you can easier manage the behavior of your dumb components. I’ll explain this by a small example:
We have a list of translations. Every translation has a language which has to be unique inside the list. So when adding a new translation we want all the languages who are already present filtered out of the language choices. If we would retrieve the list of languages inside the adding component, then the adding would need to have a notion of either the list of translations or the component managing the translations list. We don’t want any of both situations because this would decrease the reuse of the adding component. By retrieving this data from the smart component, we can work around this. Because the smart component knows the translations list, it can easily filter out the existing languages before passing them to the adding component.
Just because the fact that smart components are your single source of truth makes them hard even impossible to reuse. It is more likely that you will reuse your dumb components with other services then vice versa. This is also the reason why you will have little smart components and many dumb components inside your application.

Dumb or Presentational Components

As you could have guessed the dumb components are the opposite of the smart component. Instead of retrieving data, the data is passed to them and if they make changes to the data, these changes are exposed using action/events. As you can see these components have a well-defined interface with all their in- and outputs. This makes the purpose of your component very clear and will help you to improve your understanding of your application even better for you and your team.
The  goal of a dumb component is that you can easily reuse it inside your application. That is why it’s good to keep your dumb small. This will increase the chance you will be able to reuse it. This should also be a trigger to help you decided whether you should split your code up into a component or not.
Another way to help you decide how to split up into dumb components is by keeping the design principle “Separation of concerns” in mind. Every concern can be a potential component. My opinion on this is to be pragmatic on this. If your not planning on reusing some parts then don’t just split them up jus because they would be great components. Keep them in a single component, and if the day comes that you can extract some reusable components, you can still refractor them.
Because dumb components are small and well defined they are also very easy to test. You don’t need to mock anything because you can just pass the data directly to your component.


By implementing this way of working into my current project, I was able to extract a lot of reusable components that in other cases would have been copied with minor adjustments. Now they live as separate components that can be easily maintained.
It also made my code more readable. Instead of big chunks of codes in a single controller, I now have my well-defined components implementing their own concerns. By hiding the implementations details inside the components my smart component is very clean in code but still very clear and readable.
In my next post I will handle on how I implemented this approach inside an angular 1.5 application.

Wednesday, April 13, 2016

Project setup in Angular 1.x

Since 6 months now I have been working on an Angular 1.x project. In the start I have been struggling with the setup of the project, but I found a good way to start with the Angular 1 Style guide of John Papa. Combined with an module loader I optimized this setup.

Below you will find an example how to structure your application. The examples I will provide will use es6, but the concept behind it can easily be used with module loaders like commonJS and requireJS. In these cases just the imports of the files and the export of the file it self is handle a bit different.



As you can see in every folder structure I follow the same pattern. I have the following files:

  • index.js
    • The entry point of the folder
  • *.module.js
    • The definition of the angular module
  • *.routes.js
    • The routing definitions
  • other files
    • Controllers
    • Directives
    • Components
    • Services
    • constants

    • ….


The *.module.js file is used to define the angular module. In here we import angular and all the modules on which the module should depend on. If we are adding internal dependencies, we can do this by importing the index file of that module. This index file will init the module and expose the name of the module. (example: feature.module.js)

The module it self is exported so we can use it when we want to add services, controllers, components, … to it.


   1: import angular from "angular";
   2: // import external dependencies necessary in the module
   4: export default angular.module("myApp.feature.subFeature", []);


   1: import angular from "angular";
   2: import subFeatureModule from "./subFeature/index";
   3: // import external dependencies necessary in the module.
   5: export default angular.module("myApp.feature", [subFeatureModule]);


   1: import angular from "angular";
   2: import featureModule from "./feature/index";
   3: import "angular-route";
   4: // import external dependencies necessary in the module.
   6: export default angular.module("myApp", [featureModule, "ngRoute"]);


The *.routers.js is used to define the routing in your app for the feature. If you are using the new component router, this isn’t necessary because the routes will be defined inside the components.

   1: import FeatureModule from "./feature.module";
   3: FeatureModule.config(function($routeProvider, $locationProvider) {
   4:   $routeProvider
   5:   .when('/Feature', {
   6:     templateUrl: 'controller.html',
   7:     controller: 'Controller'
   8:   });
   9: });


The index.js file is the entry file for the folder. In here we import all the components, controllers, services, … we want to add to the module. We also import the *.module.js file so we can expose the module name. This way we can easily use it to add the module as a dependency in another module.

The name of this file is irrelevant, you can also use something like init. The reason I use index.js is because this is the default entry point of a folder in webpack.


   1: import "./component";
   2: import "./constant";
   3: import "./directive"
   4: import SubFeatureModule from "./subFeature.module";
   6: export default;


   1: import "./component"
   2: import "./controller";
   3: import "./service";
   4: import "./feature.routes";
   5: import featureModule from "./feature.module";
   7: export default;


   1: import MyAppModule from "./myApp.module";
   3: export default;

Other files

All the other files like (services, controllers, directives, constants, …) will import the *.module.js file. Every thing else necessary to define the service, controller, directive, … will also be present in this file. This way everything is present on a single place.


   1: import SubFeatureModule from "./subFeature.module";
   2: // other imports
   4: ComponentController.$inject = [];
   6: SubFeatureModule.component("component", {
   7:     template: "<div></div>",
   8:     controller: ComponentController,
   9: })
  11: function ComponentController(){}


   1: import SubFeatureModule from "./subFeature.module";
   2: // other imports
   4: SubFeatureModule.constant("constant", {
   5:     "key": value
   6: })


By defining the component in the *.module.js file we have a single point where we can find the definition of the module. By exposing the module we also create a single point where we keep the name of the module. You no longer need to use the name of the module if you want to add a service/controller/… to it. This eliminates typo’s in the module names.

The index.js provides a single entry point for the module. All the services/controllers/… you want to add to the module should be added in here. Also we import the *.module.js file. This gives us the ability to expose the name of the module. These names can then be use to be added as dependency without having to know it string value. Again this eliminates typo’s in the module names.

In all the other files the definition and resolving of the dependencies can be put together. This downsizes the chance on errors and dependency mismatches