Communication Between Components Using @Input() in Angular

In Angular 2 a component can share data and information with another component by passing data or events. A component can be used inside another component, thus creating a component hierarchy. The component being used inside another component is known as the child component and the enclosing component is known as the parent component. Components can communicate to each other in various ways, including:

  • Using @Input()
  • Using @Output()
  • Using Services
  • Parent component calling ViewChild
  • Parent interacting with child using a local variable

In this article, we will focus on how a child component can interact with a parent component using the @Input() property. We’ll also look into intercepting the input message and logging changes in the input message.

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Let us consider the components created in the listing below. We have created a component called AppChildComponent, which will be used inside another component.

We have also created another component called AppComponent. Inside AppComponent, we are using AppChildComponent:

In the above listings, AppComonent is using AppChildComponent, hence AppComponent is the parent component and AppChildComponent is the child component.

Passing data from parent component to child component

Let us start with passing data from the parent component to the child component. This can be done using the input property. @Input decorator or input properties are used to pass data from parent to child component. To do this, we’ll need to modify child AppChildComponent as shown in the listing below:

As you notice, we have modified the greetMessage property with the @Input() decorator. Also, we have implemented onInit, which will be used in demos later. So essentially, in the child component, we have decorated the greetMessage property with the @Input() decorator so that value of greetMessage property can be set from the parent component.

Next, let us modify the parent component AppComponent to pass data to the child component.

From the parent component, we are setting the value of the child component’s property greetMessage. To pass a value to the child component, we need to pass the child component property inside a square bracket and set its value to any property of parent component. We are passing the value of childmessage property from the parent component to the greetMessage property of the child component.

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Intercept input from parent component in child component

We may have a requirement to intercept data passed from the parent component inside the child component. This can be done using getter and setter on the input property.

Let us say we wish to intercept an incoming message in the child component, and combine it with some string. To achieve this, we created a property called _greetmessage and using @Input() decorator creating getter and setter for _greetmessage property. In the getter, we’re intercepting the input from the parent component and combining it with the string. This can be done as shown in the next listing:

In the setter, we are manipulating incoming data from the parent component and appending some text to that. Keep in mind that the behavior of the parent component would not change whether we are intercepting the message or not. To explore it further, let us take another example and create a child component, which will display names passed from the parent. If the parent passes empty name value, then the child component will display some default name. To do this, we have modified the setter in the child component. In the setter, we are checking whether the name value is passed or not. If it is not passed or it is an empty string, the value of name would be assigned to a default value. The child component can be created as shown in the listing below:

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Simplifying Two-Way Data Binding in Angular 2

There are three types of data bindings in Angular 2, they are as follows:

  1. Interpolation
  2. Event Binding
  3. Property Binding

If you are coming from Angular 1.X background, you might be wondering that where is the two-way data binding? Remember, when first time you saw AngularJS 1.X demo, and was just blown away by power of ng-model? Yes, like you, I was also very impressed by power of two-way data binding in AngularJS 1. Even though, AngularJS 1 two-way data binding was beautiful, it came with the baggage of digest cycle and $watch.

To simplify the things, Angular 2 does not have any built in Two-Way data binding. It does not mean; you cannot have two-way data binding in Angular 2 application. Come on, we cannot think of creating a modern web application without having power of two-way data binding. So, in this post, we are going to learn, how to work with two-way data binding in Angular 2.

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Two-way data binding with ngModel

Angular 2 provides us a directive ngModel to achieve two-way data binding. It is very simple and straight forward to use ngModel directive as shown in the listing below:

To use ngModel directive, we need to import FormsModule in the application. For your reference, below I am listing app.module.ts which is importing FormsModule besides other required modules.

In above demo, when typing into the input element, the input’s value will be assigned to name variable and also it would be displayed back to the view. So we are implementing two-way data binding using ngModel as shown in the below image:

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Two-way data binding without ngModel

To understand ngModel directive working, let us see how we can achieve two-way data binding without using ngModel directive. To do that, we need to use

  1. Property binding to bind expression to value property of the input element. In this demo, we are binding name variable expression to value property.
  2. Event binding to emit input event on the input element. Yes, there is an input event which will be fired whenever user will input to the input element. Using event binding, input event would be bind to an expression.

So, using the property binding and the event binding, two-way data binding can be achieved as shown in the listing below:

Same like ngModel directive demo in this demo also, when typing into the input element, the input element’s value will be assigned to name variable and also it would be displayed back to the view.

So we are implementing two-way data binding without using ngModel using the code shown in the below image:

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Let us understand few important things here:

  1. [value]=”name” is the property binding. We are binding value property of the input element with variable (or expression) name.
  2. (input)= “expression” is event binding. Whenever input event will be fired expression will be executed.
  3. “name=$event.target.value” is an expression which assigns entered value to name variable.
  4. Name variable can be accessed inside AppComponent class.

So far we have seen two-way data binding using ngModel and without ngModel. We can conclude that the directive ngModel is nothing but combination of property binding and event binding. Event binding is denoted using small bracket and property binding is denoted using square [] bracket, and if you notice syntax of ngModel is [(ngModel)], which is like a banana put into a box suggests it is combination of both event and property binding.

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Introduction to Angular 2 Components

A component is a main building block of an Angular 2 application, and an application may have any number of components. We can consider a component a particular view of the application with its own logic and data.

In AngularJS 1.0, there was the concept of controllers, $Scope, and directives to bind data and logic to the view or to create custom elements on the view. In Angular 2, components perform all the tasks that were performed by controllers, scopes and directives. Data, logic and custom elements can all be created or added to the page using components in Angular 2.

Angular 2 applications are built around components, and we can consider component as a view with its own:

  • Template
  • Application Data
  • Logic
  • Styles, and more

Let’s Create Our First Component

Let us start with creating a very simple component to display “Hello World” on the page.

appcomponent.component.ts

 

There are mainly three steps to create a component

  1. Create a class and export it. This class will contain data and the logic.
  2. Decorate the class with @component metadata. Basically, metadata describes the component and sets the value for different properties, so that a TypeScript class can be used as an Angular 2 component.
  3. Import the required libraries and modules to create the component.

Three steps discussed above can be visualized in the diagram below:

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As you can see, the Angular 2 component consists of:

  • A TypeScript Class to hold data and the logic;
  • HTML template and styles to display data in the app. It is also called as a view, which is seen by the user on the screen to interact.
  • Metadata which defines the behavior of a component. Component metadata is applied to the class using the @Component decorator. Different behavior of the component can be passed as properties of the object, which is and input parameter of the @Component decorator.

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Exploring Attribute Directives in Angular 2

Attribute Directives are used to change the behavior, appearance or look of an element on a user input or via data from the service. Essentially, there are three types of directives in Angular 2:

  1. Component
  2. Structural directives
  3. Attribute directives

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In this post, we will learn how to create Attribute Directives in Angular 2. So let’s say we want to change the background color of an element; in that case we would apply the attribute directive to the element.

Create first Attribute directive

Let’s start with creating the Attribute Directive. To do this, we need to create a class and decorate it with @directive decorators. A simple attribute directive to change the color of an element can be created as shown in the next listing:

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Different ways of injecting dependency in an AngularJS Application

When you start learning the very first characteristics of AngularJS, you may come across something called Dependency Injection (DI): the premise that AngularJS injects dependencies whenever an application needs them. As a developer, our task is only to pass the dependency to the module and everything else will be taken care by AngularJS.

To create a controller, we pass $scope object and other dependencies to the module’s controller function. For example, to create a ProductController, we are passing $scope object and Calculator service dependencies. As a developer our job is to pass the dependencies and AngularJS will inject them whenever the application needs them.

As a developer, we really don’t care about how AngularJS injects dependencies – we don’t need to know how the injection process works to develop applications.  However, it is better if we know different ways of passing dependencies. In AngularJS, dependencies can be passed in three possible ways. They are as follows:

  • Passing a dependency as Function Arguments
  • Passing a dependency as Array Arguments
  • Passing a dependency using the $inject service

Let us explore these options one by one.

Passing a dependency as a Function Argument

Perhaps most of the time you pass a dependency as a function argument, which is perfectly fine. For example, we pass a $scope object to create a controller as shown in the listing below:

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What is a Provider () in AngularJS?

The provider() function allows us to create a configurable service where we can set input per application for the service created using the provider (). For example, if we need to set API key to access a service on the application level, we can set that in the module config and pass input to the provider using the $provide service. All the others ways to create services internally use the $provide service.

Creating a service using $provide service in module.config

Let us start by creating a very simple service using the provider() function.

	
app.config(function ($provide) {
    $provide.provider('globalsetting', function () {
        this.$get = function () {
            var appname = "Lawyer App";
            return {
                appName: appname
            };
        }
    })
});

Let’s explore what is going on in the above snippet. To create a service using provider, we need to use the $provide service. The provider function of the $provide service takes two parameters: the name of the service and the function. A provider function must have a $get function. To create a simple service using the provider(), we need to perform following five steps:

  1. Inject the $provide service in the app config method
  2. Create a provider using the provider() function
  3. Pass two parameters to the provider() function: the name of the service and a function
  4. The provider function must contain a $get function
  5. Return an object literal from the $get function

We can use the globalsetting service created using the provider by injecting it in a controller as shown in the listing below:

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How to Create a YouTube AngularJS Directive

I have often seen, developers come across requirement to embed a YouTube video in an AngularJS application. In this post, we will learn to create a simple YouTube AngularJS directive and also to use some of the popular directive from the GitHub.
We will follow step by step approach to create the YouTube custom directive. So let us start with create module and the controller.

var myApp = angular.module('myApp', []);
myApp.controller('VideoController', function ($scope) {
    $scope.video = 'zRtPUIumXcY';
});

Next we will create a custom directive with the isolated scope. Let us go ahead and create YouTube directive with following characteristics

  • Directive will be used as an Element
  • Directive will work in the isolated scope
  • Directive replace property is set to true, such that user will not able to view the directive information in the browser
  • In the template, iframe is used to play the YouTube video.
  • In the link function, we are watching the object passed to directive. Whenever value of object changed, directive will play the different video from the YouTube.

By putting all above together, directive is created as shown in the listing below:


myApp.directive('angularYoutube', function ($sce) {
    return {
        restrict: 'E',
        scope: { video: '=' },
        replace: true,
        template: '<div style="height:300px;"><iframe style="overflow:hidden;height:100%;width:100%" width="100%" height="100%" src="{{url}}" frameborder="1" allowfullscreen></iframe></div>',
        link: function (scope) {
            scope.$watch('video', function (newVal) {
                if (newVal) {
                    scope.url = $sce.trustAsResourceUrl("http://www.youtube.com/embed/" + newVal);
                }
            });
        }
    }
})

As you might notice in above listing that in the template, we are using the iframe to play the YouTube video. Also we are watching the value passed to the directive and constructing the URL using the $sce service of AngularJS.
On the view, angularYoutube directive can be used as shown in the listing below:

<div ng-controller="VideoController">
        <angular-youtube video="video"></angular-youtube>
    </div>

As of now, we should able to play a video in AngularJS application. Right now we are passing hard coded video code from the controller. We can allow user to pass the video code, just by using an input textbox as shown in the listing below:


<div ng-controller="VideoController">
        <input type="text" ng-model="video" placeholder="enter video code here to play"/>
        <hr/>
        <angular-youtube video="video"></angular-youtube>
    </div>


Here we have created a very simple custom AngularJS directive to embed the YouTube video in the AngularJS application.
For advanced scenarios, you may want to use Angular YouTube Embed . I find it very useful for advanced scenarios. I hope you find this post useful. Thanks for reading.

How to create Custom Filters in AngularJS

Have you ever used filters with the ng-repeat directive as shown in the listing below?

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If so, then you’ve used a filter in an AngularJS application. AngularJS provides us many in-built directives like search. If required, AngularJS also allows us to create custom filters, which we’ll explore in this post.

AngularJS gives us a simple API to create a custom filter. You’ll remember that we use app.controller() to create controllers and app.module() to create modules. In exactly the same way, AngularJS has given us the angular.filter API to create a custom filter in AngularJS.

A custom filter can be created using the following syntax:

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How to work with the Ignite UI Chart in an AngularJS application

In this post we will learn how to work with the Ignite UI chart in an AngularJS application. Although I will use the ASP.NET Web API to pull data from the database, you can use REST service or a Web API created on any stack with the Ignite UI charts in your AngularJS application. This article is divided in two sections:

  1. Section 1 : creating ASP.NET Web API using Code First approach
  2. Section 2 : using Ignite UI in AngularJS application

If you already have or know how to create the Web API REST Service (Note: from here on out in this post, we’ll use the term “Web API” to refer both to REST Service and Web API), you can jump to section 2 of this article. On the other hand if you need help in how to create ASP.NET Web API start from the section 1. Here’s a high level flow of diagram of an application::

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Validating User Input on a Form in Angular JS

I have often seen entry level developers struggling with user input validation in AngularJS single page applications. In this post, I will give a quick but useful introduction of validations in AngularJS; consider this post as a base learning document from which you can do further learning.

Let’s start with an example as shown in the image below. You have a registration form with three fields with the following restrictions.

  1. Name : Required
  2. Email : Required and type Email
  3. Password : Required , type password and minimum length 6

We want to validate the rules mentioned above at the client side. There are two ways client side validation can be done in an AngularJS based single page application:

  1. Using the HTML5 validations
  2. Using the AngularJS validation directives
  3. A Combination of both

HTML5 validation

Here we’ve created the Add user form shown above using the mark up listed below:

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