include ../../../../_includes/_util-fns
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The Developers Guide is a practical guide to Angular for experienced programmers who
are building client applications in HTML and JavaScript.
figure
img(src="/resources/images/devguide/intro/people.png" alt="Us" align="left" style="width:200px; margin-left:-40px;margin-right:10px" )
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We are on a journey together to understand how Angular works and, more importantly,
how to make it work for us. We look at our application requirements and we see problems to solve.
* How do we get data onto the screen and handle user interactions?
* How do we organize our code into manageable, cohesive chunks of functionality that work together?
* What are the essential Angular building blocks and how do they help?
* How do we minimize routine, mechanical coding in favor of declarative, higher level constructs without losing control?
This chapter begins the journey. It's an introduction to Angular in two parts.
1. [How to read this guide](#how-to-read)
2. [An architecture overview](#architecture)
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# How to Read this Guide
The chapters of this guide target an Angular feature and how to use it to solve a programming problem.
Every chapter includes code snippets ... snippets we can reuse in our own applications.
Typically, these snippets are excerpts from a running sample application that accompanies the chapter.
We can often find a link to a live version of that sample near the top of the chapter ... like this one.
[Live Example](/resources/live-examples/intro/ts/src/plnkr.html)
This link opens in a browser and runs the sample code supporting this chapter's architecture overview.
We can inspect, modify, save, and download the code.
A few chapters are written as tutorials and are clearly marked as such.
Most chapters are *not* tutorials and do not explain how to build the accompanying sample.
These chapters highlight key points in code but generally don't include the entire source.
We can get that by way of the live link.
We don't have to read this guide straight through.
The "[Cheat Sheet](cheatsheet.html)" is a handy map to Angular overall.
A few early chapters are arranged sequentially and best read together to establish a foundation in Angular.
But most chapters stand on their own. We can browse to any of them as our interest or some necessity moves us.
There is a learning path for those of us who are new to Angular.
It starts *outside* the guide with the [QuickStart](../quickstart). The QuickStart is the "Hello, World" of Angular 2.
It shows us how to setup the libraries and tools we'll need to write *any* Angular app.
It ends with a "proof of life", a running Angular app.
The [Tutorial](../tutorial) is our next stop.
It walks us step-by-step from where QuickStart leaves off to a simple data-driven app. Simple, yes, but with
the essential characteristics we'd expect of a professional application:
a sensible project structure, data binding, master/detail, services, dependency injection, navigation, and remote data access.
The final iteration of the "[Tour of Heroes](#toh)" is a positive answer to that most important question:
***can we build an Angular 2 application that does what we need it to do?***
We suggest continuing from the "Tour of Heroes" tutorial to the early core chapters of this Developers Guide in the following order:
1. [Displaying Data](displaying-data.html)
1. [User Input](user-input.html)
1. [Forms](forms.html)
1. [Dependency Injection](dependency-injection.html)
We might pause at that point to absorb what we've learned and perhaps experiment on our own before diving into [Template Syntax](template-syntax.html).
That chapter is rather dense. It's also essential reading at some point and
we will return to it frequently as we compose our HTML templates, both for understanding and as a reference.
Follow your own star from there, wherever it leads.
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# Architecture Overview
Angular 2 is a framework to help us build client applications in HTML and JavaScript.
The framework consists of several cooperating libraries, some of them core and some optional.
We write applications by composing HTML *templates* with Angularized-markup,
writing *component* classes to manage those templates, adding application logic in *services*,
and handing the top root component to Angular's *bootstrapper*.
Angular takes over, presenting our application content in a browser and responding to user interactions
according to the instructions we provided.
figure
img(src="/resources/images/devguide/intro/airplane.png" alt="Us" align="left" style="width:200px; margin-left:-40px;margin-right:10px" )
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Of course there is more to it than this.
We're cruising at high altitude in this overview.
We're looking for landmarks. We should expect the object below to be fuzzy and obscured by occasional clouds.
Details become more clear and precise when we land in the chapters themselves.
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An Angular 2 application rests on seven main building blocks:
1. [Component](#component)
1. [Template](#template)
1. [Metadata](#metadata)
1. [Data Binding](#data-binding)
1. [Directive](#directive)
1. [Service](#service)
1. [Dependency Injection](#dependency-injection)
figure
img(src="/resources/images/devguide/intro/overview.png" alt="overview" style="margin-left:-40px;" )
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Learn these seven and we're on our way.
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## The Component
figure
img(src="/resources/images/devguide/intro/hero-component.png" alt="Component" align="left" style="width:200px; margin-left:-40px;margin-right:10px" )
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A **Component** controls a patch of screen real estate that we could call a *view*.
The shell at the application root with navigation links, that list of heroes, the hero editor ...
they're all views controlled by Components.
We define a Component's application logic - what it does to support the view - inside a class.
The class interacts with the view through an API of properties and methods.
A `HeroListComponent`, for example, might have a `heroes` property that returns an array of heroes
that it acquired from a service.
It might have a `selectHero()` method that sets a `selectedHero` property when the user click on a hero from that list.
It might be a class like this:
+makeExample('intro/ts/src/app/hero-list.component.ts', 'class')
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Angular creates, updates, and destroys components as the user moves through the application.
The developer can take action at each moment in this lifecycle through optional [Lifecycle Hooks](#).
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We're not showing those hooks in this example
but we are making a mental note to find out about them later.
We may wonder who is calling that constructor? Who provides the service parameter?
For the moment, have faith that Angular will call the constructor and deliver an
appropriate `HeroService` when we need it.
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## The Template
figure
img(src="/resources/images/devguide/intro/template.png" alt="Template" align="left" style="width:200px; margin-left:-40px;margin-right:10px" )
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We define a Component's view with its companion **template**. A template is a form of HTML
that tells Angular how to render the Component.
A template looks like regular HTML much of the time ... and then it gets a bit strange. Here is a
template for our `HeroList` component
+makeExample('intro/ts/src/app/hero-list.component.html')
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We recognize `
` and `
`.
But there's other markup that no one told us about in school.
What is`*ng-for`, {{hero.name}}, `(click)`, `[hero]`, and ``?
These are examples of Angular's [template syntax](template-syntax.html).
We will grow accustomed to that syntax and may even learn to love it.
We'll begin to explain it in a moment.
Before we do, focus attention on the last line.
The `` tag is a custom element representing the `HeroDetailComponent`.
The `HeroDetailComponent` is a *different* component than the `HeroListComponent` we've been reviewing.
The `HeroDetailComponent` (code not shown) presents facts about a particular hero, the
hero that the user selects from the list presented by the the `HeroListComponent`.
The `HeroDetailComponent` is a **child** of the the `HeroListComponent`.
figure
img(src="/resources/images/devguide/intro/component-tree.png" alt="Metadata" align="left" style="width:300px; margin-left:-40px;margin-right:10px" )
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Notice how `` rests comfortably among the HTML elements we already know.
We can mix ... and will mix ... our custom components with native HTML in the same layouts.
And in this manner we can and will compose complex component trees to build out our richly featured application.
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## Angular Metadata
figure
img(src="/resources/images/devguide/intro/metadata.png" alt="Metadata" align="left" style="width:150px; margin-left:-40px;margin-right:10px" )
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Metadata tells Angular how to process a class.
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[Looking back](#component-code) at the `HeroListComponent`, we see that it's just a class.
There is no evidence of a framework, no "Angular" in it at all.
In fact, it really is *just a class*. It's not a component until we *tell Angular about it*.
We tell Angular that `HeroListComponent` is a component by attaching **metadata** to the class.
The easy way to attach metadata in TypeScript is with a **decorator**.
Here's some metadata for `HeroListComponent`:
+makeExample('intro/ts/src/app/hero-list.component.ts', 'metadata')
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Here we see the `@Component` decorator which (no surprise) identifies the class
immediately below it as a Component class.
A decorator is a function. Decorators often have a configuration parameter.
The `@Component` decorator takes a required configuration object with the
information Angular needs to create and present the component and its view.
Here we see a few of the possible `@Component` configuration options:
* `selector` - a css selector that tells Angular to create and insert an instance of this component
where it finds a `` tag in *parent* HTML.
If the template of the application shell (a Component) contained
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>Angular inserts an instance of the `HeroListComponent` view between those tags.
* `templateUrl` - the address of this component's template which we showed [above](#the-template).
* `directives` - an array of the Components or Directives that *this* template requires.
We saw in the last line of our template that we expect Angular to insert a `HeroDetailComponent`
in the space indicated by `` tags.
Angular will do so only if we mention the `HeroDetailComponent` in this `directives` array.
* `providers` - an array of **dependency injection providers** for services that the component requires.
This is one way to tell Angular that our component's constructor requires a `HeroService`
so it can get the list of heroes to display. We'll get to dependency injection in a moment.
figure
img(src="/resources/images/devguide/intro/template-metadata-component.png" alt="Metadata" align="left" style="height:200px; margin-left:-40px;margin-right:10px" )
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The `@Component` function takes the configuration object and turns it into metadata that it attaches
to the component class definition. Angular discovers this metadata at runtime and thus knows how to do "the right thing".
The template, metadata, and component together describe the view.
We apply other metadata decorators in a similar fashion to guide Angular behavior.
The `@Injectable`, `@Input`, `@Output`, `@RouterConfig` are a few of the more popular decorators
we'll master as our Angular knowledge grows.
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The architectural take-away is that we must add metadata to our code
so that Angular knows what to do.
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## Data Binding
Without a framework, we would be responsible for pushing data values into the HTML controls and turning user responses
into actions and value updates. Writing such push/pull logic by hand is tedious, error-prone and a nightmare to
read as the experienced jQuery programmer can attest
figure
img(src="/resources/images/devguide/intro/databinding.png" alt="Data Binding" style="width:220px; float:left; margin-left:-40px;margin-right:20px" )
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Angular supports **data binding**,
a mechanism for coordinating parts of a template with parts of a component.
We add binding markup to the template HTML to tell Angular how to connect both sides.
There are four forms of data binding syntax. Each form has a direction - to the DOM, from the DOM, or in both directions -
as indicated by the arrows in the diagram.
:marked
We saw three forms of data binding in our [example](#template) template:
+makeExample('intro/ts/src/app/hero-list.component.1.html', 'binding')(format=".")
:marked
* The {{hero.name}} "[interpolation](displaying-data.html#interpolation)"
displays the component's `hero.name` property value within the `
` tags.
* The `[hero]` [property binding](template-syntax.html#property-binding) passes the `selectedHero` from
the parent `HeroListComponent` to the `hero` property of the child `HeroDetailComponent`.
* The `(click)` [event binding](user-input.html#click) calls the Component's `selectHero` method when the user clicks
on a hero's name
**Two-way data binding** is an important fourth form
that combines property and event binding in a single notation using the `ng-model` directive.
We didn't have a two-way binding in the `HeroListComponent` template;
here's an example from the `HeroDetailComponent` template (not shown):
+makeExample('intro/ts/src/app/hero-detail.component.html', 'ng-model')(format=".")
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In two-way binding, a data property value flows to the input box from the component as with property binding.
The user's changes also flow back to the component, resetting the property to the latest value,
as with event binding.
Angular processes *all* data bindings once per JavaScript event cycle,
depth-first from the root of the application component tree.
figure
img(src="/resources/images/devguide/intro/component-databinding.png" alt="Data Binding" style="float:left; width:300px; margin-left:-40px;margin-right:10px" )
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We don't know all the details yet
but it's clear from these examples that data binding plays an important role in communication
between a template and its component ...
figure
img(src="/resources/images/devguide/intro/parent-child-binding.png" alt="Parent/Child binding" style="float:left; width:300px; margin-left:-40px;margin-right:10px" )
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... ***and*** between parent and child components
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## The Directive
figure
img(src="/resources/images/devguide/intro/directive.png" alt="Parent child" style="float:left; width:150px; margin-left:-40px;margin-right:10px" )
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Our Angular templates are *dynamic*. When Angular renders them, it transforms the DOM
according to the instructions given by a **directive**.
A directive is a class with directive metadata. In TypeScript we'd apply the `@Directive` decorator
to attach metadata to the class.
:marked
We already met one form of directive: the component. A component is a *directive-with-a-template*
and the `@Component` decorator is actually a `@Directive` decorator extended with template-oriented features.
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While the **component is technically a directive**,
it is so distinctive and central to Angular applications that we chose
to separate the component from the directive in our architectural overview.
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There are two *other* kinds of directives as well that we call "structural" and "attribute" directives.
They tend to appear within an element tag like attributes,
sometimes by name but more often as the target of an assignment or a binding.
**Structural** directives alter layout by adding, removing, and replacing elements in DOM.
We see two built-in structural directives at play in our [example](#template) template:
+makeExample('intro/ts/src/app/hero-list.component.1.html', 'structural')(format=".")
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* [`*ng-for`](displaying-data.html#ng-for) tells Angular to stamp out one `
` per hero in the `heroes` list.
* [`*ng-if`](displaying-data.html#ng-if) includes the `HeroDetail` component only if a selected hero exists.
**Attribute** directives alter the appearance or behavior of an existing element.
In templates they look like regular HTML attributes, hence the name.
The `ng-model` directive, which implements two-way data binding, is an example of an attribute directive.
+makeExample('intro/ts/src/app/hero-detail.component.html', 'ng-model')(format=".")
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It modifies the behavior of an existing element (typically an ``)
by setting its display value property and responding to change events.
Angular ships with a small number of other directives that either alter the layout structure
(e.g. [ng-switch](template-syntax.html#ng-switch))
or modify aspects of DOM elements and components
(e.g. [ng-style](template-syntax.html#ng-style) and [ng-class](template-syntax.html#ng-class)).
And of course we can write our own directives.
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## The Service
figure
img(src="/resources/images/devguide/intro/service.png" alt="Service" style="float:left; margin-left:-40px;margin-right:10px" )
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"Service" is a broad category encompassing any value, function or feature that our application needs.
Almost anything can be a service.
A service is typically a class with a narrow, well-defined purpose. It should do something specific and do it well.
:marked
Examples include:
* logging service
* data service
* message bus
* tax calculator
* application configuration
There is nothing specifically "Angular" about services. Angular itself has no definition of a "service".
There is no service base class, no place to register a "service".
Yet services are fundamental to any Angular application. Our components are big consumers of service.
We prefer our component classes lean. Our components don't fetch data from the server,
they don't validate user input, they don't log directly to console. They delegate such task to services.
A component's job is to enable the user experience and nothing more. It mediates between the view (rendered by the template)
and the application logic (which often includes some notion of a "model").
A good component presents properties and methods for data binding.
It delegates everything non-trivial to services.
Angular doesn't *enforce* these principles.
It won't complain if write a "kitchen sink" component with 3000 lines.
Angular does help us *follow* these principles ... by making it easy to factor our
application logic into services and make those services available to components through *dependency injection*.
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## Dependency Injection
figure
img(src="/resources/images/devguide/intro/dependency-injection.png" alt="Service" style="float:left; width:200px; margin-left:-40px;margin-right:10px" )
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"Dependency Injection" is a way to supply a new instance of a class
with the fully-formed dependencies it requires. Most dependencies are services.
Angular uses dependency injection to provide new components with the services they need.
:marked
In TypeScript, Angular can tell which services a component needs by looking at the types of its constructor parameters.
For example, the constructor of our `HeroListComponent` needs the `HeroService`:
+makeExample('intro/ts/src/app/hero-list.component.ts', 'ctor')
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When Angular creates a component, it first asks an **Injector** for
the services that the component requires.
An `Injector` maintains a container of service instances that it has previously created.
If a requested service instance is not in the container, the injector makes one and adds it to the container
before returning the service to Angular.
When all requested services have been resolved and returned,
Angular can call the component's constructor with those services as arguments.
This is what we mean by *dependency injection*.
The process of `HeroService` injection looks a bit like this:
figure
img(src="/resources/images/devguide/intro/injector-injects.png" alt="Service" )
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If the `Injector` doesn't have a `HeroService`, how does it know how to make one?
In brief, we must have previously registered a **provider** of the `HeroService` with the `Injector`.
A provider is something that can create or return a service, typically the service class itself.
We can register providers at any level of the application component tree.
We often do so at the root when we bootstrap the application so that
the same instance of a service is available everywhere.
+makeExample('intro/ts/src/app/boot.ts', 'bootstrap')
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Alternatively, we might register at a component level ...
+makeExample('intro/ts/src/app/hero-list.component.ts', 'providers')
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... in which case we get a new instance of the
service with each new instance of that component.
We've vastly over-simplified dependency injection for this overview.
We can learn the full story in the [Dependency Injection](dependency-injection.html) chapter.
The points to remember are:
* dependency injection is wired into the framework and used everywhere.
* the `Injector` is the main mechanism.
* an injector maintains a *container* of service instances that it created.
* an injector can create a new service instance from a *provider*.
* a *provider* is a recipe for creating a service.
* we register *providers* with injectors.
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# The Other Stuff
We've learned just a bit about the seven main building blocks of an Angular application
1. [Component](#component)
1. [Template](#template)
1. [Metadata](#metadata)
1. [Data Binding](#data-binding)
1. [Directive](#directive)
1. [Service](#service)
1. [Dependency Injection](#dependency-injection)
That's a foundation for everything else in an Angular application
and it's more than enough to get going.
But it doesn't include everything we'll need or want to know.
Below is a brief, alphabetical list of other important Angular features and services.
Most of them are covered in this Developers Guide (or soon will be):
>**Animations** - A forthcoming animation library makes it easy for developers to animate component behavior
without deep knowledge of animation techniques or css.
>**Bootstrap** - A method to configure and launch the root application component.
>**Change Detection** - Learn how Angular decides that a component property value has changed and
when to update the screen.
Learn how it uses **zones** to intercept asynchronous activity and run its change detection strategies.
>**Component Router** - With the Component Router service, users can navigate a multi-screen application
in a familiar web browsing style using URLs.
>**Events** - The DOM raise events. So can components and services. Angular offers mechanisms for
publishing and subscribing to events including an implementation of the [RxJS Observable](https://github.com/zenparsing/es-observable) proposal.
>**[Forms](forms.html)** - Support complex data entry scenarios with HTML-based validation and dirty checking.
>**HTTP** - Communicate with a server to get data, save data, and invoke server-side actions with this Angular HTTP client.
>**Lifecycle Hooks** - We can tap into key moments in the lifetime of a component, from its creation to its destruction,
by implementing the "Lifecycle Hook" interfaces.
>**[Pipes](pipes.html)** - Services that transform values for display.
We can put pipes in our templates to improve the user experience. For example,
this `currency` pipe expression,
:marked
>displays a price of "42.33" as `$42.33`.
>**[Testing](../testing/index.html)** - Angular provides a testing library for "unit testing" our application parts as they
interact with the Angular framework.
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# Appendix: The Hero Staffing Agency
The final phase of the tutorial Tour of Heroes app runs like this.
figure.image-display
img(src='/resources/images/devguide/toh/toh-anim.gif' alt="Tour of Heroes in Action" height="300px")
:marked
There's a backstory to the "Tour of Heroes" and every sample in this guide.
The world is full of crises large and small.
Fortunately, there are courageous heroes prepared to take on every challenge.
The shadowy "Hero Staffing Agency" matches crises to heroes.
We are contract developers, hired by The Agency to build an application to manage their operations.
The Agency maintains a stable of heroes with special powers.
We ordinary humans submit our crises as job requests. The heroes bid to take the job and The Agency
assigns each job accordingly. Our application handles every detail of recruiting, tracking and job assignment.