Managing state transitions is one of the hardest parts of building applications. This is especially true on the web, where you also need to ensure that the state is reflected in the URL. In addition, we often want to split applications into multiple bundles and load them on demand. Doing this transparently is not trivial.

The Angular router solves these problems. Using the router, you can declaratively specify application states, manage state transitions while taking care of the URL, and load bundles on demand.

In this book, I will talk about the router's mental model, its API, and the design principles behind it. To make one thing clear: this book is not about Angular. There is a lot of information about the framework available online. So if you want to get familiar with the framework first, I would recommend the following resources:

Therefore in this book, I assume you are familiar with Angular, and so I won't talk about dependency injection, components, or bindings. I will only talk about the router.

Why would you read this book if you can find the information about the router online?

This book goes far beyond a how-to-get-started guide. It is a complete description of the Angular router. The mental model, design constraints, and the subtleties of the API - everything is covered. Understanding these will give you deep insights into why the router works the way it does and will make you an Angular router expert.

In this book, you will find a number of text styles that distinguish between different kinds of information. Here are some examples of these styles and an explanation of their meaning.

Code words in text, database table names, folder names, filenames, file extensions, path names, dummy URLs, user input, and Twitter handles are shown as follows: "Then, it will instantiate ConversationCmp with MessagesCmp in it, with ComposeCmp displayed as a popup."

A block of code is set as follows:

@Component({
  template: `
    Edit
  `
})
class MessageCmp {
  public id: string;
  constructor(private route: ActivatedRoute) {
    route.params.subscribe(_ => this.id = _.id);
  }
}

When we wish to draw your attention to a particular part of a code block, the relevant lines or items are set in bold:

@Component({
  template: `
    Edit
  `
})
class MessageCmp {
  public id: string;
  constructor(private route: ActivatedRoute) {
    route.params.subscribe(_ => this.id = _.id);
  }
}

New terms and important words are shown in bold. Words that you see on the screen, for example, in menus or dialog boxes, appear in the text like this: "In order to download new modules, we will go to Files | Settings | Project Name | Project Interpreter."

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For all the examples in this book we will use MailApp, which is an application akin to Inbox or Gmail. At launch, the application displays a list of conversations, which we can browse through. Once we click on a conversation, we can see all its messages. We can also compose a new message, or view an existing message in a popup.

You can find the source code of the MailApp application here: https://github.com/vsavkin/router_mailapp.

The code in this book has been simplified and altered in places to make it more appropriate for teaching. The final MailApp code on GitHub illustrates how this teaching code can be applied in a full application.