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更新日期：2017-01-19
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What Is Angular?
AngularJS is a structural framework for dynamic web apps. It lets you use HTML as your template language and lets you extend HTML's syntax to express your application's components clearly and succinctly. Angular's data binding and dependency injection eliminate much of the code you would otherwise have to write. And it all happens within the browser, making it an ideal partner with any server technology.
Angular is what HTML would have been, had it been designed for applications. HTML is a great declarative language for static documents. It does not contain much in the way of creating applications, and as a result building web applications is an exercise in what do I have to do to trick the browser into doing what I want?
The impedance mismatch between dynamic applications and static documents is often solved with:
a library - a collection of functions which are useful when writing web apps. Your code is in charge and it calls into the library when it sees fit. E.g., jQuery.
frameworks - a particular implementation of a web application, where your code fills in the details. The framework is in charge and it calls into your code when it needs something app specific. E.g., durandal, ember, etc.
Angular takes another approach. It attempts to minimize the impedance mismatch between document centric HTML and what an application needs by creating new HTML constructs. Angular teaches the browser new syntax through a construct we call directives. Examples include:
Data binding, as in {{}}.
DOM control structures for repeating, showing and hiding DOM fragments.
Support for forms and form validation.
Attaching new behavior to DOM elements, such as DOM event handling.
Grouping of HTML into reusable components.
A complete client-side solution
Angular is not a single piece in the overall puzzle of building the client-side of a web application. It handles all of the DOM and AJAX glue code you once wrote by hand and puts it in a well-defined structure. This makes Angular opinionated about how a CRUD (Create, Read, Update, Delete) application should be built. But while it is opinionated, it also tries to make sure that its opinion is just a starting point you can easily change. Angular comes with the following out-of-the-box:
Everything you need to build a CRUD app in a cohesive set: Data-binding, basic templating directives, form validation, routing, deep-linking, reusable components and dependency injection.
Testability story: Unit-testing, end-to-end testing, mocks and test harnesses.
Seed application with directory layout and test scripts as a starting point.
Angular's sweet spot
Angular simplifies application development by presenting a higher level of abstraction to the developer. Like any abstraction, it comes at a cost of flexibility. In other words, not every app is a good fit for Angular. Angular was built with the CRUD application in mind. Luckily CRUD applications represent the majority of web applications. To understand what Angular is good at, though, it helps to understand when an app is not a good fit for Angular.
Games and GUI editors are examples of applications with intensive and tricky DOM manipulation. These kinds of apps are different from CRUD apps, and as a result are probably not a good fit for Angular. In these cases it may be better to use a library with a lower level of abstraction, such as jQuery.
The Zen of Angular
Angular is built around the belief that declarative code is better than imperative when it comes to building UIs and wiring software components together, while imperative code is excellent for expressing business logic.
It is a very good idea to decouple DOM manipulation from app logic. This dramatically improves the testability of the code.
It is a really, really good idea to regard app testing as equal in importance to app writing. Testing difficulty is dramatically affected by the way the code is structured.
