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Dart 常见问题和解答

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Contents

Updated June 2019

This page collects some of the top questions we’ve heard from the community since Dart was open sourced.

General

Q. Is there a specification for Dart?

Yes. Dart 1 has a formal specification owned by Ecma TC52.

Dart 2.x is currently being specified; the specification is available from the Dart language specification page.

Q. How are you taking input on changes to Dart?

We listen to feedback and issues, and we review patches from contributors. A contributor with a good track record can become a committer to the repository. Google engineers will also be working in the public repository, making visible changes. The project is lucky to have received many external patches and has welcomed distributed committers.


Language

Q. Isn’t Dart a lot like Java?

Dart has some similarities with Java. See the Intro to Dart for Java Developers codelab for examples of some of the differences between Dart and Java.

Q. How does Dart relate to Go?

Dart and Go are both language projects started at Google, but they are independent and have different goals. As a result, they make different choices, and the languages have very different natures, even while we all try to learn from each others’ work.

Q. Why isn’t Dart more like Haskell / Smalltalk / Python / Scala / other language?

Various reasons, depending on the language being asked about.

For languages that are quite different from JavaScript: it’s important for Dart to compile to efficient JavaScript. Our experience in GWT is that if the source language is too different from JavaScript, it creates some cases where complex output code is needed to emulate the source language’s behavior. This can cause performance to vary in ways that are not transparent to the programmer.

For languages that are compiled to native code: it’s important that Dart compiles efficiently to machine code, and thus it shares a number of aspects with other compiled languages.

For languages that are “more dynamic” than Dart: Dart deliberately trades off some of this arbitrary runtime modification for the goal of better performance and more productive tools.

Q. Why isn’t Dart syntax more exciting?

We did throw in some nice syntactic features such as this. constructor args and => for one-line functions, but we’d agree that Dart chooses familiarity over excitement. One team member’s personal testimonial: “I wish it had a little more razzle dazzle but I can’t deny that literally on my first day of writing Dart code, I was productive in it.”

Q. Does Dart have reflection capabilities?

For servers and command-line scripts, we have reflection support from the mirrors API. There is no support for mirrors when using Dart to write web or Flutter apps (more info).

Q. Can Dart add tuples, pattern matching, non-nullable types, partial evaluation, optional semicolons, …?

Future releases might be able to include (some of) those features, although we can’t include everything. Some features don’t fit the basic nature of the language, and some don’t play well with other features. Simplicity is the single most important gift we can give to future programmers.

Please look at the language funnel and language issues list to see if your request is already there. If it is, let us know that you care and give it a thumbs up. Otherwise, go ahead and add a new request issue (see the language evolution process for details). Make a thoughtful argument for your feature. Sample code with and without your feature is good evidence; a sizeable codebase that shows the need is even better evidence.

Don’t be surprised if the Dart language team says “no” by default. It’s far more painful to remove a language feature than to add it, so Dart is likely to add the most obvious features first, and then revisit the next tier later. And there simply are more possible language features in the world that can fit into any single language without making a total hash of it. But we do very much appreciate suggestions and evidence. We hope you’ll see our appreciation through careful design choices and fair communication about them.


Types

Q. Is Dart a statically typed language?

Yes, Dart 2 is statically typed. For more information, read about Dart’s type system.

With its combination of static and runtime checks, Dart has a sound type system, which guarantees that an expression of one type cannot produce a value of another type. No surprises!

Even with type-safe Dart, you can annotate any variable with dynamic if you need the flexibility of a dynamic language. The dynamic type itself is static, but can contain any type at runtime. Of course, that removes many of the benefits of a type-safe language for that variable.

Q. Why are generics covariant?

Covariant generics fit a common intuition that programmers have, and very often this intuition is correct, such as in the common “read-only” use of a generic. Although this intuition isn’t always correct, Dart is erring on the side of convenience by having covariant generics.

The only other reasonable default variance would be invariance. While having only invariant generics would definitely prevent more errors, it would also prevent a lot of valid programs or require conversion every time you have a list of “apples”, and someone just wants “fruits”.

We are familiar with a variety of ways that languages try to mark or infer variance. We feel that variance inference systems add too much complexity for their benefit in Dart.

Again, we’re trying to be pragmatic, and we think the outcome is reasonable.


Usage and tools

Q. Does Dart support JSON?

Yes. See the JSON converters in the dart:convert library.

Q. Can Dart run on the server?

Yes. See Dart on the Server for details.

Q. How do I use third party code, or share code?

You can find many packages on the pub.dev site a service for hosting packages of Dart code. Use the pub command to package your code and upload to the site.

Q. Do I need to use a particular editor or IDE to write Dart code?

Nope. You can try out Dart code with DartPad, and then use your favorite editor or IDE for development. Some full-featured IDEs such as IntelliJ IDEA, WebStorm, and Visual Studio Code have Dart plugins. Open source Dart plugins also exist for a number of editors. For more information, see the Dart tools.

Q. Can I build an Android app with Dart?

Yes! You can build an Android app that also works on iOS from a single codebase using Flutter, which is powered by the Dart platform.

Q. What are some real-world production deployments of Dart?

Google AdWords, AdSense, AdMob, and the Google Assistant all use Dart. A significant portion of Google’s revenue flows through these apps. Inside or outside of Google, every Flutter app uses Dart.


Native execution

Q. Can I compile Dart code to native code?

Yes. For programs targeting devices (mobile, desktop, server, and more), Dart Native includes both a Dart VM with JIT (just-in-time) compilation and an AOT (ahead-of-time) compiler for producing machine code.

Flutter is a sample framework that uses Dart’s native compilation capability to produce fast native apps.

Q. Can I compile a Dart program for running in a terminal?

Yes. Dart programs can be compiled to native x64 machine code for running in a Terminal/Command Prompt on desktop operating systems such as Windows, macOS, and Linux. For more details, see the dart2native documentation.


Web: general

Q. What browsers do you support as JavaScript compilation targets?

The production compiler (dart2js) supports Internet Explorer 11 and the last two major releases of the following browsers:

  • Chrome
  • Edge
  • Firefox
  • Safari

The development compiler (dartdevc) supports only Chrome.

Q. Is Dart supported by my browser?

Although no production browsers can execute Dart code directly, all modern browsers can execute Dart code that’s been compiled to JavaScript.

Q. How do I debug an app?

For setup details and a walkthrough, see Debugging Dart Web Apps.

The debugging section of the dart2js documentation has some tips for specific browsers.

Q. What web frameworks can I use with Dart?

You can use the low-level HTML API defined by core libraries such as dart:html, or you can use a framework such as AngularDart. During Google I/O 2019 we announced a technical preview of Flutter for web that lets you take Flutter UI code and business logic and run it directly in the browser.

Q. Will the Dart VM get into Chrome?

No. Dart is designed to compile to JavaScript to run across the modern web.


Web: JavaScript and other technologies

Q. How does Dart code interoperate with JavaScript libraries?

Although Dart and JavaScript are completely separate languages with separate VMs, they can interoperate. For more information, see JavaScript and TypeScript interop.

Q. I have a large JavaScript codebase. How can I migrate it to Dart?

Try migrating one major feature at a time, and use the JavaScript interoperability library only when necessary.

Q. How does Dart compare with using the Closure compiler on JavaScript?

The idea of optional type annotations is similar. Dart’s are nicer syntactically.

Compare the following Closure compiler code:

// Closure compiler code

/**
 * @param {String} name
 * @return {String}
 */
makeGreeting = function(name) {
  /** @type {String} */
  var greeting = 'hello ' + name;
  return greeting;
}

With the following Dart code:

// Dart code

String makeGreeting(String name) {
  var greeting = 'hello $name';
  return greeting;
}

Q. How does Dart compare with CoffeeScript?

Both Dart and CoffeeScript are inspired by JavaScript, and both can be translated back to it. They make different choices, particularly in the flavor of their syntax. As a language we think it’s fair to say that Dart differs semantically from JavaScript more than CoffeeScript does; that may result in a less line-for-line translation, but we believe Dart-generated JavaScript can have excellent size and speed.

Dart introduces new semantics, while CoffeeScript retains the semantics of JavaScript.

If you like CoffeeScript for its more structured feel than raw JavaScript, you may like Dart’s static type annotations.

Q. What does Google think of TypeScript?

TypeScript and Dart have similar goals; they make building large-scale web apps easier. However, their approaches are fairly different. TypeScript maintains backwards compatability with JavaScript, whereas Dart purposely made a break from certain parts of JavaScript’s syntax and semantics in order to eradicate large classes of bugs and to improve performance. The web has suffered from too little choice for too long, and we think that both Dart and TypeScript are pointing to a brighter future for web developers. You can read a more complete response on our blog.

Q. I have a large app written in GWT. How do I port it to Dart?

Java and Dart are syntactically similar, so this might be easier than you think. You can rely on the Dart analyzer to flag any syntax problems. Alternatively, you may consider porting one feature at a time to Dart and using the JavaScript interoperability library as the common middle ground. Be sure to watch Dart-JavaScript interoperability, a talk from Dart Developer Summit 2016.


Web: JavaScript compilation

Q. Will any valid Dart code compile to JavaScript, or are there limitations?

We intend for any valid Dart code to compile to JavaScript. Of course, some libraries only run on the server or in Flutter. For example, the dart:io library provides access to operating system files and directories with APIs not available to the browser.

Q. Why does Dart have two JavaScript compilers, dartdevc and dart2js?

The two compilers have different use cases. You don’t usually have to worry about which compiler you’re using, because the webdev tool chooses the right compiler for your use case. When you’re developing your app, webdev chooses dartdevc, which supports incremental compilation so you can quickly see the results of your edits. When you’re building your app for deployment, webdev chooses dart2js, which uses techniques such as tree shaking to produce optimized code.

Q. How can dart2js produce JavaScript that runs faster than handwritten JavaScript?

Think of dart2js as a real compiler, which can analyze your entire program and make optimizations that you probably can’t or won’t do. Just like gcc can output efficient code by moving code around, dart2js can take advantage of Dart’s structured nature to implement global optimizations.

We don’t claim that all Dart code will run faster than handwritten JavaScript, when compiled to JavaScript, but we’re working to make the common cases fast.

Q. How can I write Dart code that compiles to performant JavaScript?

See Helping dart2js generate better code. Just be aware that this information might change as the implementation of dart2js changes.

Q. Why is the code for “Hello, World” so big, compared to the original Dart code after compilation to JavaScript?

We believe that it’s important to create small and efficient JavaScript from Dart, but most developers don’t write “Hello, World” apps. It’s all relative, and with tree shaking (dead code elimination), minification, and compression, Dart apps can be compiled to JavaScript fairly efficiently.

Kevin Moore saw improvements in the size of the generated JavaScript from his real-world HTML5 game.

The dart2js team strives to generate smaller output, but is more focused on real-world apps instead of trivial examples.

Q. How are floating point numbers handled when compiled to JavaScript?

JavaScript has only one number representation: an IEEE-754 double-precision floating-point number. This means that any number—integer or floating point—is represented as a double. JavaScript has typed data arrays, and the mapping from native Dart typed lists to JavaScript typed arrays is trivial.

Q. How are integers handled when compiled to JavaScript?

Because all numbers are stored as doubles, integers are restricted to a 53-bit precision. Integer values in the range of -253 to 253 can be stored without loss of accuracy. Because JavaScript VMs play tricks with the internal representation of numbers (similar to those described above), staying within smi range is still good practice.

Q. How are typed lists handled when compiled to JavaScript?

JavaScript offers typed arrays that are compatible with Dart’s typed lists. The mapping is trivial—for example, Float32List becomes a Float32Array. The one exception today is that dart2js does not support 64-bit integers and thus does not support Int64List or Uint64List. Dart code compiled via dart2js results in a runtime exception if either of those lists is used.

Q. Why not compile Dart to asm.js instead of JavaScript?

Asm.js is a very restricted subset of JavaScript best suited as a compilation target for C compilers. It does not include JavaScript objects or direct access to the DOM. Essentially, it allows only arithmetic operations and manipulations on typed arrays.

While it is possible to implement the features that Dart requires, they would incur a large overhead in both speed and size, compared to relying on the already existing features provided by the underlying JavaScript engine. For example, any JavaScript machine comes with a garbage collector; implementing another one in asm.js would increase the output size, and be noticeably slower than the well-tuned garbage collectors of modern JavaScript VMs.