关于flutter:Dart-的编程语言之美-4-个超级特性

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原文

https://reprom.io/the-beauty-...

参考

https://dart.dev/codelabs/asy...
https://medium.com/flutter-co...
https://dart.dev/guides/langu...
https://api.dart.dev/stable/2...

注释

在浏览 Flutter 时，我读到最多的毛病之一就是应用 Dart 编程语言。它还没有 Kotlin 那么成熟，这是我读到的最常被提及的论点之一。在我看来(我抵赖这可能会引起争议) ，Dart 是一种平凡的语言，我不会在 Flutter 创立应用程序时为其余任何语言扭转它，我是在 Kotlin 业余地创立 android 应用程序之后说的，顺便说一句，这也是一种优雅而漂亮的语言。

在这篇文章中，我打算展现我最喜爱的 Dart 编程语言的 4 个个性，没有特定的程序; 让咱们看看咱们如何利用这个古代工具:

Null safety

最近在 2.12 版本中增加(包含在 Flutter 2.0 中)。在任何伪装松软和高效的古代语言中，空平安都是必须的。这就是为什么 Dart 团队始终致力于实现声音 null 平安，这意味着咱们能够有能够为空的类型和不能为空的类型，如果咱们尝试在前面执行一个不平安的操作，咱们会在应用程序构建之前失去一个编译谬误:

// This is a String that can be nullString? nullVar;// This String cannot be null, the compiler forces me// to set it a value because of its non-nullable nature.String nonNullVar = 'I am not null';// Alternatively, I can specify that the value will be// set later, but the property continues to be non-nullable.late String lateNonNullVar;// If I want to call a method on an instance of a type// that can be null, I need first to do a runtime check that// its value is not null.if (nullVar != null) {  nonNullVar.toLowerCase();}// Or call it using the '?' operator, which means that the// method will only be called if the instance is not null:nullVar?.toLowerCase();// If the type is not nullable I can safely call any// method on it directly.nonNullVar.toLowerCase();// Always remember to initialize late vars, or you// will get an exception when trying to access its members.lateNonNullVar = 'some value';lateNonNullVar.toLowerCase();

Async / await

就像在 Javascript 中咱们有 Promises，在 Dart 中咱们有 Futures，其中 async/await 是次要的关键词，这给了咱们开发者一个简略而弱小的办法来解决异步操作:

应用 Futures，咱们能够轻松地进行以后操作流，期待某个异步操作实现，而后持续工作。

// To specify that a function will perform an asynchronous// operation (like doing a network request or reading from// a database) we mark it with the 'async' keyword:asyncFunction() async {  // ...}// Use the 'await' keyword to stop the flow until the function// has completed its task:await asyncFunction();// You must declare a function as 'async' if it contains// calls to other async functions:main() async {  await asyncFunction();}// If the async function returns a value, wrap it within// a Future. For instance, the following function// would do a network call and return its result:Future<NetworkResult> doNetworkCall() async {  // ...}final result = await doNetworkCall();// But what if the network request fails and an exception// is thrown? Just wrap the call in a try/catch block:late NetworkResult result;try {  result = await doNetworkCall();} on NetworkException catch (e) {  // Handle error}

须要指出的是，即便应用 async/await 关键字，所有的操作都是在同一个线程中执行的，如果咱们须要具体的性能要求，咱们能够应用 isolates 产生代替线程。

定义函数参数的多种办法

在 Dart 中，咱们在定义函数参数时有多个选项:

// You can define mandatory parameters as you do in// many other languages, specifying their type and setting a label:functionWithMandatoryParameters(String someString, int someNumber) {  // ...}// You are forced to send the defined parameters// when using the function:functionWithMandatoryParameters('some_string', 46);// You can however specify that the parameters are optional:// (note that the type must be defined as nullable, precisely because// there's no guarantee that the caller will send a value)functionWithOptionalParams(  {String? optionalString, int? optionalNumber}) {  // ...}// You can call this function without sending any values,// or specifying a value for an optional parameter with its label:functionWithOptionalParams();functionWithOptionalParams(optionalString: 'some_string');functionWithOptionalParams(  optionalString: 'some_string', optionalNumber: 46);// When defining optional parameters, you can set a default value// that will be used in case that there is no value sent by the caller:functionWithDefaultValue({String someString = 'default'}) {  // ...}// The value of someString is 'default'functionWithDefaultValue();// The value of someString is 'some_string'functionWithDefaultValue(someString: 'some_string');// Lastly, you can even define mandatory named parameters with the// 'required' keyword, this is useful to enhance code readability.createUser(  {required String username,  required String name,  required String surname,  required String address,  required String city,  required String country}) {// ...}createUser(  username: 'Ghost',  name: 'John',  surname: 'Doe',  address: '3590  Mill Street',  city: 'Beaver',  country: 'US');

Composition with mixins

软件开发中最不风行的趋势之一是重组而不是继承，这意味着应用相似组件的元素向类增加性能，而不是从父类继承。这种办法容许咱们轻松地增加封装的性能，而无需解决简单的继承层次结构。

例如，假如您有一个登录逻辑，您可能心愿在应用程序中的不同地位应用它。您能够应用这个逻辑创立一个组件(mixin) ，而后在须要时重用它:

abstract class AuthUtils {  Future<User> login() async {    // Here we can add the login logic that will later be reused    // in any class that ads this mixin.  }}class LoginPage extends StatefulWidget {  LoginPage({Key? key}) : super(key: key);  @override  _LoginPageState createState() => _LoginPageState();}class _LoginPageState extends State<LoginPage> with AuthUtils {  @override  Widget build(BuildContext context) {    return FutureBuilder<User>(      future: login(), // Calling the mixin function      builder: (BuildContext context, AsyncSnapshot<User> snapshot) {        // ...      },    );  }}

这里的长处是，咱们能够得心应手地增加任意多的 mixin，而不是仅通过应用继承从一个父类继承。

总结

这些只是 Dart 提供给开发者的多个有用个性中的 4 个。如果你想理解更多，我倡议你去参观 Dart 语言之旅，它以一种十分敌对的形式解释了这门语言的每一个细节。

https://dart.dev/guides/langu...

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