Dart中的typedef是什么？ [英] What is a typedef in Dart?

问题描述

我已经阅读了说明，并且我知道它是函数类型的别名。

I have read the description, and I understand that it is a function-type alias.

• typedef，或者函数类型别名，为函数类型提供一个名称，您可以在声明字段和返回类型时使用该名称。在将函数类型分配给变量时，typedef会保留类型信息。

• A typedef, or function-type alias, gives a function type a name that you can use when declaring fields and return types. A typedef retains type information when a function type is assigned to a variable.

http://www.dartlang.org/docs/spec/latest/dart-language-specification.html#kix.yyd520hand9j

但是我该如何使用呢？为什么用函数类型声明字段？什么时候使用？它解决什么问题？

But how do I use it? Why declaring fields with a function-type? When do I use it? What problem does it solve?

我想我需要一个或两个真实的代码示例。

I think i need one or two real code examples.

推荐答案

Dart中typedef的一种常见用法是定义一个回调接口。例如：

A common usage pattern of typedef in Dart is defining a callback interface. For example:

typedef void LoggerOutputFunction(String msg);

class Logger {
  LoggerOutputFunction out;
  Logger() {
    out = print;
  }
  void log(String msg) {
    out(msg);
  }
}

void timestampLoggerOutputFunction(String msg) {
  String timeStamp = new Date.now().toString();
  print('${timeStamp}: $msg');
}

void main() {
  Logger l = new Logger();
  l.log('Hello World');
  l.out = timestampLoggerOutputFunction;
  l.log('Hello World');
}

运行上述示例将产生以下输出：

Running the above sample yields the following output:

Hello World

2012-09-22 10：19：15.139：Hello World

Hello World
2012-09-22 10:19:15.139: Hello World

typedef行表示LoggerOutputFunction接受String参数并返回void。

The typedef line says that LoggerOutputFunction takes a String parameter and returns void.

timestampLoggerOutputFunction与该定义匹配，因此可以分配给

timestampLoggerOutputFunction matches that definition and thus can be assigned to the out field.

让我知道您是否需要另一个示例。

Let me know if you need another example.

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