Dart_Language_Sample

Dart

跨平台

发布日期: 2018-08-10

文章字数: 855

阅读时长: 4 分

阅读次数:

1. Hello world！

每个应用都有一个main()函数，Dart的主函数如下：

void main() {
    print('Hello, World!');
}

2. Variables

Dart语言是类型安全的强类型语言，因此，在定义属性时需要定义类型。
如果属性有初始化值时，Dart会默认推测出此属性的类型，因此不需要显式地声明类型；
如果属性没有初始化值，则必须要为属性显式声明类型。

var name = 'Voyager I';
var year = 1977;
var antennaDiameter = 3.7;
var flybyObjects = ['Jupiter', 'Saturn', 'Uranus', 'Neptune'];
var image = {
    'tags': ['saturn'],
    'url': '//path/to/saturn.jpg'
};

3. Control Flow Statement

Dart的控制流类似Java、JavaScript、Kotlin等语言，这里不再累述！

if (year >= 2001) {
    print('21st century');
} else if (year >= 1901) {
    print('20th century');
}

for (var object in flybyObjects) {
    print(object);
}

for (int month = 1; month <= 12; month++) {
    print(month);
}

while (year < 2016) {
    year += 1;
}

4. Functions

Dart的函数支持

int fibonacci(int n) {
if (n == 0 || n == 1) return n;
    return fibonacci(n - 1) + fibonacci(n - 2);
}

var result = fibonacci(20);

Dart使用 => 表示拥有单行表达式的函数的简写的指示器

flybyObjects.where((name) => name.contains('turn')).forEach(print);

5. Comments

Dart的注释使用比较新颖！

// This is a normal, one-line comment.

/// This is a documentation comment, used to document libraries,
/// classes, and their members. Tools like IDEs and dartdoc treat
/// doc comments specially.

/* Comments like these are also supported. */

6. Imports

Dart的导包类似JavaScript

// Importing core libraries
import 'dart:math';

// Importing libraries from external packages
import 'package:test/test.dart';

// Importing files
import 'path/to/my_other_file.dart';

7. Classes

Dart定义类、构造器、属性

定义一个拥有两个构造器，一个函数，三个属性的类（注意下面的定义两个构造器的新颖方式）

    class Spacecraft {
        String name;
        DateTime launchDate;

        // Constructor, with syntactic sugar for assignment to members.
        Spacecraft(this.name, this.launchDate) {
            // Initialization code goes here.
        }

        // Named constructor that forwards to the default one.
        Spacecraft.unlaunched(String name) : this(name, null);

        int get launchYear =>
            launchDate?.year; // read-only non-final property

        // Method.
        void describe() {
            print('Spacecraft: $name');
            if (launchDate != null) {
            int years =
                DateTime.now().difference(launchDate).inDays ~/
                    365;
            print('Launched: $launchYear ($years years ago)');
            } else {
            print('Unlaunched');
            }
        }
    }

-----

使用这个类


    var voyager = Spacecraft('Voyager I', DateTime(1977, 9, 5));
    voyager.describe();

    var voyager3 = Spacecraft.unlaunched('Voyager III');
    voyager3.describe();

8. Inheritance

Dart的继承

Dart的继承不同与Java语言（Dart的每个类都默认是接口；Dart不仅可以实现”单继承“还可以实现“多继承”）

class Orbiter extends Spacecraft {
    num altitude;
    Orbiter(String name, DateTime launchDate, this.altitude)
        : super(name, launchDate);
}

9. Mixins

Dart的多继承通过“extends with”和“implements”实现，请往下看！

定义一个类

class Piloted {
    int astronauts = 1;
    void describeCrew() {
        print('Number of astronauts: $astronauts');
    }
}

多继承

class PilotedCraft extends Spacecraft with Piloted {
    // ···
}

10. Interface and Abstract class

Dart没有单独定义接口的关键词，Dart的每个类默认都是接口，都可以使用implements被实现

Dart的抽象类类似Java

定义一个类/一个接口

class MockSpaceship implements Spacecraft {
    // ···
}

定义抽象类

abstract class Describable {
    void describe();

    void describeWithEmphasis() {
        print('=========');
        describe();
        print('=========');
    }
}

11. Async

Dart的异步类似JavaScript、Kotlin等语言！

const oneSecond = Duration(seconds: 1);
// ···
Future<void> printWithDelay(String message) async {
    await Future.delayed(oneSecond);
    print(message);
}

12. Exception

Dart的异常类似Java、Kotlin等语言，如下图

主动抛出异常

if (astronauts == 0) {
    throw StateError('No astronauts.');
}

捕获异常、处理异常类似Java，不同的是多了on (on 和 catch的区别是on没有参数，catch有参数)

try {
    for (var object in flybyObjects) {
        var description = await File('$object.txt').readAsString();
        print(description);
    }
} on IOException catch (e) {
    print('Could not describe object: $e');
} finally {
    flybyObjects.clear();
}

未完待续。。。