了解Dart中的括号(颤振) [英] Understanding Brackets in Dart (Flutter)

问题描述

我对 Dart (Flutter)中的括号感到困惑.哪个括号()，{}，[]"用于什么?

I have many confusion about Brackets in Dart(Flutter). Which bracket "(), {}, []" is used for what?

推荐答案

• ()可以对表达式进行分组:

  • () can group expressions:

    var x = (1 + 2) * 3;
    

    或可以指定函数的参数列表:

    or can designate parameter lists for functions:

    var getAnswer = () => 42;
    
    int square(int x) => x * x;
    

    或可以指定函数调用:

    var answer = getAnswer();
    var squared = square(4);
    

    or是某些关键字语法的一部分.这包括(但不限于) if ， assert ， for ， while ， switch ， catch :

    or is part of the syntax to some keywords. This includes (but is not limited to) if, assert, for, while, switch, catch:

    if (condition) {
      ...
    }
    
    assert(condition);
    
    for (var item in collection) {
      ...
    }
    
    
    while (condition) {
      ...
    }
    
    switch (value) {
      ...
    }
    
    try {
      ...
    } on Exception catch (e) {
      ...
    }
    

  • [] 本身会创建 列出文字:

  • [] by itself creates List literals:

    var list = [1, 2, 3];
    var emptyList = []; // Creates a List<dynamic>.
    

    或在对象上使用时，调用 operator [] ，通常访问集合的元素:

    or when used on an object, calls operator [], which usually accesses an element of a collection:

    var element = list[index];
    var value = map[key];
    

    或在参数列表中，指定可选的 positional 参数:

    or in a parameter list, specifies optional positional parameters:

    int function(int x, [int y, int z]) {
      return x + y ?? 0 + z ?? 0;
    }
    
    function(1, 2);
    

    在dartdoc注释中

    或，创建对其他符号的链接引用:

    or in dartdoc comments, creates linked references to other symbols:

    /// Creates a [Bar] from a [Foo].
    Bar fooToBar(Foo foo) {
      // ...
    }
    

  • {} 可以创建代码块，将行分组在一起并限制变量范围.这包括(但不限于)函数体，类声明， if - else 块， try - catch 块， for 块， while 块， switch 块等:

  • {} can create a block of code, grouping lines together and limiting variable scope. This includes (but is not limited to) function bodies, class declarations, if-else blocks, try-catch blocks, for blocks, while blocks, switch blocks, etc.:

    class Class {
      int member;
    }
    
    void doNothing() {}
    
    void function(bool condition) {
      {
        int x = 'Hello world!';
        print(x);
      }
    
      if (condition) {
        int x = 42; // No name collision due to separate scopes.
        print(x);
      }
    }
    

    或单独创建 Set 或 Map 文字:

    or by itself can create Set or Map literals:

    var set = {1, 2, 3};
    var emptySet = <int>{};
    
    var map = {'one': 1, 'two': 2, 'three': 3};
    var emptyMap = {}; // Creates a Map<dynamic, dynamic>
    

    或在参数列表中，指定可选的命名参数:

    or in a parameter list, specifies optional named parameters:

    int function(int x, {int y, int z}) {
      return x + y ?? 0 + z ?? 0;
    }
    
    function(1, y: 2);
    

    或创建枚举:

    enum SomeEnumeration {
      foo,
      bar,
      baz,
    }
    

    或 String 中的

    用于消除插值表达式的歧义:

    or in Strings is used to disambiguate interpolated expressions:

    var foo = 'foo';
    var foobar = '${foo}bar';
    
    var s = '${function(1, 2)}';
    

  • <>> 在函数或类声明中成对使用时会创建

  • <> when used as a pair in function or class declarations creates generics:

    class GenericClass<T> {
      T member;
    }
    
    T function<T>(T x) {
      // ...
    }
    

    或使用泛型时指定显式类型:

    or specifies explicit types when using generics:

    var map = <String, int>{};
    

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