Flutter / Dart-（）{}与（）=&gt之间的差异； {} [英] Flutter/Dart - Difference between () {} and () => {}

问题描述

在Flutter / Dart中，示例有时显示粗箭头，有时不显示。以下是示例：

In Flutter/Dart the examples sometimes show fat arrow and sometimes dont. Here are examples:

RaisedButton(
  onPressed: () {
    setState(() {
      _myTxt = "Text Changed";
    });
  },

在其他地方可以看到：

void main() => runApp(MyApp());

推荐答案

胖箭头语法只是返回表达式的简写方式，类似于（）{返回表达式；} 。

根据文档。

The fat arrow syntax is simply a short hand for returning an expression and is similar to (){ return expression; }.
According to the docs.

注意：仅表达式而不是语句-可以出现在箭头（=>）和分号（;）之间。例如，您不能在其中放置if语句，但是可以使用条件表达式

Note: Only an expression—not a statement—can appear between the arrow (=>) and the semicolon (;). For example, you can’t put an if statement there, but you can use a conditional expression

void main(){
    final cls = TestClass();
    cls.displayAnInt((){
       //you can create statements here and then return a value
       int num1 = 55;
       int num2 = 1;
       int sum = num1 + num2;
       return sum;
    });
   cls.displayAnInt(() => 55 + 1); // simply return an int expression
}
class TestClass{

    displayAnInt(makeIntFunc){

       int intValue = makeIntFunc();
       print('The int value is $intValue');
    }
}

从上面的代码中，您可以看到在以下情况下可以执行多行语句

From the code above, You can see that multiline statement can be made when the callback function is used and then a value is returned, while the fat arrow simply has an expression with no return keyword.

考虑您对不支持dart中多行语句的粗箭头的回答。这是可以理解的，因为这样做（）=> {somtheing} 表示您正在返回地图，并且可能会看到类似（）=>的内容。 {’name’：’John’，‘age’：25} 而不是（）=> {_myTxt =文字已更改； _myTxt =从不介意； } 。

Considering your answer about fat arrows not supporting multiline statements in dart. This is quite understandable since doing () => {somtheing} would imply you are returning a map and it would expect to see something like () => {'name':'John', 'age':25} and not () => { _myTxt = "Text Changed";_myTxt = "Never Mind"; } .

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