问题描述
在阅读飞镖代码时，我经常看到一些仅带有下划线_参数调用的函数。它使我烦恼了一段时间，并且由于flutter改善了其分析消息，因此我有了一些线索...但是我觉得我不太了解这个概念：-（
When reading dart code I often see some functions called with just an underscore _ parameter. It's bugging me out for some time now and since flutter has improved its analysis messages I have some clues... but I feel like I don't really grasp this concept :-(
昨天我为测试编写了以下内容：
Yesterday I wrote the following for a test :
when(mockDevice.getLocalPath()).thenAnswer(() async => fileFolder);
并获得以下分析
添加下划线时效果很好。
When adding underscore it's working perfectly.
when(mockDevice.getLocalPath()).thenAnswer((_) async => fileFolder);
我遇到的最令人恐惧的例子来自@remi rousselet编写的提供程序包
The most frightenning example I meet come from provider package written by @remi rousselet
builder: (_, counter, __) => Translations(counter.value),
来自省der示例：
Widget build(BuildContext context) {
return MultiProvider(
providers: [
ChangeNotifierProvider(builder: (_) => Counter()),
ProxyProvider<Counter, Translations>(
builder: (_, counter, __) => Translations(counter.value),
),
],
child: Foo(),
);
}
class Translations {
const Translations(this._value);
final int _value;
String get title => 'You clicked $_value times';
}
推荐答案
下划线通常表示您不会在块内使用该参数，这只是编写代码的好方法，例如：
Underscore is normally an indication that you are not going to use that parameter inside the block it is just a good way to write code, for instance:
method(int useful, int useless) {
// say I am only going to use 'useful' in this block
}
上面的代码也可以写成：
Above code can also be written as:
method(int useful, int _) {
// using '_' means I'm not going to use 2nd parameter in the block
}
现在回答您的问题：
Answer to your question now:
builder: (_, counter, __) => Translations(counter.value),
表示您有3个参数 _ ，计数器和 __ ，仅计数器是您正在使用的，因此第一个和第三个参数分别用 _ 和 __ 表示。这是编写代码的更简洁的方法。
means you have 3 parameters _, counter and __, and only counter is what you are using, so 1st and 3rd parameters are denoted with _ and __. This is just cleaner way to write code.
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