“行动"一词是什么意思使用 Play 框架在 Scala 函数定义中做什么? [英] What does the word "Action" do in a Scala function definition using the Play framework?

问题描述

我正在开发 Play 应用程序，我刚刚开始使用 Scala.我看到在下面的函数中的等号之后和大括号之前有这个词Action.

I am developing Play application and I've just started with Scala. I see that there is this word Action after the equals sign in the function below and before curly brace.

def index = Action {
  Ok(views.html.index("Hi there"))
}

这段代码有什么作用?我已经看到它与 def index = { 一起使用，但没有与大括号前的单词一起使用.

What does this code do? I've seen it used with def index = { but not with the word before the curly brace.

我假设函数的名称是 index.但是我不知道 Action 这个词在这种情况下有什么作用.

I would assume that the name of the function is index. But I do not know what the word Action does in this situation.

推荐答案

其他答案处理您的具体情况.不过，你问的是一般情况，所以我会尝试从这个角度回答.

The other answers deal with your specific case. You asked about the general case, however, so I'll attempt to answer from that perspective.

首先，def 用于定义方法，不是函数(现在最好了解这种区别).但是，您是对的，index 是该方法的名称.

First off, def is used to define a method, not a function (better to learn that difference now). But, you're right, index is the name of that method.

现在，与您可能熟悉的其他语言(例如 C、Java)不同，Scala 允许您使用表达式定义方法(如使用赋值运算符语法所建议的那样，=).也就是说，= 之后的所有内容都是一个表达式，每次调用该方法时都会将其计算为一个值.

Now, unlike other languages you might be familiar with (e.g., C, Java), Scala lets you define methods with an expression (as suggested by the use of the assignment operator syntax, =). That is, everything after the = is an expression that will be evaluated to a value each time the method is invoked.

所以，在 Java 中你必须说:

So, whereas in Java you have to say:

public int three() { return 3; }

在 Scala 中，你可以说:

In Scala, you can just say:

def three = 3

当然，表达式通常更复杂(如您的情况).它可能是一个代码块，就像您更习惯看到的那样，在这种情况下，该值是该块中最后一个表达式的值:

Of course, the expression is usually more complicated (as in your case). It could be a block of code, like you're more used to seeing, in which case the value is that of the last expression in the block:

def three = {
   val a = 1
   val b = 2
   a + b
}

或者它可能涉及对其他对象的方法调用:

Or it might involve a method invocation on some other object:

def three = Numbers.add(1, 2)

事实上，后者正是您的具体示例中发生的事情，尽管它需要更多的解释才能理解原因.这涉及到两个魔法:

The latter is, in fact, exactly what's going on in your specific example, although it requires a bit more explanation to understand why. There are two bits of magic involved:

1. 如果一个对象有一个 apply 方法，那么你可以把它当作一个函数来对待.例如，你可以说 Add(1, 2) 当你真正的意思是 Add.apply(1,2)(假设有一个 Add> 带有 apply 方法的对象，当然).需要明确的是，它不必是使用 object 关键字定义的对象.任何具有合适 apply 方法的对象都可以.
2. 如果一个方法有一个 by-name 参数(例如，def ifWaterBoiling(fn: => Tea))，那么你可以像 <代码>ifWaterBoiling { makeTea }.该块中的代码是惰性求值的(并且可能根本不求值).这相当于编写 ifWaterBoiling({ makeTea }).{ makeTea } 部分只是定义了一个表达式，该表达式被传入，未计算，用于 fn 参数.

1. If an object has an apply method, then you can treat the object as if it were a function. You can say, for example, Add(1, 2) when you really mean Add.apply(1,2) (assuming there's an Add object with an apply method, of course). And just to be clear, it doesn't have to be an object defined with the object keyword. Any object with a suitable apply method will do.
2. If a method has a single by-name parameter (e.g., def ifWaterBoiling(fn: => Tea)), then you can invoke the method like ifWaterBoiling { makeTea }. The code in that block is evaluated lazily (and may not be evaluated at all). This would be equivalent to writing ifWaterBoiling({ makeTea }). The { makeTea } part just defines an expression that gets passed in, unevaluated, for the fn parameter.

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