调用[myFunction]的结果未被使用 [英] Result of call to [myFunction] is unused

问题描述

在Obj-C中，通常的做法是使用便利功能来执行常见操作，例如为视图配置自动布局：

  func makeConstraint（withAnotherView：UIView） - > NSLayoutConstraint 
 {
 //创建一些约束
 // ... 
 
 //返回创建的约束
 return NSLayoutConstraint（）
 } 
  

如果您只需设置约束并忘记它，可以调用：

[view1 makeConstraint：view2]

稍后存储约束以便您可以删除/修改它，您可以执行以下操作：

  NSLayoutConstraint * c; 
c = [view1 makeConstraint：view2] 
  

我想在swift中执行此操作，但如果我调用了上面的函数，并没有捕获返回的约束，我得到了警告：

 调用'makeConstraint（withAnotherView :)'未使用
  

非常烦人。有什么办法让Swift知道我并不总是想要捕获返回值？

注意：我知道这一点。这是丑陋的，而不是我要找的：

  _ = view1.makeConstraint（withAnotherView：view2）
  

解决方案

这是Swift 3中引入的行为。使用 @warn_unused_result 显式注释函数，以便告诉编译器结果应该被调用者使用，现在这是默认行为。

你可以在函数中使用 @discardableResult 属性来通知编译器返回值不必被'消耗'调用者。

  @discardableResult 
 func makeConstraint（withAnotherView：UIView） - > NSLayoutConstraint {
 
 ... //做有副作用的事情
 
 return NSLayoutConstraint（）
} 
  

  view1。 makeConstraint（view2）//无警告
 
 let constraint = view1.makeConstraint（view2）//按预期工作
  

您可以在演化提案。

In Obj-C, a common practice was to use convenience functions to perform common operations, like configuring auto layout for views:

func makeConstraint(withAnotherView : UIView) -> NSLayoutConstraint
{
   // Make some constraint 
   // ...

   // Return the created constraint
   return NSLayoutConstraint()
}

If you just needed to set the constraint and forget about it, you could call:

[view1 makeConstraint: view2]

If you wanted to store the constraint later so that you could remove/modify it, you would do something like:

NSLayoutConstraint * c;
c = [view1 makeConstraint: view2]

I want to do this in swift, but if I call the above function and do not capture the returned constraint, I get the warning:

Result of call to 'makeConstraint(withAnotherView:)' is unused

VERY annoying. Is there some way to let Swift know that I don't always want to capture the return value?

NOTE: I know about this. It is ugly and not what I'm looking for:

_ = view1.makeConstraint(withAnotherView: view2)

解决方案

This is behaviour that has been introduced in Swift 3. Instead of having to explicitly annotate functions with @warn_unused_result in order to tell the compiler that the result should be used by the caller, this is now the default behaviour.

You can use the @discardableResult attribute on your function in order to inform the compiler that the return value doesn't have to be 'consumed' by the caller.

@discardableResult
func makeConstraint(withAnotherView : UIView) -> NSLayoutConstraint {

   ... // do things that have side effects

   return NSLayoutConstraint()
}

view1.makeConstraint(view2) // No warning

let constraint = view1.makeConstraint(view2) // Works as expected

You can read about this change in more detail on the evolution proposal.

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