Squeak Smalltalk中块的比较

Question

我正在使用Squeak编程，需要比较两个代码块，如下所示：（toRunBlock是一个实例变量）

~~~Other code~~~
toRunBlock := [nil].
~~~Other code~~~

但是在某个时候，我需要将它与另一块代码进行比较：

(toRunBlock = [nil]) ifTrue: [
    "Run some code if toRunBlock hasn't been overwritten"
].

但是那个检查总是返回错误，我找不到一种方法来检查它们是否相等。有人能帮我解决吗？

- TheCompModder

---

为什么需要比较两个块？难道这不是一个建模问题吗？ - Leandro Caniglia

Answer 1

正如@LeandroCaniglia所指出的那样，您不必比较块。以下是两种解决问题的方法，而无需比较块：

1. initialize the variable to nil. In your accessor method you initialize it lazily:

   toRunBlock
       ^ toRunBlock ifNil: [ [] ]
   

   Now, when you look at the variable toRunBlock it will be nil unless #toRunBlock has been sent or the block as been set by other means.

   Your code would become:

   toRunBlock ifNil: [
       "Run some code if toRunBlock hasn't been overwritten"
   ].
   

2. use additional state by setting an instance variable you can check. This could be your setter method for example:

   toRunBlock: aBlock
       toRunBlock := aBlock.
       hasToRunBlockBeenSet := true
   

   And to check you could use a method like this:

   hasToRunBlockBeenSet
       ^ hasToRunBlockBeenSet ifNil: [ false ]
   

   Your code would become:

   self hasToRunBlockBeenSet ifTrue: [
       "Run some code if toRunBlock hasn't been overwritten"
   ].
   

第二种方法可以说更加可靠。

- Max Leske

Answer 2

除了引用相等性外，BlockClosures并没有定义相等性。尽管两个块可能表现相同，但它们仍然不同，因为它们是闭包而不仅仅是代码片段。每个块都有一个对上下文（方法激活或另一个块）的引用，该上下文是创建该块的地方，因此您的两个[nil]至少在这方面不同。

您只能通过在其他地方存储先前的块来检查块是否仍与之前相同。假设[nil]在您的情况下是某种默认值，您可以将该[nil]块存储在另一个实例变量或类池变量（例如defaultRunBlock）中，并将toRunBlock（按引用）与该变量进行比较，以检查toRunBlock是否已更改。

Object subclass: #YourClass
    instanceVariableNames: 'toRunBlock defaultRunBlock' 
        "or as class variable:"
    classVariableNames: 'DefaultRunBlock'
    poolDictionaries: ''
    category: 'Kernel-Methods'

initialize
    defaultRunBlock := [nil]

otherCode
    toRunBlock := defaultRunBlock

whereYouCompareThem
    toRunBlock == defaultRunBlock ifTrue: [ "..." ]

如果您想检查自上次调用后toRunBlock是否发生了更改，您还可以将先前的值存储在额外的实例变量中。

注：该段内容为关于IT技术的讨论，建议根据具体上下文进行翻译。

whereYouCompareThem
    toRunBlock == previousRunBlock ifTrue: [ "..." ].
    previousRunBlock := toRunBlock

- JayK