如何模拟“一次分配” Scala 中的 var?
假设我们正在处理此上下文:
object AppProperties {
private var mgr: FileManager = _
def init(config: Config) = {
mgr = makeFileManager(config)
}
}
此代码的问题是 AppProperties
中的任何其他方法都可能重新分配 mgr
。是否有一种技术可以更好地封装 mgr
,使其感觉像是其他方法的 val
?我考虑过类似的事情(受到这个的启发答案):
object AppProperties {
private object mgr {
private var isSet = false
private var mgr: FileManager = _
def apply() = if (!isSet) throw new IllegalStateException else mgr
def apply(m: FileManager) {
if (isSet) throw new IllegalStateException
else { isSet = true; mgr = m }
}
}
def init(config: Config) = {
mgr(makeFileManager(config))
}
}
...但这对我来说感觉相当沉重(初始化让我想起了太多的C++:-))。还有其他想法吗?
This is a follow-up question to my previous initialization variable question.
Suppose we're dealing with this context:
object AppProperties {
private var mgr: FileManager = _
def init(config: Config) = {
mgr = makeFileManager(config)
}
}
The problem with this code is that any other method in AppProperties
might reassign mgr
. Is there a technique to better encapsulate mgr
so that it feels like a val
for the other methods? I've thought about something like this (inspired by this answer):
object AppProperties {
private object mgr {
private var isSet = false
private var mgr: FileManager = _
def apply() = if (!isSet) throw new IllegalStateException else mgr
def apply(m: FileManager) {
if (isSet) throw new IllegalStateException
else { isSet = true; mgr = m }
}
}
def init(config: Config) = {
mgr(makeFileManager(config))
}
}
... but this feels rather heavyweight to me (and initialization reminds me too much of C++ :-)). Any other idea?
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您可以使用隐式来做到这一点,使隐式仅在应该能够重新分配的方法中可用。查看值不需要隐式,因此“变量”对其他方法是可见的:
You could do it with implicits, making the implicit available only in the method that is supposed to be able to reassign. Viewing the value doesn't require the implicit, so the "variable" is visible to other methods:
好的,这是我的建议,直接受到 的启发axel22 的,雷克斯·克尔 的,以及 Debilski< /a> 的答案:
我们获得了编译时安全性,
:=
不会被意外调用,因为我们需要对象的SetOnceCredential
,该对象仅返回一次。尽管如此,只要调用者拥有原始凭证,就可以重新分配变量。这适用于AnyVal
和AnyRef
。隐式转换允许我在许多情况下直接使用变量名,如果这不起作用,我可以通过附加()
来显式转换它。典型用法如下:
如果在同一文件中的其他某个点,我尝试获取另一个凭据并重新分配变量(如
usedAfterInit
中所示),则不会产生编译时错误,但是在运行时失败。OK, so here's my proposal, directly inspired by axel22's, Rex Kerr's, and Debilski's answers:
We get compile-time safety that
:=
is not called accidentally as we need the object'sSetOnceCredential
, which is returned only once. Still, the var can be reassigned, provided the caller has the original credential. This works withAnyVal
s andAnyRef
s. The implicit conversion allows me to use the variable name directly in many circumstances, and if this doesn't work, I can explicitly convert it by appending()
.Typical usage would be as follows:
This doesn't yield a compile-time error if at some other point in the same file, I try getting another credential and reassigning the variable (as in
calledAfterInit
), but fails at run-time.我假设您不需要使用基元有效地执行此操作,并且为了简单起见,您也不需要存储
null
(但如果这些假设不成立,您当然可以修改这个想法):并在需要该功能的任何地方使用此类。 (也许您更喜欢
apply()
而不是get
?)如果您确实希望它看起来像变量(或方法)访问而不需要额外的技巧,请使用 SetOnce私人的,以及
I assume you don't need to do this efficiently with primitives, and for simplicity that you also don't need to store
null
(but you can of course modify the idea if these assumptions are false):and just use this class wherever you need that functionality. (Maybe you would prefer
apply()
toget
?)If you really want it to look just like a variable (or method) access with no extra tricks, make the SetOnce private, and
这并不是最好的方式,也不是您所要求的,但它为您提供了一些访问封装:
Not really the nicest way and not really what you asked for but it gives you some encapsulation of access:
看看JPP的帖子我有做了另一个变体:
这一次,我们完全可以多次分配 var,但我们需要在范围内拥有正确的凭据。凭证在第一次分配时创建并返回,这就是为什么我们需要立即将其保存到
implicit val credential = mgr := new FileManager(config)
。如果凭证不正确,则不会起作用。(请注意,如果范围内有更多凭据,则隐式凭据不起作用,因为它们具有相同的类型。也许可以解决此问题,但我目前不确定。)
Looking at JPP’s post I have made another variation:
This time, we are perfectly allowed to assign the var multiple times but we need to have the correct credential in scope. The credential is created and returned on first assign which is why we need to save it immediately to
implicit val credential = mgr := new FileManager(config)
. If the credential is incorrect, it won’t work.(Note that the implicit credential does not work if there are more credentials in scope because they’ll have the same type. It might be possible to work around this but I’m not sure at the moment.)
我在想:
X 可以设置一次。
I was thinking something like:
X can be set exactly once.
这并不完全相同,但在许多情况下,“设置变量一次,然后继续使用它”的解决方案是使用或不使用特殊工厂方法的简单子类化。
It’s not exactly the same thing but in many cases, the solution for this ‘set the variable once and then keep using it’ is simple subclassing with or without a special factory method.
您始终可以将该值移动到另一个对象,仅初始化一次并在需要时访问它。
You can always move that value to another object, initialize it only once and access it when needed.