[英]Swift generic function call underlying method based on generic type
I am writing an interface to Someone Else's Code (SEC), and I have a long list of functions more or less like this: 我正在为Someone Else的代码(SEC)编写一个接口,我有一长串功能或多或少像这样:
public func readString(_ row: Row, columnName: String) -> String? {
return SEC.getString(row, columnName)
}
public func readInt(_ row: Row, columnName: String) -> Int? {
return SEC.getInt(row, columnName)
}
And so on. 等等。
What I would like to do is have a single function: 我想做的是有一个功能:
public func read<T>(_ row: Row, columnName: String) -> T? {
// call correct SEC.get* method
}
I tried a simple switch statement on T
and on T.Type
, but no dice. 我在T
和T.Type
上尝试了一个简单的switch语句,但没有骰子。 I also tried a modified version of the above: 我也试过上面的修改版:
public func read<T: ReadableType>(_ row: Row, columnName: String) -> T? {
let getter = T.getterMethod
return getter(row, columnName)
}
Where I could create an enum
of the metatypes that had SEC.get*
functions, and extend it to return the proper getter method. 我可以在哪里创建具有SEC.get*
函数的元类型的enum
,并扩展它以返回正确的getter方法。 This to me seemed ideal. 这对我来说似乎很理想。 Alas: 唉:
public enum ReadableTypes {
case String.Type // nope
case Int.self // also nope
}
I'm not sure what else to try; 我不知道还有什么可以尝试的; it's not the end of the world if I simply have a dozen read* methods, but the code that calls this method could be so dang tight if I could make it generic. 如果我只是有十几个读取方法,那么这不是世界末日,但是如果我可以使它成为通用的话,那么调用这种方法的代码就会如此紧密。
You can individually test to see what type the generic placeholder represents like this: 您可以单独测试以查看通用占位符代表的类型:
if T.self is Int.Type //...
The same sort of test can be done in a switch..case
statement. 可以在switch..case
语句中完成相同类型的测试。
Assuming SEC
is of type SECType
what I'd do is extend SECType
to have a generic get
method that keys on the return type: 假设SEC
是SECType
类型,我要做的是扩展SECType
以使用一个通用的get
方法来键入返回类型:
extension SECType {
public func get<T>(_ row: Row, _ columnName: String) -> T? {
// switch on the generic type
switch T.self {
// As much as I dislike force-unwrapping an Optional
// this is about as safe as it gets.
case is Int.Type : return getInt (row, columnName) as! T?
case is String.Type: return getString(row, columnName) as! T?
//...
default: return nil
}
}
}
Now you can write your own read
function like: 现在您可以编写自己的read
函数,如:
public func read<T>(_ row: Row, columnName: String) -> T? {
return SEC.get(row, columnName)
}
Of course you can skip doing the extension
and just do a generic function with a switch
. 当然,您可以跳过extension
,只需使用switch
执行通用功能。 However, it's harmless to add the method to the type and it makes sense for the type to have a generic method like this. 但是,将该方法添加到类型中是无害的,并且类型具有这样的泛型方法是有意义的。
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