I have a generic function that calls a web service and serialize the JSON response back to an object.
class func invokeService<T>(service: String, withParams params: Dictionary<String, String>, returningClass: AnyClass, completionHandler handler: ((T) -> ())) {
/* Construct the URL, call the service and parse the response */
}
What I'm trying to accomplish is is the equivalent of this Java code
public <T> T invokeService(final String serviceURLSuffix, final Map<String, String> params,
final Class<T> classTypeToReturn) {
}
AnyClass
as a parameter type the
right thing to do?MyObject.self
as the returningClass value, but I get a compilation error "Cannot convert the expression's type '()' to type 'String'"CastDAO.invokeService("test", withParams: ["test" : "test"], returningClass: CityInfo.self) { cityInfo in /*...*/
}
Edit:
I tried using object_getClass
, as mentioned by holex, but now I get:
error: "Type 'CityInfo.Type' does not conform to protocol 'AnyObject'"
What need to be done to conform to the protocol?
class CityInfo : NSObject {
var cityName: String?
var regionCode: String?
var regionName: String?
}
You are approaching it in the wrong way: in Swift, unlike Objective-C, classes have specific types and even have an inheritance hierarchy (that is, if class B
inherits from A
, then B.Type
also inherits from A.Type
):
class A {}
class B: A {}
class C {}
// B inherits from A
let object: A = B()
// B.Type also inherits from A.Type
let type: A.Type = B.self
// Error: 'C' is not a subtype of 'A'
let type2: A.Type = C.self
That's why you shouldn't use AnyClass
, unless you really want to allow any class. In this case the right type would be T.Type
, because it expresses the link between the returningClass
parameter and the parameter of the closure.
In fact, using it instead of AnyClass
allows the compiler to correctly infer the types in the method call:
class func invokeService<T>(service: String, withParams params: Dictionary<String, String>, returningClass: T.Type, completionHandler handler: ((T) -> ())) {
// The compiler correctly infers that T is the class of the instances of returningClass
handler(returningClass())
}
Now there's the problem of constructing an instance of T
to pass to handler
: if you try and run the code right now the compiler will complain that T
is not constructible with ()
. And rightfully so: T
has to be explicitly constrained to require that it implements a specific initializer.
This can be done with a protocol like the following one:
protocol Initable {
init()
}
class CityInfo : NSObject, Initable {
var cityName: String?
var regionCode: String?
var regionName: String?
// Nothing to change here, CityInfo already implements init()
}
Then you only have to change the generic constraints of invokeService
from <T>
to <T: Initable>
.
If you get strange errors like "Cannot convert the expression's type '()' to type 'String'", it is often useful to move every argument of the method call to its own variable. It helps narrowing down the code that is causing the error and uncovering type inference issues:
let service = "test"
let params = ["test" : "test"]
let returningClass = CityInfo.self
CastDAO.invokeService(service, withParams: params, returningClass: returningClass) { cityInfo in /*...*/
}
Now there are two possibilities: the error moves to one of the variables (which means that the wrong part is there) or you get a cryptic message like "Cannot convert the expression's type ()
to type ($T6) -> ($T6) -> $T5
".
The cause of the latter error is that the compiler is not able to infer the types of what you wrote. In this case the problem is that T
is only used in the parameter of the closure and the closure you passed doesn't indicate any particular type so the compiler doesn't know what type to infer. By changing the type of returningClass
to include T
you give the compiler a way to determine the generic parameter.