Call a method from a String in Swift

Question

Calling a method from its name (in a String format) can be sometimes useful.
In Swift it is recomended to change behavior and to use closures to do something "dynamically", so for example you can have a dictionary of functions, with the name as the key, and the implementation as the value.
However, sometimes you want to simply know "how to do it", and this is the reason of this question.
So, how to call dynamically a Swift method starting from it's name as string?

In Objective C it was simple:

[self performSelector:NSSelectorFromString(@"aSelector")];

But performSelector is banned in Swift
Is there any alternative?

question from:https://stackoverflow.com/questions/24245262/call-a-method-from-a-string-in-swift

Answer 1

In Swift, you should use closures and change your approach. However, if you want to use performSelector to dynamically call a method given only it's String signature, altough it's not supported natively, I've found how to do it.

It is possible to create a C alternative to performSelector that:

• works even on native swift classes (non objective-c)
• takes a selector from string

However it's not so straightforward to implement a complete version of it, and it's necessary to create the method in C.

in C we have dlsym(), a function that returns a pointer to a function given the char symbol. Well, reading this interesting post: http://www.eswick.com/2014/06/inside-swift/ I've learned a lot of interesting things about swift.

Swift instance methods are plain functions with a specific signature, like this

_TFC14FirstSwiftTest12ASampleClass13aTestFunctionfS0_FT_CSo8NSString

where the "self" value is passed as the last parameter

in short you can call it directly from the c side without any kind of bridging, it is sufficient to rebuild the correct function signature. In the signature above, there is the name of the project (FirstSwiftTest) and the lenght (14), the name of the class (ASampleClass) and the lenght (12), the name of the function (aTestFunction) and the lenght (13), then other values as the return type ecc ecc. For other details look at the previous link

The function above, is the representation of this:

class ASampleClass
{
    func aTestFunction() -> NSString
    {
        println("called correctly")
        return NSString(string: "test")
    }

}

Well, on the c side, I was able to create this function

#include <stdio.h>
#include <dlfcn.h>

typedef struct objc_object *id;

id _performMethod(id stringMethod, id onObject)
{
    // ...
    // here the code (to be created) to translate stringMethod in _TFC14FirstSwiftTest12ASampleClass13aTestFunctionfS0_FT_CSo8NSString
    // ...

    id (*functionImplementation)(id);
    *(void **) (&functionImplementation) = dlsym(RTLD_DEFAULT, "_TFC14FirstSwiftTest12ASampleClass13aTestFunctionfS0_FT_CSo8NSString");

    char *error;

    if ((error = dlerror()) != NULL)  {
        printf("Method not found 
");
    } else {
        return functionImplementation(onObject); // <--- call the function
    }
    return NULL
}

And then called it on the swift side

    let sampleClassInstance = ASampleClass()

    println(_performMethod("aTestFunction", sampleClassInstance))

The function resulted in these statement printed on the log:

called correctly
test

So it should be not so difficult to create a _performMethod() alternative in C that:

• creates automatically the function signature (since it seems to have a logic :-)
• manages different return value types and parameters

EDIT

In Swift 2 (and maybe in Beta3, I didn't try) It seems that performSelector() is permitted (and you can call it only on NSObject subclasses). Examining the binary, It seems that now Swift creates static functions that can be specifically called by performSelector.

I created this class

class TestClass: NSObject {
    func test() -> Void {
        print("Hello");
    }
}

let test = TestClass()
let aSel : Selector = NSSelectorFromString("test")
test.performSelector(aSel)

and now in the binary I find

000000010026d830 t __TToFC7Perform9TestClass4testfT_T_

At this time, I don't understand well the reasons behind this, but I'll investigate further