LombaX LombaX - 4 months ago 15
Swift Question

Call a method from a String in Swift

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?

Answer

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 \n");
    } 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