How to make a callback function pointer from C++ to Swift? - c++

Question

There are many tutorials to make this on web but none are clear and objective, because thats I'm prefer show my case.
I'm developing a iOS app that use a C API to connect to a service web and it have callback functions, my task is simple, I must get events generateds in C code on Swift code. To this I tried some ways, the current way that I'm trying is the follow.
Scenario
I've three files: wrapper.cpp, Bridging-Header.h and ViewController.swift
On Briding-Header.h I declared a function's pointer.
Bridging-Header.h
void callback_t(void(*f)(unsigned char*));
On wrapper.cpp I wrote my code to connect to web and use callback functions. So this is the callback method to get state connection when is disconnected.
wrapper.cpp
void callback_web_disconnected(unsigned char *c) {
printf("Disconnected %s",c);
// Here I want invoke a method to Swift code
callback_t(r); // But this not works
}
It's not compiles, error message: Use of undeclared identifier 'callback_t'.
If I try write: void callback_frame(unsigned char*);, linker command failed with exit code 1 error occurs.
ViewController.swift
func callback_t(_ f: ((UnsafeMutablePointer<UInt8>?) -> Void)!) {
print("ARRIVED ON VIEWCONTROLLER!")
// Here I want get the error code (unsigned char*) passed as parameter
}
I can't import Bridging-Header.h file on wrapper.cpp because cause many conflicts.

Answer 1

First, callback_t is not an identifier. I don't see it typedef anywhere..
Second, you need some way of telling C++ that the callback is your swift function. To do that, I pass it as a parameter to the C++ function similar to how we do it in Objective-C and Swift.. Otherwise you need to store the callback in a global variable somewhere and have C++ access it.
Using the first method of passing the callback as a parameter:
First in the C++ header (Foo.h) I did (Do NOT remove the ifdef stuff.. the compiler uses C linkage when importing to Swift but when compiling the C++ side, it'll be mangled so to make it use C linkage, we extern "C" the code):
#ifndef Foo_hpp
#define Foo_hpp
#include <stdio.h>
#ifdef __cplusplus
extern "C" {
#endif
typedef void(*callback_t)(const char *);
void callback_web_disconnected(callback_t);
#ifdef __cplusplus
} // extern "C"
#endif
#endif /* Foo_hpp */
Then in the implementation file (Foo.cpp) I did:
#include "Foo.h"
#include <thread>
#include <iostream>
#ifdef __cplusplus
extern "C" {
#endif
void callback_web_disconnected(callback_t callback)
{
std::thread t = std::thread([callback] {
std::this_thread::sleep_for(std::chrono::seconds(2));
if (callback)
{
callback("Hello World");
}
});
t.detach();
}
#ifdef __cplusplus
} // extern "C"
#endif
Then in ViewController.swift I did:
import UIKit
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
callback_web_disconnected({
if let ptr = $0 {
let str = String(cString: ptr)
print(str)
}
})
}
}
It works fine. The Swift code gets called from C++ after 2 seconds has passed.
Using the second method of storing the callback in a global variable (which I despise but let's not get into that)..
In Foo.h I did:
#ifndef Foo_hpp
#define Foo_hpp
#include <stdio.h>
#ifdef __cplusplus
extern "C" {
#endif
typedef void(*callback_t)(const char *);
callback_t globalCallback; //Declare a global variable..
void callback_web_disconnected();
#ifdef __cplusplus
} // extern "C"
#endif
#endif /* Foo_hpp */
In Foo.cpp I did:
#include "Foo.h"
#include <thread>
#include <iostream>
#ifdef __cplusplus
extern "C" {
#endif
void callback_web_disconnected()
{
std::thread t = std::thread([] {
std::this_thread::sleep_for(std::chrono::seconds(2));
if (globalCallback)
{
globalCallback("Hello World");
}
});
t.detach();
}
#ifdef __cplusplus
} // extern "C"
#endif
In ViewController.swift, I did:
import UIKit
class ViewController: UIViewController {
override func viewDidLoad() {
super.viewDidLoad()
// Do any additional setup after loading the view, typically from a nib.
//Set the globalCallback variable to some block or function we want to be called..
globalCallback = {
if let ptr = $0 {
let str = String(cString: ptr)
print(str)
}
}
//Trigger our test.. I guess your C++ code will be doing this anyway and it'll call the globalCallback.. but for the sake of this example, I've done it here..
callback_web_disconnected()
}
}