Swift and call C++ function with &int argument

Question

I study how to use C++ with Swift in one project.

I have C++ class with interface

class SwypeDetect { public: SwypeDetect(); void asyncFunction(int &a); };

and implementation

SwypeDetect::SwypeDetect() {} void SwypeDetect::asyncFunction(int &a) { a = 1; sleep(3); a = 10; sleep(3); a = 100; }

asyncFunction just change value of argument a three time each three seconds. Of course I create Objective-C wrapper

@interface Wrapper() @property (nonatomic, assign) SwypeDetect detector; @end @implementation Wrapper - (instancetype) init { self = [super init]; if (self) { _detector = SwypeDetect(); } return self; } - (void)asyncFunction:(int *)a { _detector.asyncFunction(*a); } @end

And then use this wrapper in Swift class

class ViewController: UIViewController { let queue = DispatchQueue(label: "wrapper queue", attributes:.concurrent) var valueA: Int32 = 0 { didSet { print("new valueA \(valueA) on time \(Date())") } } var detector: Wrapper? { didSet { if let detector = detector { queue.async { detector.asyncFunction(&self.valueA) } } } } override func viewDidLoad() { super.viewDidLoad() detector = Wrapper() } }

I expect that didSet block of valueA would be call three times, but in console I see only call with last change of valueA: "new valueA 100 on time 2018-03-26 11:50:18 +0000". What could I do to change this behaviour?

Swift registers the changed value only on return from detector.asyncFunction(&self.valueA). — Martin R
– Martin R, Commented Mar 26, 2018 at 12:32
I see that, but how can I observe changes while before return from detector.asyncFunction(&self.valueA)? — FuzzzzyBoy
– FuzzzzyBoy, Commented Mar 26, 2018 at 12:34

Vyacheslav · Accepted Answer · 2018-03-26 12:52:38Z

You have to say somewhere 'the variable is changed'. You can do it using closures/blocks/lambdas for instance

ViewController.swift

import UIKit class ViewController: UIViewController { let queue = DispatchQueue(label: "wrapper queue", attributes:.concurrent) var valueA: Int32 = 0 { didSet { print("new valueA \(valueA) on time \(Date())") } } var detector: Wrapper? { didSet { if let detector = detector { queue.async { detector.asyncFunction(&self.valueA) { value in print("iteration value: \(value)") } } } } } override func viewDidLoad() { super.viewDidLoad() detector = Wrapper() } }

Wrapper.h

#import <Foundation/Foundation.h> @interface Wrapper : NSObject - (instancetype) init; - (void)asyncFunction:(int *)a progressHandler: (void(^)(int))progressHandler; @end

Wrapper.mm

#import "Wrapper.h" #import "SwypeDetect.hpp" @interface Wrapper() @property (nonatomic, assign) SwypeDetect detector; @end @implementation Wrapper - (instancetype) init { self = [super init]; if (self) { _detector = SwypeDetect(); } return self; } - (void)asyncFunction:(int *)a progressHandler: (void(^)(int))progressHandler { _detector.asyncFunction(*a , progressHandler); } @end

SwipeDetect.cpp

#include "SwypeDetect.hpp" #include <unistd.h> #include <iostream> SwypeDetect::SwypeDetect() {} void SwypeDetect::asyncFunction(int &a, std::function<void(int)> f) { std::cout << "Come up and C++ me some time." << std::endl; a = 1; f(a); sleep(1); a = 10; f(a); sleep(1); a = 100; f(a); }

SwypeDetect.hpp

#include <stdio.h> #include <functional> class SwypeDetect { public: SwypeDetect(); void asyncFunction(int &a, std::function<void(int)> f); };

testsetsetset-Bridging-Header.h

#import "Wrapper.h"

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