How to access iOS photo library / use Objective-C properly?

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3

I figured out how to achieve this. For everyone interested, here is the code:

MyViewController.h

#pragma once

#if PLATFORM_IOS
#import <UIKit/UIKit.h>

// UIImagePickerControllerDelegate to respond to user interactions
// UINavigationControllerDelegate because we want to present the photo library modally
@interface MyViewController : UIViewController <UIImagePickerControllerDelegate, UINavigationControllerDelegate>

// function to run in iOS main thread
+ (void) runSelectPhoto;

@end
#endif

MyViewController.cpp

// ...

#if PLATFORM_IOS
#include "IOSAppDelegate.h"
#import <Foundation/Foundation.h>
#endif

#include "MyViewController.h"

#if PLATFORM_IOS

@interface MyViewController()
@end

@implementation MyViewController

+ (MyViewController*)GetDelegate
{
    static MyViewController * Singleton = [[MyViewController alloc] init];
    return Singleton;
}

-(void)selectPhoto
{
    // create the Photo Library display object
    UIImagePickerController *picker = [[UIImagePickerController alloc] init];
    // type of picker interface to be displayed by the controller
    picker.sourceType = UIImagePickerControllerSourceTypePhotoLibrary;
    // delegate object which receives notification when the user picks an image or exits the picker interface
    picker.delegate = self;
    // show object
    // the UIImagePickerController is ONLY supported in portrait mode (seestackoverflow.com/a/16346664/4228316)
    [[IOSAppDelegate GetDelegate].IOSController presentViewController : picker animated : NO completion : nil];
}

+(void)runSelectPhoto
{
    // perform this action on the iOS main thread
    [[MyViewController GetDelegate] performSelectorOnMainThread:@selector(selectPhoto) withObject:nil waitUntilDone : NO];
}

#pragma mark - Image Picker Controller delegate methods

- (void)imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo : (NSDictionary *)info{

    // get the chosen original image
    UIImage *chosenImage = info[UIImagePickerControllerOriginalImage];
    // ...

    // call C++ functions on the game thread
    [FIOSAsyncTask CreateTaskWithBlock : ^ bool(void)
    {
        // call C++ functions
        // ...
        return true;
    }];

    // get rid of UIImagePicker
    [picker dismissViewControllerAnimated : YES completion : NULL];
}

-(void)imagePickerControllerDidCancel:(UIImagePickerController *)picker{
    [picker dismissViewControllerAnimated : YES completion : NULL];
}

@end
#endif

The method to show the image picker is called with

#if PLATFORM_IOS
	NSLog(@"Show image picker");
	[MyViewController runSelectPhoto];
#endif