How to use a native image chooser in iOS - not perfected, just proof of concept.

main.kv

BoxLayout:
    Button:
        text: "hello"
        on_release:
            app.pick_image()
    Button:
        text: "update"
        on_release:
            app.update()

main.m snippet

#import <Photos/Photos.h>

@interface NativeImagePicker : UIViewController
@property (strong, nonatomic) UIImagePickerController *imagePickerController;
@end
@implementation NativeImagePicker

-(id)init {
    NSLog(@"initializing NativeImagePicker");
    return self;
}

- (int)displayImagePicker {
    PHAuthorizationStatus status = [PHPhotoLibrary authorizationStatus];
    if(status == PHAuthorizationStatusNotDetermined) {
        // Request photo authorization
        [PHPhotoLibrary requestAuthorization:^(PHAuthorizationStatus status) {
            // User code (show imagepicker)
        }];
    } else if (status == PHAuthorizationStatusAuthorized) {
        // User code
    } else if (status == PHAuthorizationStatusRestricted) {
        // User code
    } else if (status == PHAuthorizationStatusDenied) {
        // User code
    }
    UIImagePickerController* imagePicker = [[UIImagePickerController alloc]init];
    // Check if image access is authorized
    if([UIImagePickerController isSourceTypeAvailable:UIImagePickerControllerSourceTypePhotoLibrary]) {
        imagePicker.sourceType = UIImagePickerControllerSourceTypePhotoLibrary;
        // Use delegate methods to get result of photo library -- Look up UIImagePicker delegate methods
        imagePicker.delegate = self;
        // Must show our imagepicker above the root view
        UIViewController *top = [UIApplication sharedApplication].keyWindow.rootViewController;
        [top presentViewController:imagePicker animated:YES completion: nil];
    }
    return 0;
}
    - (void) imagePickerController:(UIImagePickerController *)picker didFinishPickingMediaWithInfo:(NSDictionary *)info {
        // Save the image as "cached.png" in a folder accessible by the app (user_data_dir in python/kivy)
        UIImage *image = info[UIImagePickerControllerOriginalImage];
        NSData *imageData = UIImagePNGRepresentation(image);
        
        NSArray *paths = NSSearchPathForDirectoriesInDomains(NSDocumentDirectory, NSUserDomainMask, YES);
        NSString *documentsDirectory = [paths objectAtIndex:0];
        
        NSString *imagePath = [documentsDirectory stringByAppendingPathComponent:[NSString stringWithFormat:@"%@.png",@"cached"]];
        
        NSLog(@"pre writing to file");
        if (![imageData writeToFile:imagePath atomically:NO])
        {
            NSLog(@"Failed to cache image data to disk");
        }
        else
        {
            NSLog(@"the cachedImagedPath is %@",imagePath);
        }
        [picker dismissViewControllerAnimated:YES completion:nil];//{
        // <#code#>
        //}];
    }

@end

main.py

import sys
sys.path.append("/".join(x for x in __file__.split("/")[:-1]))
from kivy.app import App
from kivy.utils import platform
if platform == 'ios':
    from pyobjus import autoclass
from kivy.uix.image import Image
from kivy.properties import ObjectProperty, StringProperty
import pyrebase



class MainApp(App):
    native_image_picker = ObjectProperty(None)
    image_path = StringProperty("")

    def on_start(self):
        if platform == 'ios':
            self.native_image_picker = autoclass("NativeImagePicker").alloc().init()

    def update(self):
        print("Updating image...")

        folder = "/".join(x for x in self.user_data_dir.split("/")[:-1])
        image_path = folder + "/" + "cached.png"
        image = Image(source=image_path, nocache=True)
        self.root.add_widget(image)


    def pick_image(self):
        if platform == 'ios':
            self.native_image_picker.displayImagePicker()




MainApp().run()