How to use delegate in objective C and swift Viewcontroller

Question

My project is combined with one viewcontroller written in objective C ( objCViewController ) and the other is written by swift ( SwiftViewCOntroller ). I have several variable NSString. I would like to update the string in objCViewController and access it in SwiftViewController by using delegate because I need to change between these two viewcontroller continuously and keep updating the string.

Here is the code:

objCViewController.h

#import <UIKit/UIKit.h>

@protocol MyDelegate <NSObject>
@end

@interface objCViewController : UIViewController{
  NSString * stringbeingpassed;
}

@property (nonatomic, weak) id<MyDelegate> delegate;
@property (nonatomic, retain) NSString * stringbeingpassed;
@end

objCViewController.m

@implementation objCViewController
@synthesize delegate;
@synthesize stringbeingpassed;

- (void)updatestring {
  //update string in this method
  NSString * newstring = @"testing";

  if (delegate != nil && [delegate respondsToSelector:@selector(stringUpdated:)]) {
    [delegate stringUpdated: newstring];
}

}

bridging header.h:

#import "objCViewController.h"

SwiftViewController.swift:

protocol MyDelegate {
func stringUpdated(newMessage: String)
}

import UIKit
@objc class SwiftViewController: UIViewController, MyDelegate{
override func viewDidLoad() {
    super.viewDidLoad()
}

func stringUpdated(newMessage:String) {
let newMessage = "sent string"
}

I have tried to use delegate but I have no idea how to use it. I'm completely new to swift and objective C

Q1. I would like to ask how can I assign my newstring to delegate in objCViewController and then pass it to SwiftViewController .

Q2. Another question is that how can I retrieve the data in delegate in SwiftViewController . What should I add?

Q3. Anything else that I have missed in defining delegate? Do I need to define it in both viewcontroller? Thank you.

Answer 1

It's hard to say whether a delegate is the right tool for the job due to the minimal information provided. In fact I might suggest researching better ways to architect your app.

That being said, it would be helpful to see the code for your protocol MyDelegate . If this protocol doesn't already it will need a 'func' that accepts a 'String' as an argument/parameter. For now let's call this function stringUpdated . With that code that's provided you're going to need to set an instance of SwiftViewController to your property delegate within objCViewController . That way when updatestring is called you can do something like this:

- (void)updatestring {
    //update string in this method
    NSString * newstring = @"testing";

    if (delegate != nil && [delegate respondsToSelector:@selector(stringUpdated:)]) {
        [delegate stringUpdated: newstring]
    }
}

In SwiftViewController you'll have to adopt the protocol like so:

@objc class SwiftViewController: UIViewController, MyDelegate {

and then adhere to the protocol by implementing the function stringUpdated .

Update

Your protocol is still missing the method. It should look like this:

@protocol MyDelegate
- (void) stringUpdated:(NSString *)updatedString;
@end

Answer 2

let me answer Q3 first Q3:Anything else that I have missed in defining delegate? Do I need to define it in both viewcontroller? No you haven't to define MyDelegate in both viewControllers, just add it to the objCViewControlle

Now here under I had modified you code

objCViewController.h

@protocol MyDelegate <NSObject>
- (void) stringUpdated:(NSString *)updatedString;
- (NSString *) getString;
@end

@interface objCViewController : UIViewController{
  NSString * stringbeingpassed;
}

@property (nonatomic, weak) id<MyDelegate> delegate;
@property (nonatomic, retain) NSString * stringbeingpassed;
@end

bridging header.h:

#import "objCViewController.h"

SwiftViewController.swift:

import UIKit
@objc class SwiftViewController: UIViewController, MyDelegate{
var thisIsObjectiveVCString:String?
override func viewDidLoad() {
    super.viewDidLoad()
}

func stringUpdated(newMessage:String) {
thisIsObjectiveVCString = newMessage
}

func getString() ->String {
return thisIsObjectiveVCString
}
}

Q2: how can I retrieve the data in delegate in SwiftViewController. What should I add?

on objCViewControlle

if (delegate != nil && [delegate respondsToSelector:@selector(getString)]) {
        NSString * theString = [delegate getString];
    }

Q1: how can I assign my newstring to delegate in objCViewController and then pass it to SwiftViewController

actually on objCViewController if you call updatestring then you will update SwiftViewController