Simple example...
Let's say the child view controller has a UISlider
and we want to pass the value of the slider back to the parent via a delegate.
In the child view controller's header file, declare the delegate type and its methods:
ChildViewController.h
#import <UIKit/UIKit.h>
// 1. Forward declaration of ChildViewControllerDelegate - this just declares
// that a ChildViewControllerDelegate type exists so that we can use it
// later.
@protocol ChildViewControllerDelegate;
// 2. Declaration of the view controller class, as usual
@interface ChildViewController : UIViewController
// Delegate properties should always be weak references
// See http://stackoverflow.com/a/4796131/263871 for the rationale
// (Tip: If you're not using ARC, use `assign` instead of `weak`)
@property (nonatomic, weak) id<ChildViewControllerDelegate> delegate;
// A simple IBAction method that I'll associate with a close button in
// the UI. We'll call the delegate's childViewController:didChooseValue:
// method inside this handler.
- (IBAction)handleCloseButton:(id)sender;
@end
// 3. Definition of the delegate's interface
@protocol ChildViewControllerDelegate <NSObject>
- (void)childViewController:(ChildViewController*)viewController
didChooseValue:(CGFloat)value;
@end
In the child view controller's implementation, call the delegate methods as required.
ChildViewController.m
#import "ChildViewController.h"
@implementation ChildViewController
- (void)handleCloseButton:(id)sender {
// Xcode will complain if we access a weak property more than
// once here, since it could in theory be nilled between accesses
// leading to unpredictable results. So we'll start by taking
// a local, strong reference to the delegate.
id<ChildViewControllerDelegate> strongDelegate = self.delegate;
// Our delegate method is optional, so we should
// check that the delegate implements it
if ([strongDelegate respondsToSelector:@selector(childViewController:didChooseValue:)]) {
[strongDelegate childViewController:self didChooseValue:self.slider.value];
}
}
@end
In the parent view controller's header file, declare that it implements the ChildViewControllerDelegate
protocol.
RootViewController.h
#import <UIKit/UIKit.h>
#import "ChildViewController.h"
@interface RootViewController : UITableViewController <ChildViewControllerDelegate>
@end
In the parent view controller's implementation, implement the delegate methods appropriately.
RootViewController.m
#import "RootViewController.h"
@implementation RootViewController
- (void)tableView:(UITableView *)tableView didSelectRowAtIndexPath:(NSIndexPath *)indexPath {
ChildViewController *detailViewController = [[ChildViewController alloc] init];
// Assign self as the delegate for the child view controller
detailViewController.delegate = self;
[self.navigationController pushViewController:detailViewController animated:YES];
}
// Implement the delegate methods for ChildViewControllerDelegate
- (void)childViewController:(ChildViewController *)viewController didChooseValue:(CGFloat)value {
// Do something with value...
// ...then dismiss the child view controller
[self.navigationController popViewControllerAnimated:YES];
}
@end
Hope this helps!