I am trying to figure out delegation in iOS. Basically, I have classA which contains methodA. I also have classB which I would like to call methodA from.
To be specific, I have a class called ViewControllerRootHome and class called ViewControllerRootHomeLeftPanel. The ViewControllerRootHome has a method in it called, movePanelToOriginalPosition I would like to call this method from the ViewControllerRootHomeLeftPanel class.
Any help would be greatly appreciated. Ohh forgot to mention I'm still using Objective-C for the project.
I'll give this an attempt.
Let's say you've got a ViewController called ViewControllerA, and another ViewController called ViewControllerB. We want to call a method inside A from B. How are we going to achieve this?
Simple. We will define a protocol inside B that A will comply to. Let me do that right here.
#import ...
#protocol myProtocol; // Declare Protocol
#interface ViewControllerB : UIViewController
#property (nonatomic, weak)id <myProtocol> myDelegate; // Create Delegate property
#end // Notice this is AFTER the #end of the #interface declaration
#protocol myProtocol <NSObject> // Define Protocol
-(void)doSomething;
#end
Okay, now you have defined a protocol called myProtocol that you wish to use inside ViewControllerA
Let us use it there. We will have to do several things: One, comply to the protocol. And two, set our current VC as it's delegate!
#import ...
#import "ViewControllerB" // IMPORT the VC with the Protocol
#interface ViewControllerA : UIViewController <myProtocol> // Conform to Protocl
#property (nonatomic)ViewControllerB *viewControllerB;
#end
Notice I've defined a property of type ViewControllerB. You will need to have a reference to ViewControllerB in some shape or form. This is usually easy to achieve because you normally create an instance of ViewControllerB from ViewControllerA. Otherwise it will need to be set externally or passed to ViewControllerA upon initialization and you set it as a property there.
Inside ViewControllerA.m, set ViewControllerA as it's delegate:
self.ViewControllerB.myDelegate = self;
Now, all you have to do is define the method from the protocol inside ViewController A so it can be called:
-(void)doSomething
{
...
}
This is all you need to do. However, please note that if you have TWO ViewControllers complying to each other's protocols, you will likely have to declare the protocols inside their own header files.
Edit: How to call the method.
If you want to call the method defined inside the protocol. You will do so inside ViewControllerB, like so:
if ([self.myDelegate respondsToSelector:#selector(doSomething)])
{
[self.myDelegate doSomething];
}
Related
Normally I set up my delegates like normal:
#interface CAKGameViewController : UIViewController <CAKGameSceneDelegate>
then follow up in the CAKGameScene.h with
#protocol CAKGameSceneDelegate;
#interface CAKGameScene : SKScene <SKPhysicsContactDelegate>
...(properties, etc)
#protocol CAKGameSceneDelegate <NSObject>
#optional
- (void)getStarted;
#end
But as most of us know as a property you have to setup shop like this:
#property (nonatomic, weak, readwrite) id<CAKGameSceneDelegate> myDelegate;
My question is, I want to use the actual SKSceneDelegate (self.delegate, not (self.myDelegate) and go like this:
#interface CAKGameViewController : UIViewController <SKSceneDelegate>
Now this is fine and I can use self.delegate to reference the controller, but the problem is I can't seem to figure out how to customize the CAKGameViewController, i.e. setup the protocol further, or customize the protocol, for my methods and/or properties. :(
Any assistance would be helpful.
Thanks
I finally found a suitable workaround to use and assign the SKScene delegate. The trick is to cast (of course right)!?
You interface the GameViewController and conform to protocols
in CAKGameViewController.h
#interface CAKGameViewController : UIViewController
<SKSceneDelegate, CAKGameSceneDelegate>
in CAKGameScene.h
#protocol CAKGameSceneDelegate <NSObject>
// any #property;
// or - (void)methods;
#end
in CAKGameViewController.m you assign the delegate using self:
self.scene.delegate = self; //this is the Apple delegate property
then whenever you want to use
something = self.delegate.property; // <or>
[self.delegate method]; // from the CAKGameSceneDelegate protocol
you cast
something = ((id<CAKGameSceneDelegate>)self.delegate).property; // <or>
[((id<CAKGameSceneDelegate>)self.delegate) method]; // from non-apple protocol
what I did was just make a #define
#define delegateHelper ((id<CAKGameSceneDelegate>)self.delegate)
then I can:
something = delegateHelper.property;
[delegateHelper method];
I just figure if you are going to have a property called delegate, like most cocoa/objective-c objects do, to make things work, the delegate should be the delegate, you shouldn't have to use myDelegate, or something silly. delegateHelper may not be much better, but for me I can live with this workaround.
You can extend the original delegate in the protocol definition:
#protocol CAKGameSceneDelegate <SKSceneDelegate>
//extend SKSceneDelegate with new functions here
#end
Or in Swift:
protocol CAKGameSceneDelegate: SKSceneDelegate
{
//extend SKSceneDelegate with new functions here
}
You do not have to redefine delegate if you do it this way.
I have three viewControllers. When I'm on the third view controller I want to send a message to the first one. I'm using a protocol and trying to set the delegate for this.
viewControllerC.h
#protocol ViewControllerCDelegate
- (void)performAction;
#end
...
#property (nonatomic, strong) id<ViewControllerCDelegate> delegate;
viewControllerA.h
#interface ViewControllerA : UIViewController <ViewControllerCDelegate>
viewControllerA.m
...
- (void)performAction {
NSLog(#"Action was performed");
}
So the only problem is, I can't set the delegate from the third to the first. How can I set viewControllerC's delegate to viewControlerA?
Here's an image to describe it:
You'd have to pass along the A controller through to the B controller to be set as the delegate for C when it is created. Kind of messy.
In this case though it might make more sense to use a notification model where Controller A listens for a NSNotification that the action was completed on C.
EDIT: edited for clarity
Disclaimer: I'm new and pretty bad. But I have tried very hard and read lots of stuff to figure this out, but I have not...
I think my whole delegate pattern would work, except I can't figure out how to set the delegate property of ViewController to self in the MatchLetter class. The reason is because I can't figure out how to call code there. It's not a view controller, so viewDidLoad or prepareForSegue won't work.
This is what I've got:
ViewController.h
#import <UIKit/UIKit.h>
#class ViewController;
#protocol letterMatchProtocol <NSObject>
- (BOOL) isLetterMatch:(char) firstLetter;
#end
#interface ViewController : UIViewController
#property (nonatomic, weak) id <letterMatchProtocol> delegate;
#end
ViewController.m
#import "ViewController.h"
#interface ViewController ()
#end
#implementation ViewController
- (void)viewDidLoad {
[super viewDidLoad];
char c = 'a';
// This is the method I want to delegate to MatchLetter, to have a BOOL returned
BOOL returnValue = [self.delegate isLetterMatch:c];
}
#end
MatchLetter.h
#import <Foundation/Foundation.h>
#import "ViewController.h"
#interface Delegate : NSObject <letterMatchProtocol>
#end
MatchLetter.m
#import "MatchLetter.h"
#implementation Delegate
// this is the code I think I need to run here, to set the delegate property...
// ViewController *viewController = [ViewController new];
// viewController.delegate = self;
// ... so that isLetterMatch can be run here from ViewController.m
// But I don't know where to put this code, or how to get it to run before the ViewController
// especially since there are no segues or views to load.
- (BOOL) isLetterMatch:(char)firstLetter {
if (firstLetter == 'a') {
return YES;
}
else {
return NO;
}
}
#end
Can somebody please tell me the best way to proceed? Thanks for reading
You asked "Where to set delegate = self? Or should I just use a different design pattern?".
Answer: Don't. An object should never be it's own delegate.
Your code is quite a mess.
Don't name a class "Delegate". A delegate is a design pattern. The whole point of a delegate is that any object that conforms to a particular protocol ("speaks the language") can serve as the delegate. You don't need to know what class of object is serving as the delegate, but only that it speaks the language you need.
An analogy: When you call the operator, you don't care who is working the operator desk. You don't care about his/her gender, religion, ethnic background, how tall they are, etc. You just care that they speak your language.
Likewise, when you set up a delegate, it doesn't matter what type of object gets set as the delegate. All that matters is that the object that is the delegate conforms to the protocol for that delegate.
A table view can have ANY object serve as it's delegate, as long as that object conforms to the UITableViewDelegate protocol. You usually make you view controller be the table view's delegate, but you don't have to. You could create a custom class that manages your table views, and have it be the delegate. There is no "TableViewDelegate" object class. There is instead a UITableViewDelegate protocol, and any object that conforms to the protocol can act as a table view's delegate.
Edit: Your question is confusing. I think what you're proposing is that your Delegate class would create a view controller and make itself the delegate for the view controller.
If that's what you are talking about, your thinking is backwards. The view controller is using the Delegate class as a helper class. Any given instance of a view controller class can create an instance of the Delegate class and set it as it's delegate if it desires. You might have 3 instances of ViewController at one time, each with it's own instance of your Delegate class.
Thus, the ViewController object is the one that should create and set up an instance of Delegate if it needs one:
- (void) viewDidLoad;
{
self.delegate = [[Delegate alloc] init];
//other setup here
}
I'm currently learning Obj-C and more specifically - protocols. I will need to make up a scenario here for my question to make sense.
First example (with a delegate).
I'm a UIView subclass which requests information from it's controller to be displayed (or rather how the information should be displayed). I declare a protocol and make a delegate reference object (or whatever it's called):
#property (nonatomic, weak) id <protocolName> dataSource;
My controller conforms to this protocol and implements the required method.
View sends messages to the Controller, and the Controller answers and everything is fine.
This as far as I know is delegation through protocols and I believe I understand it.
But let's consider another scenario.
I'm a class which is the brain for a simple level-based game. I say when to show the menu or when to start playing a game level. But I need to know when a level is completed.
Which means this class needs to be ready to receive messages from anyone that implements the protocol, for example from another class which is responsible for the current level. Now this is what I don't understand. How the protocol should look like and where/how to implement it?
Let me know if my question still doesn't make sense. Thanks!
Delegate is a one-to-on relationship method for sending message between entities.
If you want to receive the same message from different class, you can use NSNotification that is a one-to-many relationship.
Look at the NSNotificationCenter class, especially postNotification method (for sending messages) and addObserver method (used when received a notification)
EDIT :
Here's an example code for delegation.
Let's take your game level-based example. You have a LevelManager class and FirstLevel class. If you want the LevelManager to be notice when the FirstLevel ended, you have to declare a protocol in your FirstLevel class
// FirstLevel.h
#protocol SomeProtocol <NSObject>
-(void)levelDidEnd;
#end
#interface FirstLevel : NSObject
#property (nonatomic, weak) id<SomeProtocol> delegate;
#end
Then somewhere in your FirstLevel.m, call your protocol method when the level ended
// FirstLevel.m
- (void)playGame {
if ([player isDead]) {
[delegate levelDidEnd];
}
}
The next step is to implement your protocol to your LevelManager :
// LevelManager.h
#interface LevelManager : NSObject <SomeProtocol>
#property (strong, nonatomic) FirstLevel *firstLevel;
#end
And then, set your firstLevel.delegate and implement your protocol method
// LevelManager.h
- (void)viewDidLoad {
firstLevel.delegate = self;
}
- (void) didLevelEnd {
// do actions here like showing the menu
}
You still can go with delegation:
Let the level be a object with an delegate of type id<LevelDelegate>
#protocol LevelDelegate <NSObject>
-(void)completedLevel:(Level *)level;
#end
The BrainController conforms to LevelDelegate and creates all levels. for each level it set itself as delegate.
If a level recognizes it has finished, it calls [self.delegate completedLevel:self];.
The BrainController gets informed and does what ever is necessary.
I have one delegate ExampleDelegate and I have one UITableViewController and detail UIViewController both implementing that delegate:
#interface ClassA : UITableViewController <ExampleDelegate>
and:
#interface ClassB : UIViewController <ExampleDelegate>
and delegate:
#protocol ExampleDelegate <NSObject>
-(void)notifyUser;
#end
#interface Example : NSObject
#property (nonatomic, retain) id delegate;
-(id)initWithDelegate:(id<ExampleDelegate>) delegate;
#end
At first I'm initializing the instance of delegate from ClassA and its working fine but once I navigate to the ClassB there also I'm creating one instance for the delegate when I came back to the ClassA the delegate instance get retained from ClassB, so the function inside the ClassB gets called always instead of ClassA's function.
Can anyone point me out what I'm doing wrong and how to get this working?
You need to make sure that you nil out your delegates once they're not needed - this will help you achieve 2 things:
Pass the correct class the proper delegation that it needs to handle when it is visible.
Prevent crashes so that when a class gets deallocated and is defined as a delegate, you might get a "message sent to deallocated instance" crash.
In your case, once ClassB goes back to ClassA, you must make sure that ClassA has that delegate defined and that ClassB's delegate property is nil.
Do you intend for Class B to be the delegate of Class A so that it can call back with the -notifyUser method? If that's the case, you don't need the initializer and you should have the delegate property on class B declared as...
#property (assign, nonatomic) id <ExampleDelegate> delegate;
Then, if I'm following you correctly, when you create the detail controller (Class B), you will set it's delegate property to self (Class A). Now when Class B needs to communicate back up to Class A it simply needs to call -notifyUser on it's delegate like so...
// Something happened that you want to communicate back up the chain
[self.delegate notifyUser];
When you use this pattern, the protocol is typically declared on the class that also implements the property for it to be set on, in this case that is Class B.
Edit:
In both classes I have declared delegates as strong variable which needs to be declared as weak as I came through some doc so by changing this it solved the problem.
And thanks for you all the answers which helped a lot.