Beginning cocoa programming: delegates

By the way, I know how to get my example to work: I can just delete my AppController class, then go into IB and create a MyDelegate instance, and then drag a connection from the window to the MyDelegate instance. What I want to know is why calling setDelegate: is not sufficient to setup a delegate.

I constructed another simple example of a delegate: I created an app with one lone NSTextField on the window. I wanted to see if setDelegate: would work with a different widget(NSTextField v. NSWindow). My goal was to see if I could setup a delegate using setDelegate: on an NSTextField, such that a delegate method would be called when I typed in the text field. I looked through the documentation, and I decided to to see if implementing this delegate method would work:

- (BOOL)control:(NSControl *)control textShouldBeginEditing:(NSText *)fieldEditor;

I created a new app(deleting the classes that Xcode automatically creates, as well as the instance in the MainWindow.xib window in IB), and then I created this class:

//MyDelegate.h

import <Cocoa/Cocoa.h>

@interface MyDelegate : NSObject <NSTextFieldDelegate> {

}

- (BOOL)control:(NSControl *)control

textShouldBeginEditing:(NSText *)fieldEditor;

@end

//MyDelegate.m

@implementation MyDelegate

- (BOOL)control:(NSControl *)control

textShouldBeginEditing:(NSText *)fieldEditor

{

NSLog(@"***In delegate method...");

return YES;

}

@end

1) In IB, I created an instance of MyDelegate by dragging an Object from the Library onto the MainWindow.xib window. I entered the name MyDelegate in the TextField Identity Inspector.

2) In IB, I ctrl-clicked on the NSTextField, and then I dragged a connection from NSTextField's 'delegate' outlet to the MyDelegate instance(in the MainWindow.xib window).

3) Saved everything, build&run, and when I typed in the text field, I saw the logged message in the Debugger Console, as expected.

Next, I attempted to create code that uses setDelegate: to achieve the same thing as above.

4) In IB, I ctrl-clicked on the NSTextField and deleted the connection from the 'delegate' outlet to the MyDelegate instance.

5) I made these changes to the code:

//MyDelegate.h

#import <Cocoa/Cocoa.h>

@interface MyDelegate : NSObject <NSTextFieldDelegate> {

IBOutlet NSTextField* textField;

}

- (id)init;

- (BOOL)control:(NSControl *)control

textShouldBeginEditing:(NSText *)fieldEditor;

@end

//MyDelegate.m

#import "MyDelegate.h"

@implementation MyDelegate

- (id) init

{

if (![super init]) {

return nil;

}

[textField setDelegate:self];

return self;

}

- (BOOL)control:(NSControl *)control

textShouldBeginEditing:(NSText *)fieldEditor

{

NSLog(@"***In delegate method...");

return YES;

}

@end

6) In IB, I ctrl-clicked on the MyDelegate instance in the MainWindow.xib window, and then I dragged a connection from the outlet 'textField' to the NSTextField(in the app's main window).

7) I saved in IB, then build&run (save everything), and this time when I typed in the textfield, the message was not logged to the Debug Console.

What is the matter with setDelegate:? Why doesn't setDelegate: succeed in setting up the delegate so that I can see the message logged to the console?

Thanks

I think I found the answer to what's happening with setDelegate:, which by the way no one on the internet seems to be aware of, including the author of the book. At this link:

http://developer.apple.com/library/mac/#documentation/Cocoa/Reference/Applicatio nKit/Protocols/NSNibAwaking_Protocol/Reference/Reference.html%23//apple_ref/occ/ instm/NSObject/awakeFromNib

...it says:

===

An

awakeFromNib

awakeFromNib

===

Huh?!! You mean it's NOT guaranteed that the objects pointed to by a class's instance variables exist??? Let's read on:

===

During the instantiation process, each object in the archive is unarchived and then initialized with the method befitting its type....

Once all objects have been instantiated and initialized from the archive, the nib loading code attempts to reestablish the connections between each object’s outlets and the corresponding target objects. If your custom objects have outlets, an

NSNib

Finally, after all the objects are fully initialized, each receives an

awakeFromNib

===

So that means that inside an init() method, you can't send messages to (i.e call methods on) other objects--including those objects that are pointed to by your class's instance variables. During the initialization phase, i.e. in your init() methods, you have to assume that no other objects exist (unless you specifically created those objects inside your init() method). As a result, if you want to call methods on other objects, you have to do it in awakeFromNib at such time when all the other objects in your app are guaranteed to exist.

I modified my code to call setDelegate: in my class's awakeFromNib method. Now the code works as expected and I am able to see the delegate method log its message in the Debug Console:

//MyDelegate.h

#import <Cocoa/Cocoa.h>

@interface MyDelegate : NSObject <NSTextFieldDelegate> {

IBOutlet NSTextField* textField;

}

- (id)init;

- (void)awakeFromNib;

- (BOOL)control:(NSControl *)control

textShouldBeginEditing:(NSText *)fieldEditor;

@end

//MyDelegate.m

#import "MyDelegate.h"

@implementation MyDelegate

- (id) init

{

if (![super init]) {

return nil;

}

//[textField setDelegate:self];

return self;

}

- (void)awakeFromNib

{

[textField setDelegate:self];

}

- (BOOL)control:(NSControl *)control

textShouldBeginEditing:(NSText *)fieldEditor

{

NSLog(@"***In delegate method...");

return YES;

}

@end

Hmmm...I think a good author/teacher would have mentioned that slightly important fact.