Binding NSTextField to NSString - objective-c

Question

I have a very simple question I hope someone can answer as I start understanding bindings. I want to programmatically change my NSString value and have the NSTextField update to that value thru bindings. I have a NSTextField and NSLabel. To represent the value of the myString properly changing I have a NSButton.
I have NSTextField's Value bound to myString property of App Delegate with Continuously Update Value checked.
I have NSLabel's Value bound to myString Property of App Delegate.
I have NSButton outlet hooked to setDefault method.
When I type in NSTextField the NSLabel updates as expected but when I click the button the myString property is updated but not in the NSTextField.
What do I need to do to get the NSTextField update to the myString property????
AppDelegate.h
#interface AppDelegate : NSObject<NSApplicationDelegate>
{
NSString *myString;
}
#property (assign) IBOutlet NSWindow *window;
#property NSString *myString;
- (IBAction)setDefault:(id)sender;
#end
AppDelegate.m
#implementation AppDelegate
#synthesize window = _window;
#synthesize myString;
- (void)applicationDidFinishLaunching:(NSNotification*)aNotification
{
myString = #"This is a string";
}
- (IBAction)setDefault:(id)sender
{
NSLog(#"%#", myString);
myString = #"This is a string";
NSLog(#"%#", myString);
}
#end

Answer 1

It shouldn't be
myString = #"This is a string";
but this:
self.myString = #"This is a string";
both in -applicationDidFinishLaunching: and in -setDefault:. Don't forget to specify self in your NSLog statements as well. You'd probably like to specify a different string in -setDefault: so that you can actually see that a change is taking place.
One other thing: You're effectively saying that you want to assign to myString, but that's not appropriate for an object. Instead of:
#property NSString *myString;
you should instead use
#property (copy) NSString *myString;
or at least
#property (retain) NSString *myString;
The former is preferred because passing a NSMutableString instance effectively copies it as a NSString, while passing a NSString instance simply retains it.
Good luck to you in your endeavors.

Answer 2

I recommend that you prefix your member variables. This way you can distinguish between setting the member directly or using the setter. In your example I would do the following.
#interface AppDelegate : NSObject<NSApplicationDelegate>
{
NSString *m_myString;
}
#property (assign) IBOutlet NSWindow *window;
#property NSString *myString;
- (IBAction)setDefault:(id)sender;
#end
...
#implementation AppDelegate
#synthesize window = _window;
#synthesize myString = m_myString;
- (void)applicationDidFinishLaunching:(NSNotification*)aNotification
{
self.myString = #"This is a string";
}
- (IBAction)setDefault:(id)sender
{
NSLog(#"%#", m_myString);
self.myString = #"This is a string";
NSLog(#"%#", m_myString);
}
#end
Notice, that I changed the #synthesize to assign the member variable.
To clarify:
self.myString = #"This is a string";
.. is an alternative syntax for ...
[self setMyString:#"This is a string"];
You can also set the member directly ...
[self willChangeValueForKey:#"myString"];
m_myString = #"This is a string";
[self didChangeValueForKey:#"myString"];
But then you need to "inform" observers of the binding as illustrated above.