On Feb 12, 2010, at 1:31 PM, Donald Klett wrote:
> Once again, I am not understanding some aspect of Objective C and/or Cocoa.
>
> I created a simple class that contains two NSTextField objects. I used IB to
> connect the Controller object with the two text fields. The code follows.
> This example runs correctly and does copy the value from one text field to
> the other.
>
> #import <Cocoa/Cocoa.h>
>
> @interface Controller : NSObject {
> IBOutlet NSTextField* textField;
> IBOutlet NSTextField* copyField;
> }
>
> - (IBAction) buttonTarget: (id) sender;
>
> @end
>
> #import "Controller.h"
>
> @implementation Controller
>
> - (IBAction) buttonTarget: (id) sender {
> int textValue;
>
> textValue = [textField intValue];
> [copyField setIntegerValue:textValue];
> }
>
> @end
Okay, all is groovy so far. What you've left unstated in the explanation
above is that the
two text fields must have been allocated *somewhere*. I can make a SWAG that
you
dragged a couple of text fields from IB's palette onto the (content view) of
the main window
that shows up when you open MainMenu.xib, yes ?
If that is the case, you have two (instantiated and correctly * initialised by
IB) text fields that
you can connect to from your (presumably also instantiated and initialised by
IB) Controller
object.
> Now if I extend this to two objects (Controller and View), the resulting code
> does not execute correctly. Again, I used IB to connect the View object to
> the two text fields. Using the debugger I find that the two NSTextField
> objects have not been allocated (both have nil values). The code follows:
>
> #import <Cocoa/Cocoa.h>
> #import "View.h"
>
Minor detail here. Use @class forward references in .h files instead
of importing the class's .h file in your Controller's .h file. In other
words,
delete the #import "View.h line and add a @class View; line . . .
This tells the compiler you'll be referencing the View class in your .h file
with an unspoken promise you will include the View.h file in your
Controller's .m
file . . .
> @interface Controller : NSObject {
>
> View* view;
> }
>
> - (IBAction) buttonTarget: (id) sender;
>
> @end
>
> #import "Controller.h"
> #import "View.h"
>
> @implementation Controller
>
> - (id) init {
> if (self = [super init]) {
> view = [[View alloc] init];
> }
> return self;
> }
Below, you have View as a sub-class of NSObject. Why?
If View is supposed to be a sub-class of NSView, your [[View alloc] init]
above needs to be [[View alloc] initWithFrame: . . .]
>
> - (IBAction) buttonTarget: (id) sender {
> [view copyFieldValue];
> }
>
> @end
>
> #import <Cocoa/Cocoa.h>
>
> @interface View : NSObject {
>
> IBOutlet NSTextField* textField;
> IBOutlet NSTextField* copyField;
>
> }
>
> - (void) copyFieldValue;
>
> @end
Based on this declaration, do you have a View object instantiated in your
MainMenu.xib,
and did you then connect the two outlets to the two text fields from that
already instantiated object?
If so, after then instantiating a View object in your Controller, your
Controller is now talking to
the wrong View object and the one from the xib is being ignored . . .
>
> #import "View.h"
>
>
> @implementation View
>
> - (void) copyFieldValue {
> [copyField setIntegerValue:[textField intValue]];
> }
>
> @end
>
> I have no idea what I am doing wrong. Any help would be most appreciated.
> Thanks in advance.
I hope this helps a little. One of the more common beginner mistakes
is to instantiate an object in the xib, and then instantiate a completely
new version in code . . .
And as (I think) someone already pointed out, if you have a class whose
name is View, anybody reading your code superficially would expect it to
*be* something that relates to existing NSView or UIView classes and have
View-like behaviour . . .
A useful exercise to do is to look at your xib objects and draw a picture of
the objects and their connections --- that essentially defines your 'Application
Architecture', and a picture can quickly show up anomalies in the object
graph --- either orphan objects as I suspect in the case, or objects that aren't
really doing anything useful . . .
Cheers,
. . . . . . . . Henry
=================================================
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