Copying text between UITextField's

[iant]

I'm trying to create a new view on a button press and copy the text from one text field into the other.
The view gets created and displayed but if I try copy the string it crashes with:

"-[DistanceViewController ftCandles]: unrecognized selector sent to instance 0x4e44330"

	
mySecondViewController = [[DistanceViewController alloc] 
					initWithNibName:@"mySecondViewController"
					bundle:nil];
mySecondViewController.ftCandles.text = FtCandles.text; // this crashes

The data in FtCandles.text is good.
I'm guessing I have to do a cast, or the initialization isn't ready?
Sorry if this is moronic.
Any ideas?

[smithdale87]

you're reading the exception message wrong. It saying that your new view controller doesnt have a property "ftCandles", or perhaps you forgot to synthesize it.

[Duncan C]

Originally Posted by iant

I'm trying to create a new view on a button press and copy the text from one text field into the other.
The view gets created and displayed but if I try copy the string it crashes with:

"-[DistanceViewController ftCandles]: unrecognized selector sent to instance 0x4e44330"

	
mySecondViewController = [[DistanceViewController alloc] 
					initWithNibName:@"mySecondViewController"
					bundle:nil];
mySecondViewController.ftCandles.text = FtCandles.text; // this crashes

The data in FtCandles.text is good.
I'm guessing I have to do a cast, or the initialization isn't ready?
Sorry if this is moronic.
Any ideas?

What you are trying to do is very, very bad practice.

In the MVC design pattern, only the controller object should interact with the view object. In iOS, the view controller is the controller object. The UIView objects that the view controller maintains are the view objects. (Makes sense, right?)

If you want to pass a value from one view controller to another, add a string property to the target view controller. Then, in the viewWillAppear method for your second view controller, install the text in whatever view you want to use to display the value (or not, as your view controller sees fit.)

There are lots of reasons to do it this way. First, the target view controller's views may not exist at the time you are trying to do this. A view controller's views only get loaded after the view controller is displayed, and they can go away at any time if the view controller is not the front view controller (the system will unload a view controller's views if it runs low on memory.)

The more subtle, but more important, reason not to muck with another view controller's innards has to do with good design. If one view controller mucks about inside another view controller, those two view controllers are now tightly bound to each other. Any change you make to the target view controller may break the other view controller, and require you to go back and make changes. That's bad. Very bad. The end of that road is "spaghetti code." Spaghetti code is a big mess, all tangled together. As the app gets bigger and more complex, it gets harder and harder to figure out what's going on, or make changes.

This concept is called encapsulation, and it's a key part of object oriented design.

When you're designing an app, think about giving your classes a fairly high-level, clean public interface that is based on the job they do in your application, not the messy details of HOW they do that job. Then write your classes so the details are all internal to the object. That way, you are free to change the internals of any class to your heart's content, and as long as the public interface is the same, the other classes in your application are not affected.

Another HUGE advantage to this approach is code reuse. If you design your code to be modular, you can pick up a class from one application and drop it into another application and it works.

Ok, lecture over.

[smithdale87]

+1

[iant]

Thanks Guys,
Yeah I figured it was a little hackie.
Duncan, a few questions.
The "viewWillAppear" method does not exist by default. Should I write it or is it safe to set the string in viewDidLoad()?

Also are you suggesting I add a separate string property (other that the text fields text?). I'm guessing this stores the string until its can be set from the view?

If I create the 2nd view on the fly from a button press the view controllers will need pointers to one another (this seems like bad practice for code separation and since pointers may be null). Is there a better way of doing this?
Should I just do the setting in the app delegate to avoid the views having to store pointers to one another?
Ian

[Meredi86]

as far as your viewWillAppear method goes you can just drop that in no worries. I generally add mine after viewDidLoad but thats more so i can locate them easily when moving about my project. Simply go to the line you want to add it on and type:

- (void) view

You should find then that all of the optional and preset view methods will pop up for you to choose from, select viewWillAppear and hit enter. It is safe to do this and there is no reason you shouldnt. If Duncan suggests placing your code in viewWillAppear then he is probably right that there is where it should go. ViewDidLoad is different to viewWillAppear. I dont know the semantics but i think "Load" is done first when the app says it wants that view, it is only called once unless you remove it completely, so at times it can not be called as opposed to viewWillAppear, i believe this is called after viewDidLoad, but is called every time that that view appears on the screen. In essence if you unloaded your view then viewDidLoad will be called, if you moved away from your view without unloading it then viewDidLoad isnt called as the view is already loaded so you need to use "Appear" - At least that is my understanding of it.

Ill leave Duncan to address your other issues as i dont want to contradict him with something that is wrong :P

[Duncan C]

Originally Posted by iant

Thanks Guys,
Yeah I figured it was a little hackie.
Duncan, a few questions.
The "viewWillAppear" method does not exist by default. Should I write it or is it safe to set the string in viewDidLoad()?

Also are you suggesting I add a separate string property (other that the text fields text?). I'm guessing this stores the string until its can be set from the view?

If I create the 2nd view on the fly from a button press the view controllers will need pointers to one another (this seems like bad practice for code separation and since pointers may be null). Is there a better way of doing this?
Should I just do the setting in the app delegate to avoid the views having to store pointers to one another?
Ian

The standard view controller template should include an viewWillAppear method. The signature is like this:

- (void)viewWillAppear:(BOOL)animated

The reason you want to use that rather than viewDidLoad is that viewWillAppear gets called every time the system displays your view controller. The viewDidLoad method only gets called when the view controller's views are created (the first time the view controller is asked to display it's views, and any time thereafter if the views get unloaded from memory. Views usually get unloaded in low memory conditions, although you can also code your view controller to unload them manually.)

It's easier to create your view controllers at launch time. They typically don't take that much memory. It's their views and data that are memory-intensive. I'd suggest coding your view controllers, and then creating instances of each of them in your app's main nib file.

The cleanest way to share data across your app is with a data container singleton. Do a search on singletons to read about it. In brief, the idea is to create an object with properties for all the data you want to share in your program. You then create a class method with a signature like this:

+ (MyAppDataSingleton) sharedDataSingleton;

The implementation would look like this;

@implementation MyAppDataSingleton
static MyAppDataSingleton* _theSingleton = nil;

+ (MyAppDataSingleton*) sharedDataSingleton;
{
	if (!_theSingleton)
		_theSingleton = [[[self class] alloc] init];
	return _theSingleton;
}
@end;

Then, anywhere in your code you need to access shared data, you use code like this:

[MyAppDataSingleton sharedDataSingleton].someProperty

In your example, you could make the string that you're passing between view controllers a property of your data container. You'd then set it in one view controller, and use it in the viewWillAppear: method of your other view controller.

[iant]

Cheers Everyone. Some solid suggestions there!