Set string from a method with class level access

Question

I want to do the following

+(void)DoThis
{
    [textView setString:@"qwertyasdfzxcvb."];
    [textView setNeedsDisplay:YES];
}

However, It refuses to set the string. Because this works,

-(void)DoThis
{
    [textView setString:@"qwertyasdfzxcvb."];
    [textView setNeedsDisplay:YES];
}

I come to the conclusion that the string refuses to be set in a method with class level access. So how do I allow a string to be set from a +(void)?

@ jojaba @property(nonatomic, retain) IBOutlet NSTextView *textView; — evdude100, May 18 '11 at 17:49

score 2 · Answer 1 · answered May 18 '11 at 17:14

2

Assuming textView is an instance variable, there is no way to access it. At best, you can

+ (void)DoThis:(UITextView*)aTextView {
    [aTextView setString:@"qwertyasdfzxcvb."];
    [aTextView setNeedsDisplay:YES];
}

answered May 18 '11 at 17:14

Deepak Danduprolu

44,595
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How would I hook aTextView in IB? – evdude100 May 18 '11 at 17:47
What is it that you are trying to achieve exactly that requires a class method and not an instance method? – Deepak Danduprolu May 18 '11 at 17:52
Another file will call the method. – evdude100 May 18 '11 at 17:53
Can't you create an instance of the class? – Deepak Danduprolu May 18 '11 at 18:03
1

Without an instance of the class holding `textView`, how do you think 'textView' will exist? Can you explain the broader picture of what you are trying to achieve so that we can understand this better? – Deepak Danduprolu May 18 '11 at 18:19
the "LoadWorld" class method in World.m calls the class method "GetInventoryItems" in Item.m Of GetInventoryItems, It figures out which items the player is holding. It then will print them into NSTextView. – evdude100 May 18 '11 at 18:23
print them into an instance of `NSTextView`. At some point you need to pass an instance. – albertamg May 18 '11 at 18:46
@albertamg No, because PrintInventoryItems will depend on class variables – evdude100 May 18 '11 at 18:52
Why aren't you doing `[self loadWorld];`? – Deepak Danduprolu May 18 '11 at 18:58

score 2 · Answer 2 · answered May 18 '11 at 17:18

What you are most likely trying to do is set an instance variable from a class method. This does not work because class objects are not instances of the class and therefore cannot access instance variables. So your method + (void)DoThis does not have access to the textView instance variable because the class object is not an instance of the class.

score 2 · Answer 3 · edited May 23 '17 at 12:26

2

You can not set an instance variable from a class method. If you want to declare a "class variable" you have to make a static global variable inside the .m file of the class because there are no class variables in Objective-C. This is a C static variable, not in the sense Java or C# consider it. Here, "static" ensures that the variable can not be used outside of that file.

edited May 23 '17 at 12:26

Community

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1

answered May 18 '11 at 17:18

albertamg

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You can't make a property static. – evdude100 May 18 '11 at 17:51
No, you can't. As I said, if you want to declare a "class variable" you have to make a static global variable inside the .m file – albertamg May 18 '11 at 17:56
You can not set an instance variable from a class method because it does not make sense. Why not use an instance method? – albertamg May 18 '11 at 18:16
Because I need to call the method from a different location or another class method. – evdude100 May 18 '11 at 18:17

score 1 · Accepted Answer · answered May 18 '11 at 19:04

I think what you may be looking for is the singleton/shared pattern, which allows any code to access a particular instance of a class without being passed that instance.

This model is used throughout Cocoa. For example, NSFileManager has a class method defaultManager which returns the shared instance of the class. You call an instance method on this shared instance by first invoking defaultManager, so for example to call the instance method isDeletableFileAtPath you write:

[[NSFileManager defaultManager] isDeletableFileAtPath:path]

In your case your DoThis becomes:

+ (void) DoThis
{
    EvDudeClass *shared = [EvDudeClass sharedInstance];
    [shared.textView setString:@"qwertyasdfzxcvb."];
    [shared.textView setNeedsDisplay:YES];
}

And you need to add a sharedInstance method to your class. There a number of ways to do this depending on how you wish to use your class - Is it a singleton? Do you just need one shared instance and other non-shared ones? Do you just want to share the instance created in the NIB? As you mention IBOutlet here is an implementation which does the latter:

static EvDudeClass *privateSharedInstance = nil;

- (void) awakeFromNib
{
    privateSharedInstance = self; // save this instance so it can be shared
    // other init stuff
}

+ (EvDueClass *)sharedInstance { return privateSharedInstance; }

Set string from a method with class level access

4 Answers4