Call a method on a property

Question

Is there any way to call or pass a method on a property. I understand how to set and get properties but how do I manipulate them? I'm trying to remove punctuation on string using object oriented programing. The act of removing punctuation from an input string is written as a method.

main.m

TDItem *newItem = [[TDItem alloc] init];

[newItem setItemString:@"Get the mail next Tuesday!"];
NSLog(@"\nCreated Item: %@", [newItem itemString]);

NSString *itemStringWithoutPunctuation = [[NSString alloc] init];
[newItem itemStringWithoutPunctuation:[newItem itemString]];
[newItem setItemString:itemStringWithoutPunctuation];
NSLog(@"\nCreated Item: %@", [newItem itemString]);

TDItem.h

@interface TDItem : NSObject

@property NSString *itemString;


// Formating methods
- (NSString *)itemStringWithoutPunctuation:(NSString *)itemString;

TDItem.m

- (NSString *)itemStringWithoutPunctuation:(NSString *)itemString
{
NSString* itemStringWithoutPunctuation = [[itemString componentsSeparatedByCharactersInSet:[[NSCharacterSet letterCharacterSet] invertedSet]] componentsJoinedByString:@" "];
return itemStringWithoutPunctuation;
}

The debug console prints out a blank for the new itemString value.

Debuger

Created Item: Get the mail next Tuesday!
Created Item: 

If this the completely wrong what to go about changing a property value?

Answer 1

To answer your question:

NSString *itemStringWithoutPunctuation = [[NSString alloc] init]; // note: immutable string
[newItem itemStringWithoutPunctuation:[newItem itemString]]; // does NOT save the result!
[newItem setItemString:itemStringWithoutPunctuation]; // sets itemString to the empty string
NSLog(@"\nCreated Item: %@", [newItem itemString]);

Instead, do this:

NSString* itemStringWithoutPunctuation = [newItem itemStringWithoutPunctuation:[newItem itemString]];
[newItem setItemString:itemStringWithoutPunctuation];
NSLog(@"\nCreated Item: %@", [newItem itemString]);

Note: Properties have a more convenient syntax

Since itemString is a property, you can access it much more cleanly with the . syntax:

newItem.itemString = @"Hello, world" ;
NSLog ( @"The string is: %@" , newItem.itemString ) ;

Note: Alternate place to put the code

Why is itemStringWithoutPunctuation an instance method on your NewItem class? It doesn't make sense, especially requiring you to pass that string in.

You may want to, instead, do this:

@interface NSString (MyCustomAdditions)
- (NSString*) stringByRemovingPunctuation ;
@end

@implementation NSString (MyCustomAdditions)
- (NSString*) stringByRemovingPunctuation {
    return [[self componentsSeparatedByCharactersInSet:[[NSCharacterSet letterCharacterSet] invertedSet]] componentsJoinedByString:@" "];
}
@end

// elsewhere...
NSString* itemStringWithoutPunctuation = [newItem.itemString stringByRemovingPunctuation] ;
newItem.itemString = itemStringWithoutPunctuation ;

Alternatively, you could do this:

@interface TDItem : NSObject
@property NSString* itemString ;
- (NSString*) itemStringWithoutPunctuation ;
@end

// elsewhere
TDItem * item = [ [TDItem alloc] init ] ;
NSLog ( @"The string is: %@" , item.itemString ) ;
NSLog ( @"The string, without punctuation, is: %@" , [item itemStringWithoutPunctuation] ) ;

However, I'd caution you: your code to remove punctuation probably isn't doing what you think it's doing, but you'll discover this soon and can fix it then.