Objective-C类方法中的实例变量访问

Question

I wonder how can I do a thing that I always done in Java.

public class ObjectHolder {

 private List<Object> holder;

 public ObjectHolder() {
  // initialize object holder...
 }

 public boolean addObject(Object obj) {
  // add object and return true/false
 }

}

The good of this approach is that holder is not accessible for direct manipulation. I am trying to understand how this can be achieved in Objective-c, in particular I made this Objective-c class interface:

@interface ObjectHolder : NSObject {
    NSMutableArray *holder;
}

+(ObjectHolder*)holderWithObjects:(NSArray*)objs;
-(BOOL)addObject:(NSObject*)obj;

@end

But this is where my questions start, in particular within the class method holderWithObjects:

+(ObjectHolder*)holderWithObjects:(NSArray*)objs {
     ObjectHolder *oh = [[[ObjectHolder alloc] init] autorelease];
     oh->holder= // array initialization and retain
     for(NSObject obj in objs) {
      // add to holder array
      [oh->holder addObject:obj];
     }
     return oh;
}

• the holder is an instance variable, how can I avoid transforming it in @property, I know there's no 'private' concept in Objective-c, but I don't want a developer to misunderstand the correct use of the class (I hope this concept is clear).
• in Java there is a constructor where I can initialize the property, where's the best place for this in objective-c class?
• if not using @property how the memory deallocation is managed ?
• if I am not using @property, the compiler complains there's no such variable
• In class method holderWithObjects I cannot access the holder directly unless it is a property, so I end up using direct access to holder.

Answer 1

You can do it eg like this:

.h file:

@interface ObjectHolder : NSObject {
    NSMutableArray *holder;
}

+(ObjectHolder*)holderWithObjects:(NSArray*)objs;
-(void)addObject:(id)obj;

@end

.m file:

+ (ObjectHolder*)holderWithObjects:(NSArray*)objs {
    ObjectHolder *oh = [[[ObjectHolder alloc] init] autorelease];
    for(id obj in objs) {
        [oh addObject:obj];
    }
    return oh;
}

- (void)addObject:(id)obj {
    if (holder == nil) {
        holder = [[NSMutableArray alloc] init];
    }
    [holder addObject:obj];
}

- (void)dealloc {
    [holder release];
    [super dealloc];
}

Some notes:

• NSMutableArray 's -addObject: returns void, so there's really no use in returning BOOL from your method, unless you want some specific behaviour (like avoiding duplicates in the array - then maybe NSSet would be better?, etc.)
• id is more general than NSObject * and in most cases you would prefer using it, see this blog post for more info
• instance variables that are objective-C objects are nil by default, so you can check in your -addObject: for holder value and lazily initialize it when it's needed. Otherwise you can simply initialize holder in -init method - that is the equivalent of a constructor in Java
• you have to release the holder array in -dealloc method
• you can consider a method like -addObjects:(NSArray *)objs that would help you to avoid iterating through the array and adding objects one by one