Lazy loading in objective C

I heard lazy loading technique quite helpful to increase the performance of the programme. I am developing games for iPhone. I am not sure how is the way to apply lazy loading in objective C. Could anyone show me the example开发者_如何学运维 please?

Thanks in advance

---

The general pattern for lazy loading is always more or less the same:

- (Whatever *)instance
{
    if (_ivar == nil)
    {
        _ivar = [[Whatever alloc] init];
    }
    return _ivar;
}

1. In your class, add an ivar of the type you need, and initialize that to nil in the constructor;
2. Create a getter method for that ivar;
3. In the getter, test for nil. If so, create the object. Otherwise, just return the reference to it.

---

Here's an example of lazy loading from the Core Data template:

- (NSManagedObjectModel *)managedObjectModel
{
    if (managedObjectModel != nil) {
        return managedObjectModel;
    }
    managedObjectModel = [[NSManagedObjectModel mergedModelFromBundles:nil] retain];
    return managedObjectModel;
}

The first time the managedObjectModel is asked for, it is created by the code. Any time after that, it already exists (!= nil) and is just returned. That's one example of lazy loading. There are other kinds, such as lazy loading of NIB files (loading them into memory only when they're needed).

---

This would be the appropriate way according to Apple. I agree with them for multiple reasons:

• An static variable inside a method will persist over multiple calls.
• The dispatch_once function of GDC will guarantee that the given code block will only run once.
• It's thread-safe.

Objective-C:

- (AnyClass*)instance {

    static AnyClass *shared = nil;
    static dispatch_once_t onceToken;

    dispatch_once(&onceToken, ^{
        shared = [[AnyClass alloc] init];
    });

    return shared;
}

---

In your *.h class isDragging_msg and isDecliring_msg these 2 is BOOL value. and Dict_name NSMutableDictionary.

In view did load

Dict_name = [[NSMutableDictionary alloc] init];

In cell for row at index path

if ([dicImages_msg valueForKey:[[msg_array objectAtIndex:indexPath.row] valueForKey:@"image name or image link"]]) 
{ 
    cell.image_profile.image=[dicImages_msg valueForKey:[[msg_array objectAtIndex:indexPath.row] valueForKey:@"image name or image link"]];
}
else
{
    if (!isDragging_msg && !isDecliring_msg)
    {
        [dicImages_msg setObject:[UIImage imageNamed:@"Placeholder.png"] forKey:[[msg_array objectAtIndex:indexPath.row] valueForKey:@"image name or image link"]];
        [self performSelectorInBackground:@selector(downloadImage_3:) withObject:indexPath];
    }
    else
    {
        cell.image_profile.image=[UIImage imageNamed:@"Placeholder.png"];
    }
}

and for download image the function is:-

-(void)downloadImage_3:(NSIndexPath *)path
{
    NSAutoreleasePool *pl = [[NSAutoreleasePool alloc] init];

    NSString *str=[here Your image link for download];

    UIImage *img = [[UIImage alloc] initWithData:[NSData dataWithContentsOfURL:[NSURL URLWithString:str]]]; 

    [dicImages_msg setObject:img forKey:[[msg_array objectAtIndex:path.row] valueForKey:@"image name or image link same as cell for row"]];

    [tableview performSelectorOnMainThread:@selector(reloadData) withObject:nil waitUntilDone:NO];

    [pl release];
}

and at last put these methods in your class

- (void)scrollViewDidEndDragging:(UIScrollView *)scrollView willDecelerate:(BOOL)decelerate
{
    isDragging_msg = FALSE;     
    [tableview reloadData];
}

- (void)scrollViewDidEndDecelerating:(UIScrollView *)scrollView
{
    isDecliring_msg = FALSE;
    [tableview reloadData]; 
}

- (void)scrollViewWillBeginDragging:(UIScrollView *)scrollView
{
    isDragging_msg = TRUE;
}

- (void)scrollViewWillBeginDecelerating:(UIScrollView *)scrollView
{
    isDecliring_msg = TRUE; 
}