Hi!

I am currently learning Cocoa as I am porting my Win32 App across to Mac. Although most of the concept have been fairly simple to port across, from time to time I come across something which confuses me no end!

Drag and drop with NSOutlineView is one such problem…

I have found and looked at the example that comes with XCode, but I believe that the tutorial is too complex to answer the simple question of exactly how, step by step, to allow drag and drop. So, I am writing my own SIMPLE tutorial. However, I need some help…

The tutorial consists of 2 outlineviews on a window, each of which is controlled by it’s own controller. I have created a custom class (OlvNode) and I want to be able to drag OlvNode objects from the left OutlineView to the right out lineView. Simple. Right? Well, I’ve hit some snags.

In the controller for the source outlineview, I have the following code:

• (BOOL) outlineView : (NSOutlineView ) outlineView writeItems : (NSArray) items toPasteboard : (NSPasteboard*) pboard { // ‘items’ is an array that contains all the nodes we’re dragging. // For this example, lets just drag one node: draggedNode = [items objectAtIndex : 0];

  // We need to tell the Paste Board what types of data we might send to it. // Here we’ll declare: // DragDropSimplePboardType - The custom type we declared // NSStringPboardType - Simple NSString type [pboard declareTypes:[NSArray arrayWithObjects: DragDropSimplePboardType, NSStringPboardType, nil] owner:self];

  // the actual data doesn’t matter since DragDropSimplePboardType drags aren’t recognized by anyone but us!. [pboard setData : [NSData data] forType : DragDropSimplePboardType];

  // Put string data on the pboard… notice you candrag into TextEdit! [pboard setString : [draggedNode nodeCaption] forType : NSStringPboardType]; return YES; }

Then for the receiving outlineview, I call the following code:

• (unsigned int) outlineView : (NSOutlineView*) outlineView validateDrop : (id ) info proposedItem : (id) item proposedChildIndex : (int) childIndex { @try { // item is the node, so lets give it a useful name to work with OlvNode *targetNode = item;

    // This method validates whether or not the proposal is a valid one. Returns NO if the drop should not be allowed.
    //    SimpleTreeNode *targetNode = item;
    BOOL targetNodeIsValid = YES;
  	
    if (nil == targetNode)
    {
        NSLog(@"Invalid target Node");
        targetNodeIsValid = NO;
    }
  
    // Get the dragged node from the dragged node's data source
    OlvNode *_draggedNode = [info draggingSource] dataSource] draggedNode];
  	
    [[NSLog(@"Dragged Node is: '%@'", [_draggedNode nodeCaption]);
  
    // Set the item and child index in case we computed a retargeted one.
    [olv setDropItem : targetNode dropChildIndex : 0];//childIndex];
  
    return targetNodeIsValid ? NSDragOperationGeneric : NSDragOperationNone;
  

  } @catch (NSException *e) { NSLog(@”EXCEPTION: ‘%@’”, e); } return NSDragOperationNone; }

• (BOOL)outlineView : (NSOutlineView*) outlineView acceptDrop : (id ) info item : (id) targetItem childIndex : (int) childIndex { OlvNode *thisNode = targetItem;

  OlvNode *newNode = [[OlvNode alloc] init]; [newNode setNodeCaption: [thisNode nodeCaption]]; [arrNodes addObject: newNode];

  NSLog(@”Created new Node with caption ‘%@’”, [newNode nodeCaption]);

  [olv reloadData];

  return NO; }

The problem I’m having is that when I’m dragging the node over, the app crashes, and I don’t know why. I’d REALLY appreciate any help in this!!!

I’ll happily email the source to anyone who wants to see for themselves - email me at temp@jacobs.co.za to let me know you want it.

Thanks in advance!!!!!!!!!!!!!

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Posting your code and stating your application crashes doesn’t really help; you’re missing a vital piece of information. Where does it crash? :-) Run your application in the debugger and tell us what line kills it. To learn how to do this, see DebuggingTechniques, specifically the “Breaking on Exceptions” section. This technique more often than not will show you exactly what’s wrong.