How to setup a NSTableView with a custom cell using a subview

Question

I am trying to setup a NSTableView with a custom cell using an ArrayController and Bindings. To accomplish this I added a subview to the custom cell. The data connection seems to work somewhat. Though, there seems to be a redraw problem which I cannot fix. When I load the application only some of the cells are rendered. When I scroll through the rows or select one the rendering changes.

I created an example project on github to illustrate what the problem is.

The actual source code for the cell rendering can be found here:

// CustomCell.m
- (void)drawInteriorWithFrame:(NSRect)cellFrame inView:(NSView*)controlView {

  if (![m_view superview]) {
    [controlView addSubview:m_view];
  }

  // The array controller only gets wrapped data items pack by the NSObjectTransformer.
  // Therefore, objectValue returns a NSObjectWrapper.
  // Unpack the wrapper to retreive the data item.
  DataItem* dataItem = [(NSObjectWrapper*)[self objectValue] original];
  [[m_view name] setStringValue:dataItem.name];
  [[m_view occupation] setStringValue:dataItem.occupation];
  [m_view setFrame:cellFrame];
}

It seems as if the parent controlView does not redraw. Can I force it somehow?

Answer 1

This is almost certainly not a best practice way of doing this, and I'll explain why afterwards: however, it does seem to work. Replace your cell class's drawInteriorWithFrame:inView: method with the following:

- (void)drawInteriorWithFrame:(NSRect)cellFrame inView:(NSView*)controlView {
    DataItem* dataItem = [(NSObjectWrapper*)[self objectValue] original];
    [[m_view name] setStringValue:dataItem.name];
    [[m_view occupation] setStringValue:dataItem.occupation];
    [m_view setFrame:cellFrame];

    NSData *d = [m_view dataWithPDFInsideRect:[m_view bounds]];
    NSImage *i = [[NSImage alloc] initWithData:d];
    [i setFlipped:YES];

    [i drawInRect:cellFrame fromRect:NSZeroRect operation:NSCompositeSourceOver fraction:1.0];
}

The problem is that only one NSCell is created for the entire table. That's how cells are meant to work: the table view creates a cell, and calls setObject… followed by drawInterior… over and over again to get the cell to draw the whole table. That's great from an efficiency perspective (the NSCell class was designed back when 25mhz was a fast computer, so it aimed to minimise the number of object allocations), but causes problems here.

In your code, you populate a view with values, and set its frame, adding it as a subview of the table view if needed. However, since you've only got one instance of NSCell, there can only be one view: you took the single view that you had and merely moved it down the rows of the table.

To do this properly, you'd need some data structure to track all the views you added as subviews of your NSTableView, and when the cell is updating one in the drawInterior… method you'd need to look up which the correct one was and update that. You'd also need to allocate all these views in code (or at least move the view to a separate nib which you could load multiple copies of), because as it is you've only got one in your nib and copying a view is a pain.

The code I wrote is a kludge, since it's really inefficient. What I did was each time the view needs to draw, I drew the view into an off screen image buffer, and then drew the buffer into the correct place in the table view. In doing so, I avoided the problem of only having one view, since the code just takes and draws a new copy of its contents whenever it is needed.

Answer 2

EDIT: See my other answer for explanation

Have you implemented copyWithZone:? You'll need to ensure you either copy or recreate your view in that method, otherwise different cells will end up sharing a view (because NSTableView copies its cells).