Debugging with Activity Monitor

Quirky workaround of the week !

To draw a custom header in a table view, derive from NSTableHeaderCell and write your drawing code in highlight:withFrame:inView:. This works fine until you disable column selection : highlight:withFrame:inView: is never called, although the header cell obviously looks highlighted. To understand why, we can use … Activity Monitor ! As strange as it sounds, double click your process listing to get a sample button. Instead of launching Shark and getting a trace from everything, we can sample for one moment while clicking around to highlight those table header cells. Behold :

// Column selection enabled
  1 -[NSTableHeaderView drawRect:]
	1 -[NSTableHeaderView _drawColumnHeaderWithIndexes:]
	  1 -[NSTableHeaderView _drawHeaderOfColumn:]
		1 -[NSTableHeaderCell highlight:withFrame:inView:]
		  1 -[NSTableHeaderCell _drawThemeContents:highlighted:inView:]
			1 -[NSTableHeaderCell drawInteriorWithFrame:inView:]

// Column selection disabled
	3 -[NSTableHeaderView drawRect:]
	  3 -[NSTableHeaderView _drawColumnHeaderWithIndexes:]
		3 -[NSTableHeaderView _drawHeaderOfColumn:]
		  3 -[NSTableHeaderView _drawHeaderCell:withFrame:withStateFromColumn:]
			3 -[NSTableHeaderCell drawWithFrame:inView:]
			  3 -[NSTableHeaderCell _drawThemeContents:highlighted:inView:]
				3 -[NSTableHeaderCell _drawBezelWithFrame:highlighted:inView:]

Et voilà ! When column selection is disabled, highlight:withFrame:inView: is never called. _drawBezelWithFrame:highlighted:inView: is called instead. Hey, there's even some code using that very method ! Using it to draw the highlighted cell does work.

This is not foolproof as sampling just peeks at calls from time to time, not ALL calls : you might need a few runs to catch the cell redrawing. Still, Activity Monitor provides an easy way to target sampling : click on what you want sampled while the sample is being taken. It also gives quasi realtime results without needing to start Shark or wait for application termination.

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