Andrew P. Black wrote:
> The key phrase was: "If handlesMouseDown: answers false, that means that 
> your object
> doesn't handle mouse down events, _so somebody else has to decide how
> to handle them_".  So by setting handlesMouseDown to false, I was making 
> the morph react to mouse down, which is a bit counter-intuitive.  The 
> way to make it _not_ respond to mouse down is to have handlesMouseDown: 
> answer true, but to have a mouseDown: evt method that does nothing.

That is correct. It's a bit counter-intuitive because it's the world 
which implements the default behavior of dragging objects. In other 
words the action is contextual (objects in the world can be dragged) not 
builtin.

> OK, so the converse is true.  if I want my circles to handle Mouse Down, 
> then they need to answer true to handlesMouseDown: (that's easy), and 
> then there needs to be a mouseDown: evt method that does the "right 
> thing".  However, the right thing is very complicated!  I tried the 
> following, based on Tom's code:

This code seems both overly complicated as well as at least somewhat 
buggy (grabMorph:from: should only be used for owner-less morphs). Try 
the following instead:

rect := RectangleMorph new.
rect extent: 100 at 100.
circle := EllipseMorph new.
circle extent: 100 at 100.
rect addMorphCentered: circle.
rect on: #mouseDown send: #value to:["ignore drags"].
circle on: #mouseDown send: #value
        to:[circle world primaryHand grabMorph: circle].
rect openInWorld.


> I could probably find all of the bits of code that I need, to handle 
> mouse move and so on, taking care of the offset between mouse click 
> event and the origin of the Morph that I'm moving — most of the code 
> must be in HaloMorph.  But this was the Default Behavior of the circle 
> before I embedded it in the rectangle — surely there must be an easier 
> way to get that default behavior back, other than duplicating the code 
> from whereever it is hidden!

Well, by far the easiest way is to use a PasteUpMorph instead of a 
RectangleMorph - PasteUps have this behavior builtin.

Cheers,
   - Andreas