Andrew P. Black wrote:
> Well, this is much more elegant: the use of on:send:to: simplifies 
> things considerably, and, along the way, explains how to 
> use EventHandlers, which were another mystery.  But it has the same bug: 
> once the circle has been "picked up", it is no longer a submorph of the 
> rectangle.  Presumably that could be fixed by a #mouseUp handler, 
> although I tried adding
> 
> circle on: #mouseUp   send: #value
>        to:[rect addMorph: circle].
> 
> which appeared to have no effect.

Yes, indeed, it would have no effect. Once you asked the hand to grab 
the morph the hand is in control and not the morph. I think I 
misunderstood that part - if you simply want to move the circle inside 
the rectangle (without taking it out of the structure) you should be 
doing something like this:

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:[:evt| offset := circle position - evt hand position.
             circle addDropShadow].

circle on: #mouseMove send: #value:
   to:[:evt| circle position: evt hand position + offset].

circle on: #mouseUp send: #value
   to:[circle removeDropShadow].

rect openInWorld.

>> Well, by far the easiest way is to use a PasteUpMorph instead of a 
>> RectangleMorph - PasteUps have this behavior builtin.
>>
> 
> That is the answer I was looking for!  Inter alia, it explains what a 
> PasteUpMorph is for, somthing that I had never appreciated (except to 
> know that the World was one).

Well, partly. PasteUps provide the default drag and drop you see in the 
world but they *do* rip morphs out of their structure when you click on 
them. If your entities are sensitive to structural changes (or if they 
need to preserve the z-order) you'll be better off with something like 
the above.

Cheers,
   - Andreas