Richard Gaskin ambassador at
Tue Feb 1 15:28:25 EST 2011

Peter Haworth wrote:

 > I have to respectfully disagree that this isn't a bug.  The
 > mouseDoubleUp message never happens in these circumstances,
 > at least in my application, that's the bug.  Even if it did,
 > what's the point of having a mousedoubleup message if I have
 > to handle it by extra code in the mouseUp message?

I think Jacque put this succinctly:

     I don't know of any HIG that officially supports
     double-clicking a button. In fact, it's common
     to put in scripts explicitly blocking double-clicks
     from doing anything.

GUIs are driven by an implied noun-verb interaction model, in which the 
user selects an object (noun) and applies an action to it (verb).

Push buttons are commonly verb triggers - that is, they perform an 
action, usually on some object other than themselves.  The things verb 
objects act on could be consider nouns.

Consider these common operations:

- You select text in a field, then click a "B" button in a toolbar
   to make its style bold.

- You select a file object in the Finder, then select File->Open
   to open it.

The latter is a good example here, because it reminds us that most 
people use the shortcut for File->Open, which is to double-click the 
file object.

In that case, the verb object is a separate thing, the File->Open menu 
item.  Double-clicking the noun object (the file) is merely a shortcut 
for triggering the most common action performed on the object.

Note that single-clicking a file object triggers no action; it's merely 
a selection.

You'll see a similar pattern in many dialogs that contain lists with a 
default button:  you can single-click a list item to select it and then 
click the default button to apply that action to the selection, or you 
can double-click the list item to trigger the default button.

In each of these cases the verb and noun objects are different things, 
and the single-click merely selects the object but performs no action on 
it, and a double-click is merely a shortcut to a diffent action control.

So while it's common to see double-clicks used throughout many UIs, 
they're almost always a shortcut to a more visible verb trigger, and I 
can't think of any example in a popular app in which an object that 
triggers an action on double-click will also trigger a different action 
with a single-click.

There may be one, perhaps even two if we search far and wide, but here I 
would use what I like to call the Brother Guy Threshold:  "Name three". :)

In your app, the double-click acts as a sort of Undo - from your earlier 

    The single mouse click invokes the modal dialog that was the
    subject of my defaultStack problems.  The double click basically
    undoes the what the modal dialog logic did.

Given how frequently users accidentally double-click objects, you could 
consider a "Reset" or "Undo Changes" button next to the one that invokes 
the dialog, or perhaps a menu item or some other visible element.

As the OS X HIG suggests (in User Input/Double-Clicking):

     Double-clicking is most commonly used as a shortcut for other
     actions, such as pressing Command-O to open a document or
     dragging to select a word. Because not everyone is physically
     able to perform a double click, it should never be the only
     way to perform an action.

The ways this could be optimally handled will vary according to the 
specifics of the app, of course, and it may well be that yours is the 
rare case where having one push button perform two different actions may 
be best.

You're free to submit a bug report on this if you like, but don't be 
surprised if it gets flagged "Not a bug", since the only way to "fix" 
this would be to delay processing of all mouse clicks by the 
doubleClickInterval, and would only benefit the exceptional rare case 
relying on both messages in a single control.

For such unusual behaviors it seems reasonable to expect the scripter to 
craft the unique behavior they want - Craig did a great job on coming 
through with that so quickly.

  Richard Gaskin
  Fourth World
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