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Re: GUIKit - ScrollPane Tables within Wizard

• To: mathgroup at smc.vnet.net
• Subject: [mg53060] Re: GUIKit - ScrollPane Tables within Wizard
• From: Jeff Adams <jeffa at wolfram.com>
• Date: Wed, 22 Dec 2004 04:52:51 -0500 (EST)
• References: <200412150926.EAA10576@smc.vnet.net>
• Sender: owner-wri-mathgroup at wolfram.com

On Dec 15, 2004, at 3:26 AM, paulw at cwgsy.net wrote:
> The second problem I have found is that when you attempt to include an
> image within the table, it outputs some script along the lines of
> JavaObject##########Released where ###'s are replaced with the object
> id number. This does not happen using the exact same code outside of
> the wizard.

Hello,

To help track down possible sources of getting Removed[JavaObjects] in 
your use
of GUIKit I thought it might be easiest to explain what could be going 
on with
a short example definition.  Hopefully some of this insight into the 
workings
of objects in GUIKit and JLink will aid in debugging any temporary 
coding problems you
might have during development.

Typically one might create an image and set it as the icon of a button
at the time of creating the button within the definition of the button, 
like so:

Needs["GUIKit`"]

GUIRun[
   Widget["Panel", {
       Widget["Label", {"Text" -> "Icon reference demo:"}],
       Widget["Button", {"Icon" -> Widget["Icon", {"Data" -> Script[
       ExportString[ ToBoxes[a + b/c^4], "GIF",
        ConversionOptions -> {"Transparency" -> GrayLevel[1]}]]}]}]
       }]  ]


But notice what happens below if we try and create the icon widget in
one script and assign it to a Mathematica symbol and then try to use it 
in the
creation of the button sometime later on:

GUIRun[
   Widget["Panel", {
       Widget["Label", {"Text" -> "Icon reference demo:"}],
       Script[
         icn = Widget["Icon", {"Data" -> Script[ ExportString[
           ToBoxes[a + b/c^4], "GIF",
         ConversionOptions -> {"Transparency" -> GrayLevel[1]}]]}];],
       Widget["Button", {"Icon" -> Script[Print[icn]; icn]}]
       }]  ]

 From In[]:=Removed[JavaObject241387681349633]
Out[]=$Failed

This fails to work because the icon object that is created in the first 
Script is not retained by
  the system because all GUIKit Script[] blocks are automatically 
wrapped in a JavaBlock[].
You can see more documentation of JavaBlock in the J/Link Help Browser 
documentation but it
essentially releases objects, created from calls in Mathematica but not 
referenced in Java, from leaking,
and so within the second Script call, this JavaObject is already 
removed from the system (Java VM).

The recommended way of creating and managing the lifetime of widget 
objects in GUIKit is naming
them either on creation using the Name option or alternately naming 
them separately using SetWidgetReference["name", obj_].
This not only manages the lifetime of the objects automatically for you 
by GUIKit until the associated
window is closed, but also allows you to reference these named objects 
using all the other
existing GUIKit functions including the function 
WidgetReference["name"].

Below are two variants of the same functionality. Once uses the Name -> 
_ option of Widget, the other
separately registers the object using a name through the 
SetWidgetReference["name", obj_]
function. Note that the button is then created using the existing icon 
widget instance created
in the first Script block:

This technique uses the Name -> _ option

GUIRun[
   Widget["Panel", {
       Widget["Label", {"Text" -> "Icon reference demo:"}],
       Script[
         Widget["Icon", {
           "Data" -> Script[ ExportString[ ToBoxes[a + b/c^4], "GIF",
             ConversionOptions -> {"Transparency" -> GrayLevel[1]}]]},
               Name -> "myIcon"]; ],
       Widget["Button", {"Icon" -> WidgetReference["myIcon"]}]
       }]  ]

this second version instead uses the SetWidgetReference function which 
can be useful if you make separate
decisions on registering objects from when they are actually created by 
the code:

GUIRun[
   Widget["Panel", {
       Widget["Label", {"Text" -> "Icon reference demo:"}],
       Script[
         icn = Widget["Icon", {"Data" -> Script[ ExportString[ ToBoxes[a 
+ b/c^4], "GIF",
            ConversionOptions -> {"Transparency" -> GrayLevel[1]}]]}];
         SetWidgetReference["myIcon", icn]; ],
       Widget["Button", {"Icon" -> WidgetReference["myIcon"]}]
       }]  ]


For those of you familiar with J/Link you might think that
using KeepJavaObject would also work to make sure these objects are 
available.
And although this does work, it is not as clean a technique as using the
named object system of GUIKit.

For example if you were to try using just KeepJavaObject:

GUIRun[
   Widget["Panel", {
       Widget["Label", {"Text" -> "Icon reference demo:"}],
       Script[
         icn = Widget["Icon", {
               "Data" -> Script[ ExportString[ ToBoxes[a + b/c^4], "GIF",
         ConversionOptions -> {"Transparency" -> GrayLevel[1]}]]}];
         KeepJavaObject[icn]; ],
       Widget["Button", {"Icon" -> Script[icn]}]
       }]  ]

This works, but if you check the J/Link function PeekObjects[]  you 
will see this icon leaks into the system
and stays around even after the GUIKit window is closed.
To fix this you would actually have to use ReleaseJavaObject at the 
appropriate
place:

GUIRun[
   Widget["Panel", {
       Widget["Label", {"Text" -> "Icon reference demo:"}],
       Script[
         icn = Widget["Icon", {"Data" -> Script[ ExportString[ ToBoxes[a 
+ b/c^4], "GIF",
         ConversionOptions -> {"Transparency" -> GrayLevel[1]}]]}];
         KeepJavaObject[icn]; ],
       Widget["Button", {"Icon" -> Script[icn], 
Script[ReleaseJavaObject[icn]]}]
       }]   ]

And this is definitely not as convenient as the named object versions 
recommended previously.

Jeff Adams
Wolfram Research

• References:
  • GUIKit - ScrollPane Tables within Wizard
    • From: paulw@cwgsy.net