Catching changes of scene properties

Coding Python Support
1

Hi,

I’m looking for a way to track changes that happen on some blender properties, such as bpy.context.scene.render.resolution_x so my script could update other values.

A dirty workaround was to check the value in the draw part of a custom panel, but that’s anyway no more possible since recent API change prevents writing to properties at draw time.

Any clue?
Thanks in advance,
yoyoz

2

Seems to me that drivers is an ideal way to do this.

3

I’d like to give a try to this but I don’t know how to attach the driver to an existing property from the script itself. I’ve seen a post in this forum showing an elegant way to register a driver name globally, so this part should be ok.

I also hope there’re no side effect such as turning the property in blue or whatever… :wink:

4

i used to do it the draw() way either. Now i’m going with a modal operator but that’s not less dirty. Uncle Entity made a patch that made notifiers and listeners available to blenders python api. but that is still in the patch tracker waiting to be reviewed by a core dev.

my code looks as follows:


IS_RUNNING = False
class PROPERTIES_OT_maintain_renderman(bpy.types.Operator):
    bl_idname = 'renderman.maintain'
    bl_label ="maintain renderman"
    bl_options = {'REGISTER'}
    
    def modal(self, context, event):
        #print("Blender to Renderman Addon Loaded")
        maintain(context)
        return {'PASS_THROUGH'}
                
    def execute(self, context):
        wm = context.window_manager
        global IS_RUNNING
        if context.area.type == 'PROPERTIES':
            if not IS_RUNNING:
                IS_RUNNING = True
                print("adding modal handler")
                wm.modal_handler_add(self)
        return {'RUNNING_MODAL'}

the operator is started in a draw() function of a custom panel.
that’s a realy ugly way but for now it works. There must be a way to check if the modal operator is running but i don’t know how. I’ve looked at the measure panel but the way how it’s described doesn’t seem to work for me somehow.
Btw: you could also still check your values in a draw panel and then run an operator from there if needed.

5

It could also fit my need, waiting for real callback or event system…
BTW, how to you call the operator from the draw area? (I’m sorry I can’t figure out the syntax)

6

bpy.ops.renderman.maintain()

7

Thanks, it works the way I want!

The only problem I have is blender flooding the logs when exiting:

Aspect ratio fix installed
bpy stats - tot exec: 16000, tot run: 0.2427sec, average run: 0.000015sec, tot usage 1.6630%
Saved session recovery to /tmp/quit.blend
Error Totblock: 456
wmOperatorReportList len: 40 0x497e628
ASPECT_OT_fix len: 168 0x4a76118
wmOperatorPtrRNA len: 24 0x31c42f8
IDProperty group len: 96 0x496a5f8
wmOperatorReportList len: 40 0x4987dd8
ASPECT_OT_fix len: 168 0x4a62a18
wmOperatorPtrRNA len: 24 0x31c4bf8
IDProperty group len: 96 0x4969f38
wmOperatorReportList len: 40 0x4a45288
… for many pages

Here’s the modified code. It just ensures that the aspect ratio is back to 1:1 and it’s called in the draw part of my first panel. I’m only annoyed with the messages in the log…

IS_RUNNING = False
class PROPERTIES_aspect_fix(bpy.types.Operator):
    bl_idname = 'aspect.fix'
    bl_label ="Keeps 1:1 pixels"
    bl_options = {'REGISTER'}
    
    def modal(self, context, event):
        #self.maintain(context)
        rd = bpy.context.scene.render
        if rd.pixel_aspect_x != 1:
            rd.pixel_aspect_x =1
            log ('Reset pixel to square')
        if rd.pixel_aspect_y != 1:
            rd.pixel_aspect_y = 1
            log ('Reset pixel to square')
        return {'PASS_THROUGH'}
                
    def execute(self, context):
        wm = context.window_manager
        global IS_RUNNING
        if context.area.type == 'PROPERTIES':
            if not IS_RUNNING:
                IS_RUNNING = True
                print("Aspect ratio fix installed")
                wm.modal_handler_add(self)
        return {'RUNNING_MODAL'}
8

Do you Guys have a clue about how I could get rid of the nasty logs? It seems to me Blender is stacking errors and I fear some unstability if using it for a long time…
Thanks,
Yoyoz

9

I managed doing it this way, but this is broken this morning with latest builds…

I’m getting the following now:

File "/home/lionel/.blender/2.56/scripts/addons/octanerender/ui_texture.py", line 178, in draw
    bpy.ops.octane.fixtexture()
  File "/usr/share/blender/2.56/scripts/modules/bpy/ops.py", line 179, in __call__
    ret = op_call(self.idname_py(), None, kw)
SystemError: Calling operator "bpy.ops.octane.fixtexture" error, can't modify blend data in this state (drawing/rendering)