RNA: support setting default values for custom properties.

NLA requires a usable default value for all properties that
are to be animated via it, without any exceptions. This is
the real cause of T36496: using the default of 0 for a scale
related custom property obviously doesn't work.

Thus, to really fix this it is necessary to support configurable
default values for custom properties, which are very frequently
used in rigs for auxiliary settings. For common use it is enough
to support this for scalar float and integer properties.

The default can be set via the custom property configuration
popup, or a right click menu option. In addition, to help in
updating old rigs, an operator that saves current values as
defaults for all object and bone properties is added.

Reviewers: campbellbarton, brecht

Differential Revision: https://developer.blender.org/D4084

release/scripts/startup/bl_operators/wm.py

@@ -1052,6 +1052,12 @@ rna_value = StringProperty(
     maxlen=1024,
 )
 
+rna_default = StringProperty(
+    name="Default Value",
+    description="Default value of the property. Important for NLA mixing",
+    maxlen=1024,
+)
+
 rna_property = StringProperty(
     name="Property Name",
     description="Property name edit",
@@ -1089,6 +1095,7 @@ class WM_OT_properties_edit(Operator):
     data_path: rna_path
     property: rna_property
     value: rna_value
+    default: rna_default
     min: rna_min
     max: rna_max
     use_soft_limits: rna_use_soft_limits
@@ -1107,6 +1114,28 @@ class WM_OT_properties_edit(Operator):
             "hard_range": (self.min, self.max),
         }
 
+    def get_value_eval(self):
+        try:
+            value_eval = eval(self.value)
+            # assert else None -> None, not "None", see [#33431]
+            assert(type(value_eval) in {str, float, int, bool, tuple, list})
+        except:
+            value_eval = self.value
+
+        return value_eval
+
+
+    def get_default_eval(self):
+        try:
+            default_eval = eval(self.default)
+            # assert else None -> None, not "None", see [#33431]
+            assert(type(default_eval) in {str, float, int, bool, tuple, list})
+        except:
+            default_eval = self.default
+
+        return default_eval
+
+
     def execute(self, context):
         from rna_prop_ui import (
             rna_idprop_ui_prop_get,
@@ -1124,12 +1153,8 @@ class WM_OT_properties_edit(Operator):
             self.report({'ERROR'}, "Direct execution not supported")
             return {'CANCELLED'}
 
-        try:
-            value_eval = eval(value)
-            # assert else None -> None, not "None", see [#33431]
-            assert(type(value_eval) in {str, float, int, bool, tuple, list})
-        except:
-            value_eval = value
+        value_eval = self.get_value_eval()
+        default_eval = self.get_default_eval()
 
         # First remove
         item = eval("context.%s" % data_path)
@@ -1159,6 +1184,8 @@ class WM_OT_properties_edit(Operator):
         if prop_type in {float, int}:
             prop_ui["min"] = prop_type(self.min)
             prop_ui["max"] = prop_type(self.max)
+            if type(default_eval) in {float, int} and default_eval != 0:
+                prop_ui["default"] = prop_type(default_eval)
 
             if self.use_soft_limits:
                 prop_ui["soft_min"] = prop_type(self.soft_min)
@@ -1223,6 +1250,13 @@ class WM_OT_properties_edit(Operator):
         exec_str = "item.is_property_overridable_static('[\"%s\"]')" % (self.property)
         self.is_overridable_static = bool(eval(exec_str))
 
+        # default default value
+        prop_type = type(self.get_value_eval())
+        if prop_type in {int,float}:
+            self.default = str(prop_type(0))
+        else:
+            self.default = ""
+
         # setup defaults
         prop_ui = rna_idprop_ui_prop_get(item, self.property, False)  # don't create
         if prop_ui:
@@ -1230,6 +1264,10 @@ class WM_OT_properties_edit(Operator):
             self.max = prop_ui.get("max", 1000000000)
             self.description = prop_ui.get("description", "")
 
+            defval = prop_ui.get("default", None)
+            if defval is not None:
+                self.default = str(defval)
+
             self.soft_min = prop_ui.get("soft_min", self.min)
             self.soft_max = prop_ui.get("soft_max", self.max)
             self.use_soft_limits = (
@@ -1275,6 +1313,11 @@ class WM_OT_properties_edit(Operator):
         layout = self.layout
         layout.prop(self, "property")
         layout.prop(self, "value")
+
+        row = layout.row()
+        row.enabled = type(self.get_value_eval()) in {int,float}
+        row.prop(self, "default")
+
         row = layout.row(align=True)
         row.prop(self, "min")
         row.prop(self, "max")