Driver Workflow (T55145): Tweak to default expression used for new drivers created from UI

Previously, newly created drivers were set to "Scripted Expression"
mode and had their 'expression' field set to the pre-driver value of the
property, so that adding a driver didn't cause the property to immediately
lose its old value (and potentially causing havok in the scene).
However, this had the unintended consequence of making the driver setup
workflow more cumbersome, as you first had to replace that value with
the name of the driver variable before your driver would work.

This commit works around this issue by trying to combine the best of both
worlds (quite literally):  Now, the driver expression for drivers created
using Ctrl-D will be "var + <old value>".

Thus, in the simplest case, the driver will do something as soon as you fill
out the driver variable settings (e.g. just filling out the Target Object field
will do it), meaning you get your drivers working faster. Of course, it may now
be necessary to edit out the old-value, and/or it might be a bit more confusing
what/why it's there for newbies. However, the improved ease of setup, and/or
a more tangible example of how an expression may be constructed outweigh the
downsides IMO.

1 files changed, 14 insertions, 4 deletions

diff --git a/source/blender/editors/animation/drivers.c b/source/blender/editors/animation/drivers.c
index 28933f0e135..4c0e4207987 100644
--- a/source/blender/editors/animation/drivers.c
+++ b/source/blender/editors/animation/drivers.c
@@ -414,13 +414,20 @@ int ANIM_add_driver(ReportList *reports, ID *id, const char rna_path[], int arra
 			/* set the type of the driver */
 			driver->type = type;
 
-			/* creating drivers for buttons will create the driver(s) with type
+			/* Creating drivers for buttons will create the driver(s) with type
 			 * "scripted expression" so that their values won't be lost immediately,
 			 * so here we copy those values over to the driver's expression
+			 *
+			 * If the "default dvar" option (for easier UI setup of drivers) is provided,
+			 * include "var" in the expressions too, so that the user doesn't have to edit
+			 * it to get something to happen. It should be fine to just add it to the default
+			 * value, so that we get both in the expression, even if it's a bit more confusing
+			 * that way...
 			 */
 			if (type == DRIVER_TYPE_PYTHON) {
 				PropertyType proptype = RNA_property_type(prop);
 				int array = RNA_property_array_length(&ptr, prop);
+				char *dvar_prefix = (flag & CREATEDRIVER_WITH_DEFAULT_DVAR) ? "var + " : "";
 				char *expression = driver->expression;
 				int val, maxlen = sizeof(driver->expression);
 				float fval;
@@ -429,19 +436,22 @@ int ANIM_add_driver(ReportList *reports, ID *id, const char rna_path[], int arra
 					if (!array) val = RNA_property_boolean_get(&ptr, prop);
 					else val = RNA_property_boolean_get_index(&ptr, prop, array_index);
 
-					BLI_strncpy(expression, (val) ? "True" : "False", maxlen);
+					BLI_snprintf(expression, maxlen, "%s%s", dvar_prefix, (val) ? "True" : "False");
 				}
 				else if (proptype == PROP_INT) {
 					if (!array) val = RNA_property_int_get(&ptr, prop);
 					else val = RNA_property_int_get_index(&ptr, prop, array_index);
 
-					BLI_snprintf(expression, maxlen, "%d", val);
+					BLI_snprintf(expression, maxlen, "%s%d", dvar_prefix, val);
 				}
 				else if (proptype == PROP_FLOAT) {
 					if (!array) fval = RNA_property_float_get(&ptr, prop);
 					else fval = RNA_property_float_get_index(&ptr, prop, array_index);
 
-					BLI_snprintf(expression, maxlen, "%.3f", fval);
+					BLI_snprintf(expression, maxlen, "%s%.3f", dvar_prefix, fval);
+				}
+				else if (flag & CREATEDRIVER_WITH_DEFAULT_DVAR) {
+					BLI_strncpy(expression, "var", maxlen);
 				}
 			}