First stages of easier "expressions" creation...

It is now possible to create "scripted expression" drivers by simply
clicking on some property, and typing some short Python expression
prefixed with a '#'. This will result in a scripted expression driver,
with the typed-in text being created.

For example, you can click on X-Location of the default cube, and
type:
#sin(frame)
and a new driver will be created for the x-location of the cube. This
will use the current frame value, and modulate this with a sine wave.

Do note though, that the current frame is a special case here. In the
current implementation, a special "frame" driver variable, which
references the current scene frame is created automatically, so that
this simple and (assumed) common case will work straight out of the
box.

Future improvements:
- Explore possibilities of semi-automated extraction of variables from
such expressions, resulting in automated variable extraction. (Doing
away with variables completely is definitely 100% off the agenda
though)
- Look into some ways of defining some shorthands for referencing
local data (possibly related to variable extraction?)

source/blender/editors/animation/drivers.c

@@ -80,7 +80,7 @@ void free_anim_drivers_copybuf (void);
  *			1 - add new Driver FCurve, 
  *			-1 - add new Driver FCurve without driver stuff (for pasting)
  */
-static FCurve *verify_driver_fcurve (ID *id, const char rna_path[], const int array_index, short add)
+FCurve *verify_driver_fcurve (ID *id, const char rna_path[], const int array_index, short add)
 {
 	AnimData *adt;
 	FCurve *fcu;

source/blender/editors/include/ED_keyframing.h

@@ -225,6 +225,13 @@ typedef enum eCreateDriverFlags {
 
 /* -------- */
 
+/* Low-level call to add a new driver F-Curve. This shouldn't be used directly for most tools,
+ * although there are special cases where this approach is preferable.
+ */
+struct FCurve *verify_driver_fcurve(struct ID *id, const char rna_path[], const int array_index, short add);
+
+/* -------- */
+
 /* Returns whether there is a driver in the copy/paste buffer to paste */
 short ANIM_driver_can_paste(void);
 

source/blender/editors/interface/interface.c

@@ -1701,6 +1701,10 @@ int ui_set_but_string(bContext *C, uiBut *but, const char *str)
 		/* driver expression */
 		return 1;
 	}
+	else if(str[0]=='#') {
+		/* shortcut to create new driver expression (versus immediate Py-execution) */
+		return ui_but_anim_expression_create(but, str+1);
+	}
 	else {
 		/* number editing */
 		double value;

source/blender/editors/interface/interface_anim.c

@@ -6,6 +6,7 @@
 #include <stdlib.h>
 #include <string.h>
 
+#include "MEM_guardedalloc.h"
 
 #include "DNA_anim_types.h"
 #include "DNA_scene_types.h"
@@ -13,15 +14,19 @@
 
 #include "BLI_listbase.h"
 #include "BLI_string.h"
+#include "BLI_utildefines.h"
 
 #include "BKE_context.h"
+#include "BKE_animsys.h"
 #include "BKE_fcurve.h"
-
+#include "BKE_global.h"
 
 #include "ED_keyframing.h"
 
 #include "UI_interface.h"
 
+#include "RNA_access.h"
+
 #include "WM_api.h"
 #include "WM_types.h"
 
@@ -96,6 +101,74 @@ int ui_but_anim_expression_set(uiBut *but, const char *str)
 	return 0;
 }
 
+/* create new expression for button (i.e. a "scripted driver"), if it can be created... */
+int ui_but_anim_expression_create(uiBut *but, const char *str)
+{
+	bContext *C = but->block->evil_C;
+	ID *id;
+	FCurve *fcu;
+	char *path;
+	short ok=0;
+	
+	/* button must have RNA-pointer to a numeric-capable property */
+	if (ELEM(NULL, but->rnapoin.data, but->rnaprop)) {
+		if (G.f & G_DEBUG) 
+			printf("ERROR: create expression failed - button has no RNA info attached\n");
+		return 0;
+	}
+	
+	/* make sure we have animdata for this */
+	// FIXME: until materials can be handled by depsgraph, don't allow drivers to be created for them
+	id = (ID *)but->rnapoin.id.data;
+	if ((id == NULL) || (GS(id->name)==ID_MA) || (GS(id->name)==ID_TE)) {
+		if (G.f & G_DEBUG)
+			printf("ERROR: create expression failed - invalid id-datablock for adding drivers (%p)\n", id);
+		return 0;
+	}
+	
+	/* get path */
+	path = RNA_path_from_ID_to_property(&but->rnapoin, but->rnaprop);
+	
+	/* create driver */
+	fcu = verify_driver_fcurve(id, path, but->rnaindex, 1);
+	if (fcu) {
+		ChannelDriver *driver= fcu->driver;
+		
+		if (driver) {
+			/* set type of driver */
+			driver->type = DRIVER_TYPE_PYTHON;
+			
+			/* set the expression */
+			// TODO: need some way of identifying variables used
+			BLI_strncpy(driver->expression, str, sizeof(driver->expression));
+			
+			/* FIXME: for now, assume that 
+			 * 	- for expressions, users are likely to be using "frame" -> current frame" as a variable
+			 *	- driver_add_new_variable() adds a single-prop variable by default
+			 */
+			{
+				DriverVar *dvar;
+				DriverTarget *dtar;
+				
+				dvar = driver_add_new_variable(driver);
+				BLI_strncpy(dvar->name, "frame", sizeof(dvar->name));
+				
+				dtar = &dvar->targets[0];
+				dtar->id = (ID *)CTX_data_scene(C); // XXX: should we check that C is valid first?
+				dtar->rna_path = BLI_sprintfN("frame_current");
+			}
+			
+			/* updates */
+			driver->flag |= DRIVER_FLAG_RECOMPILE;
+			WM_event_add_notifier(C, NC_ANIMATION|ND_KEYFRAME, NULL);
+		}
+	}
+	
+	MEM_freeN(path);
+	
+	return ok;
+}
+
 void ui_but_anim_autokey(bContext *C, uiBut *but, Scene *scene, float cfra)
 {
 	ID *id;

source/blender/editors/interface/interface_intern.h

@@ -520,6 +520,7 @@ void ui_but_anim_add_keyingset(struct bContext *C);
 void ui_but_anim_remove_keyingset(struct bContext *C);
 int ui_but_anim_expression_get(uiBut *but, char *str, int maxlen);
 int ui_but_anim_expression_set(uiBut *but, const char *str);
+int ui_but_anim_expression_create(uiBut *but, const char *str);
 void ui_but_anim_autokey(struct bContext *C, uiBut *but, struct Scene *scene, float cfra);
 
 #endif