cmd_opacity: add relative opacity changes

This enhances the opacity command to support relative assignment as well
as the currently implemented absolute assignment. The syntax is copied
from the same format that gaps uses for relative and absolute setting.
An example usage in a sway config looks like:

// relative change (this feature)
bindsym button4 opacity plus .1
bindsym button5 opacity minus .1

// absolute change (this feature)
bindsym button4 opacity set 1
bindsym button5 opacity set .3

// old way, still supported
bindsym button4 opacity 1
bindsym button5 opacity .3

1 files changed, 18 insertions, 15 deletions

diff --git a/sway/commands/opacity.c b/sway/commands/opacity.c
index 14a07051..96e6228e 100644
--- a/sway/commands/opacity.c
+++ b/sway/commands/opacity.c
@@ -1,21 +1,13 @@
 #include <assert.h>
 #include <stdlib.h>
+#include <strings.h>
 #include "sway/commands.h"
 #include "sway/tree/view.h"
 #include "log.h"
 
-static bool parse_opacity(const char *opacity, float *val) {
-	char *err;
-	*val = strtof(opacity, &err);
-	if (*val < 0 || *val > 1 || *err) {
-		return false;
-	}
-	return true;
-}
-
 struct cmd_results *cmd_opacity(int argc, char **argv) {
 	struct cmd_results *error = NULL;
-	if ((error = checkarg(argc, "opacity", EXPECTED_EQUAL_TO, 1))) {
+	if ((error = checkarg(argc, "opacity", EXPECTED_AT_LEAST, 1))) {
 		return error;
 	}
 
@@ -25,15 +17,26 @@ struct cmd_results *cmd_opacity(int argc, char **argv) {
 		return cmd_results_new(CMD_FAILURE, "No current container");
 	}
 
-	float opacity = 0.0f;
+	char *err;
+	float val = strtof(argc == 1 ? argv[0] : argv[1], &err);
+	if (*err) {
+		return cmd_results_new(CMD_INVALID, "opacity float invalid");
+	}
 
-	if (!parse_opacity(argv[0], &opacity)) {
+	if (!strcasecmp(argv[0], "plus")) {
+		val = con->alpha + val;
+	} else if (!strcasecmp(argv[0], "minus")) {
+		val = con->alpha - val;
+	} else if (argc > 1 && strcasecmp(argv[0], "set")) {
 		return cmd_results_new(CMD_INVALID,
-				"Invalid value (expected 0..1): %s", argv[0]);
+				"Expected: set|plus|minus <0..1>: %s", argv[0]);
 	}
 
-	con->alpha = opacity;
-	container_damage_whole(con);
+	if (val < 0 || val > 1) {
+		return cmd_results_new(CMD_FAILURE, "opacity value out of bounds");
+	}
 
+	con->alpha = val;
+	container_damage_whole(con);
 	return cmd_results_new(CMD_SUCCESS, NULL);
 }