Use a prefix key to execute the key handler

The default prefix key is C-x, and can be changed in config.def.h. The
first key pressed after the prefix key will be passed the external key
handler, unless the key is Escape, which is used to cancel the prefix.

README.md

@@ -113,6 +113,10 @@ of small previews is displayed, making it easy to choose an image to open.
     N            Go [count] marked images forward
     P            Go [count] marked images backward
 
+    Ctrl-x KEY	 Execute $XDG_CONFIG_HOME/sxiv/exec/key-handler with
+                 KEY and the path of the current image as arguments.
+		 If KEY is Escape, it cancels the effect of Ctrl-x.
+
 *Thumbnail mode:*
 
     h,j,k,l      Move selection left/down/up/right [count] times

config.def.h

@@ -77,6 +77,12 @@ static const bool RENDER_WHITE_ALPHA = false;
 #endif
 #ifdef _MAPPINGS_CONFIG
 
+/* the prefix key: when pressed, the next key combo is passed to the external
+ * key handler
+ */
+#define PREFIX_KEYMASK ControlMask
+#define PREFIX_KEYSYM XK_x
+
 /* keyboard mappings for image and thumbnail mode: */
 static const keymap_t keys[] = {
 	/* modifiers    key               function              argument */

exec/key-handler

@@ -1,9 +1,9 @@
 #!/bin/sh
 
 # Example for $XDG_CONFIG_HOME/sxiv/exec/key-handler
-# Called by sxiv(1) whenever an unbound key combo is used,
-# with the key combo as its first argument and the path of the current image
-# as its second argument.
+# Called by sxiv(1) when you press the prefix key (C-x by default) followed by
+# a key combo. The key combo is passed as its first argument and the path of
+# the current image as its second argument.
 # sxiv(1) blocks until this script terminates. It then checks if the image
 # has been modified and reloads it.
 

main.c

@@ -504,6 +504,7 @@ void run_key_handler(const char *key, unsigned int mask)
 
 void on_keypress(XKeyEvent *kev)
 {
+	static bool seen_prefix_key = false;
 	int i;
 	unsigned int sh;
 	KeySym ksym, shksym;
@@ -523,6 +524,17 @@ void on_keypress(XKeyEvent *kev)
 	if (IsModifierKey(ksym))
 		return;
 
+	if (seen_prefix_key) {
+		seen_prefix_key = false;
+		if (!(MODMASK(kev->state) == 0 && ksym == XK_Escape))
+			run_key_handler(XKeysymToString(ksym), kev->state & ~sh);
+		return;
+	} else if (MODMASK(kev->state) == PREFIX_KEYMASK && ksym == PREFIX_KEYSYM) {
+		seen_prefix_key = true;
+		prefix = 0;
+		return;
+	}
+
 	if ((ksym == XK_Escape && MODMASK(kev->state) == 0) ||
 		  (key >= '0' && key <= '9'))
 	{
@@ -545,8 +557,6 @@ void on_keypress(XKeyEvent *kev)
 			break;
 		}
 	}
-	if (i == ARRLEN(keys))
-		run_key_handler(XKeysymToString(ksym), kev->state & ~sh);
 	prefix = 0;
 }