i3-msg: add support for SUBSCRIBE message type

If i3-msg is invoked with -t subscribe, it will wait for the first event
matching the given payload, before exiting.

For instance, get the number of the next focused workspace with:

    i3-msg -t subscribe '[ "workspace" ]' | jshon -e current -e num

Like inotifywait, the -m flag allows to wait indefinitely for events,
instead of exiting right after receiving the first one.

For example, continuously monitor the names of focused windows with:

    i3-msg -t subscribe -m '[ "window" ]' | jq .container.name

i3-msg/main.c

@@ -166,16 +166,18 @@ int main(int argc, char *argv[]) {
     uint32_t message_type = I3_IPC_MESSAGE_TYPE_RUN_COMMAND;
     char *payload = NULL;
     bool quiet = false;
+    bool monitor = false;
 
     static struct option long_options[] = {
         {"socket", required_argument, 0, 's'},
         {"type", required_argument, 0, 't'},
         {"version", no_argument, 0, 'v'},
         {"quiet", no_argument, 0, 'q'},
+        {"monitor", no_argument, 0, 'm'},
         {"help", no_argument, 0, 'h'},
         {0, 0, 0, 0}};
 
-    char *options_string = "s:t:vhq";
+    char *options_string = "s:t:vhqm";
 
     while ((o = getopt_long(argc, argv, options_string, long_options, &option_index)) != -1) {
         if (o == 's') {
@@ -204,25 +206,34 @@ int main(int argc, char *argv[]) {
                 message_type = I3_IPC_MESSAGE_TYPE_GET_CONFIG;
             } else if (strcasecmp(optarg, "send_tick") == 0) {
                 message_type = I3_IPC_MESSAGE_TYPE_SEND_TICK;
+            } else if (strcasecmp(optarg, "subscribe") == 0) {
+                message_type = I3_IPC_MESSAGE_TYPE_SUBSCRIBE;
             } else {
                 printf("Unknown message type\n");
-                printf("Known types: run_command, get_workspaces, get_outputs, get_tree, get_marks, get_bar_config, get_binding_modes, get_version, get_config, send_tick\n");
+                printf("Known types: run_command, get_workspaces, get_outputs, get_tree, get_marks, get_bar_config, get_binding_modes, get_version, get_config, send_tick, subscribe\n");
                 exit(EXIT_FAILURE);
             }
         } else if (o == 'q') {
             quiet = true;
+        } else if (o == 'm') {
+            monitor = true;
         } else if (o == 'v') {
             printf("i3-msg " I3_VERSION "\n");
             return 0;
         } else if (o == 'h') {
             printf("i3-msg " I3_VERSION "\n");
-            printf("i3-msg [-s <socket>] [-t <type>] <message>\n");
+            printf("i3-msg [-s <socket>] [-t <type>] [-m] <message>\n");
             return 0;
         } else if (o == '?') {
             exit(EXIT_FAILURE);
         }
     }
 
+    if (monitor && message_type != I3_IPC_MESSAGE_TYPE_SUBSCRIBE) {
+        fprintf(stderr, "The monitor option -m is used with -t SUBSCRIBE exclusively.\n");
+        exit(EXIT_FAILURE);
+    }
+
     /* Use all arguments, separated by whitespace, as payload.
      * This way, you don’t have to do i3-msg 'mark foo', you can use
      * i3-msg mark foo */
@@ -287,6 +298,24 @@ int main(int argc, char *argv[]) {
             case yajl_status_error:
                 errx(EXIT_FAILURE, "IPC: Could not parse JSON reply.");
         }
+    } else if (reply_type == I3_IPC_REPLY_TYPE_SUBSCRIBE) {
+        do {
+            free(reply);
+            if ((ret = ipc_recv_message(sockfd, &reply_type, &reply_length, &reply)) != 0) {
+                if (ret == -1)
+                    err(EXIT_FAILURE, "IPC: read()");
+                exit(1);
+            }
+
+            if (!(reply_type & I3_IPC_EVENT_MASK)) {
+                errx(EXIT_FAILURE, "IPC: Received reply of type %d but expected an event", reply_type);
+            }
+
+            if (!quiet) {
+                fprintf(stdout, "%.*s\n", reply_length, reply);
+                fflush(stdout);
+            }
+        } while (monitor);
     } else {
         if (!quiet) {
             printf("%.*s\n", reply_length, reply);

man/i3-msg.man

@@ -31,6 +31,11 @@ with an error.
 *-t* 'type'::
 Send ipc message, see below. This option defaults to "command".
 
+*-m*, *--monitor*::
+Instead of exiting right after receiving the first subscribed event,
+wait indefinitely for all of them. Can only be used with "-t subscribe".
+See the "subscribe" IPC message type below for details.
+
 *message*::
 Send ipc message, see below.
 
@@ -75,6 +80,11 @@ Gets the currently loaded i3 configuration.
 send_tick::
 Sends a tick to all IPC connections which subscribe to tick events.
 
+subscribe::
+The payload of the message describes the events to subscribe to.
+Upon reception, each event will be dumped as a JSON-encoded object.
+See the -m option for continuous monitoring.
+
 == DESCRIPTION
 
 i3-msg is a sample implementation for a client using the unix socket IPC
@@ -91,6 +101,9 @@ i3-msg border normal
 
 # Dump the layout tree
 i3-msg -t get_tree
+
+# Monitor window changes
+i3-msg -t subscribe -m '[ "window" ]'
 ------------------------------------------------
 
 == ENVIRONMENT