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--- client_server_detection [2019/08/26 04:18] – korshun
+++ client_server_detection [2019/09/05 23:54] (current) – [Writing robust multiplayer code] korshun
@@ Line 1: / Line 1: @@
-====== Clientside/Serverside detection ======
+====== Clientside/serverside detection ======
+===== Introduction =====
 You can check if a script is running serverside, clientside or in singleplayer using functions provided by ACSUtils.
 ===== Terminology =====
@@ Line 14: / Line 17: @@
 Zandronum multiplayer emulation is singleplayer. Just like in singleplayer, the same engine instance is both a server and a client.
-In regular Zandronum multiplayer, the server is only a server and clients are only clients, obviously.
+In real Zandronum multiplayer, the server is only a server and clients are only clients, obviously.
 Yet, if Zandronum were to allow hosting non-dedicated servers, where a player allows others to connect to their singleplayer game without launching a separate engine instance, the hosting player would be both a server and a client, while other players in the game would be only clients.
@@ Line 30: / Line 33: @@
 ===== Writing robust multiplayer code =====
-The definitions above enable writing ACS code that functions correctly in both singleplayer and multiplayer without singleplayer being a special case. Just make sure
+The definitions above enable writing ACS code that functions correctly in both singleplayer and multiplayer without singleplayer being a special case. Using ''IsClient'' and ''IsServer'' correctly ensures that your mod runs correctly in both singleplayer and multiplayer:
+<code acs>
+if (IsServer())
+{
+    // Run serverside gameplay code.
+}
+if (IsClient())
+{
+    // Draw hud, spawn effects.
+}
+</code>
+You can also make sure that a serverside-only function is not called on the client, and a clientside-only function is not called on the server.
+<code>
+function ServersideOnlyFunction(void)
+{
+    // writing if (IsClient()) would incorrectly display the warning in singleplayer.
+    if (!IsServer())
+        printbold(s:"Error: ServersideOnlyFunction was called clientside");
+    ...
+}
+function ClientsideOnlyFunction(void)
+{
+    // writing if (IsServer()) would incorrectly display the warning in singleplayer.
+    if (!IsClient())
+        printbold(s:"Error: ClientsideOnlyFunction was called serverside");
+    ...
+}
+</code>
+===== How it works =====
+All detection of clients and servers is founded on two functions: [[zdoom>ConsolePlayerNumber]] and [[zan>IsNetworkGame]] (formerly known as ''IsMultiplayer'').
-===== How the functions work =====
+''IsClient()'' simply checks if [[zdoom>ConsolePlayerNumber]] is not negative.
-All detection of clients and servers is founded on two functions: [[zdoom>ConsolePlayerNumber]] and [[zan>IsNetworkGame]] (formerly known as [[zan>IsMultiplayer]]).
+''IsServer()'' is more complicated. It returns true if [[zdoom>ConsolePlayerNumber]] is negative. But if it's not negative, it needs to distinguish between multiplayer emulation and real multiplayer. It checks if the multiplayer is real using [[zan>IsNetworkGame]]. But calling this function in ZDoom/GZDoom crashes the engine. Therefore, before calling ''IsNetworkGame'', it checks if the engine is Zandronum using [[source_port_detection|IsZandronum]].
-''IsClient()'' simply checks if [[zdoom>ConsolePlayerNumber]] is not -1.
+If the engine is not Zandronum, the function returns true, because in ZDoom/GZDoom multiplayer all players run gameplay code.
+If the engine is Zandronum, the function returns false if it's real multiplayer, true if it's multiplayer emulation.
+''IsServerOnly'' and ''IsClientOnly'' simply return ''IsServer() && !IsClient()'' and ''IsClient() && !IsServer()'', respectively.

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