Anatomy-of-a-Plugin.md

Anatomy of a ScriptCraft Plugin

Anything you can do using a Java-based plugin, you can do it faster and easier in JavaScript with the ScriptCraft plugin. To demonstrate this, I've recreated a commonly-used mod (homes) in javascript. The homes JavaScript plugin lets players set their current location as home and return to that location using in-game commands. They can also visit other players' homes. It's a simple plugin that demonstrates a couple of new features in ScriptCraft …

• Persistence
• Adding Player (non-operator) commands

Here, I walk you through another useful plugin which lets players modify the color of the in-game chat.

Persistence

… First persistence. Persistence is the ability to retain state after the server has shutdown and started up again. You can create a JavaScript object which will be saved at shutdown and reloaded at startup by using the built-in persist() function.

// file: scriptcraft/plugins/my-first-plugin.js
var prefs = persist('myprefs', {});
...
prefs.color = 'black';

In the example above, a new empty object is created and stored in a file called myprefs-store.json. The empty object is returned (if data is not already present in that file or the file does not exist) and any changes to the object's contents are written to the file when the server is shutdown.

The data is persisted in JSON form so it's even somewhat human-readable. Declaring a new plugin is easy. I'm going to create a new plugin called "chat" that will let players change the default color of their messages in the in-game chat window …

var store = persist('chat-colors', {players: {}});
exports.chat = { 
  setColor: function(player,chatColor) { 
    store.players[player.name] = chatColor;
  }
}

The above code doesn't do a whole lot other than let operators set a player's color choice ( /js chat.setColor(self, 'green') ). A little bit more code has to be added so that the player's text color will change when chatting with other players, but the above code will ensure the player's color setting is at least saved. The following code just ensures that when a player chats, the text will be displayed in their chosen color …

var colors = ['black', 'blue', 'darkgreen', 'darkaqua', 'darkred',
              'purple', 'gold', 'gray', 'darkgray', 'indigo',
              'brightgreen', 'aqua', 'red', 'pink',
              'yellow', 'white'];
var colorCodes = {};
var COLOR_CHAR = '\u00a7';
for (var i =0;i < colors.length;i++) 
  colorCodes[colors[i]] = i.toString(16);

var addColor = function( evt ) {
  var player = evt.player;
  var playerChatColor = store.players[ player.name ];
  if ( playerChatColor ) {
    evt.message = COLOR_CHAR + colorCodes[ playerChatColor ] + evt.message;
  }
};

if (__plugin.bukkit) {
  events.asyncPlayerChat(addColor);
} else if (__plugin.canary) {
   events.chat(addColor);
};

The next step is to declare a lookup table of colors / names and add an event handler which intercepts and inserts color codes into player's text messages.

Adding new Player Commands

The other command in ScriptCraft is the /jsp command – this lets operators expose plugins for use by regular players. To be clear, /jsp does not do any JavaScript evaluation, it just accepts parameters which are then passed on to the appropriate JavaScript plugin. So far in this example plugin we haven't provided any way for regular players to – you know – actually set their text color of choice – only operators can do this for a player using the js chat.setColor(...) JavaScript expression. Let's be clear – giving your players access to the whole API via JavaScript isn't a good idea. So how do you safely let players choose their text color? If you've written a JavaScript function and want players to be able to use that function, you expose it using the new command() function like so …

function chat_color( params, sender ){
  var color = params[0];
  if (colorCodes[color]){
    chat.setColor(sender,color);
  }else{
    echo(sender, color + ' is not a valid color');
    echo(sender, 'valid colors: ' + colors.join(', '));
  }
}
command(chat_color, colors);

… The above code adds a new subcommand to the /jsp command and also specifies autocomplete options (the last parameter – colors) for that command when the player presses the TAB key. Now the player themselves can change their chosen chat color like so …

/jsp chat_color yellow

… What I've done here is create a new plugin which lets players choose a chat color and saves that preference when the server shuts down and starts up. I've also added a new jsp sub-command – chat_color that players use to change their chat color setting. The full plugin source code is just a couple of lines of code but is a fully working plugin …

var store = persist('chat-colors', {players: {}});
exports.chat = { 
  setColor: function(player,chatColor) { 
    store.players[player.name] = chatColor;
  }
}
var colors = ['black', 'blue', 'darkgreen', 'darkaqua', 'darkred',
              'purple', 'gold', 'gray', 'darkgray', 'indigo',
              'brightgreen', 'aqua', 'red', 'pink',
              'yellow', 'white'];
var colorCodes = {};
var COLOR_CHAR = '\u00a7';
for (var i =0;i < colors.length;i++) 
  colorCodes[colors[i]] = i.toString(16);

var addColor = function( evt ) {
  var player = evt.player;
  var playerChatColor = store.players[ player.name ];
  if ( playerChatColor ) {
    evt.message = COLOR_CHAR + colorCodes[ playerChatColor ] + evt.message;
  }
};

if (__plugin.bukkit) {
  events.asyncPlayerChat(addColor);
} else if (__plugin.canary) {
   events.chat(addColor);
};

function chat_color( params, sender ){
  var color = params[0];
  if (colorCodes[color]){
    chat.setColor(sender,color);
  }else{
    echo(sender, color + ' is not a valid color');
    echo(sender, 'valid colors: ' + colors.join(', '));
  }
}

command(chat_color, colors);

… this is what I would call a minimum viable plugin and it demonstrates some of the new features of ScriptCraft – persistence (automatic), event handling, and exposing new functionality to players using the /jsp command. I hope this will give potential Minecraft modders a feel for just how easy it can be to change the game to suit their needs.