關于在cordova的插件開發的做定義消息的功能沒有官方的介紹文章，但是官方有一個這樣的例子，就是電源狀態插件。我們通過學習這個插件的編寫來學習自定義消息。

這個插件的地址為：https://github.com/apache/cordova-plugin-battery-status

安裝方法為：cordova plugin add cordova-plugin-battery-status
 
 
使用方法是：在自己的js工程里執行以下幾句話：
window.addEventListener("batterystatus", onBatteryStatus, false); function onBatteryStatus(status) {    console.log("Level: " + status.level + " isPlugged: " + status.isPlugged);}
分別是給工程添加插件自定義的消息事件，并指定消息響應函數
 
在我的測試例子中，使用了如下函數為兩個按鍵的響應函數，一個訂閱消息，一個取消訂閱：
function onBatteryStatus(info) {  alert("BATTERY STATUS:  Level: " + info.level + "isPlugged: " + info.isPlugged);}$scope.onbatteryStartBtn = function() {  alert("onStartBtn");  window.addEventListener("batterystatus",onBatteryStatus, false);};$scope.onbatteryStopBtn = function() {  alert("onStopBtn");  window.removeEventListener("batterystatus",onBatteryStatus);};
 
 
然后編譯工程：ionic build android
在手機上測試：ionic run android
 
這里顯然就是典型的回調機制，就是在自己的工程里定制消息的響應回調函數，然后當產生這個消息的話，回調函數就會被調用。
那么接下來看一下這個電池狀態插件是怎樣實現的。
 
var cordova = require('cordova'),    exec = require('cordova/exec');var STATUS_CRITICAL = 5;var STATUS_LOW = 20;var Battery = function() {    this._level = null;    this._isPlugged = null;    // Create new event handlers on thewindow (returns a channel instance)    this.channels = {      batterystatus:cordova.addWindowEventHandler("batterystatus"),      batterylow:cordova.addWindowEventHandler("batterylow"),      batterycritical:cordova.addWindowEventHandler("batterycritical")    };    for (var key in this.channels) {        this.channels[key].onHasSubscribersChange= Battery.onHasSubscribersChange;    }};function handlers() {    return battery.channels.batterystatus.numHandlers+        battery.channels.batterylow.numHandlers+        battery.channels.batterycritical.numHandlers;}/** * Event handlers for when callbacks getregistered for the battery. * Keep track of how many handlers wehave so we can start and stop the native battery listener * apPRopriately (and hopefully save onbattery life!). */Battery.onHasSubscribersChange = function() {  // If we just registered the firsthandler, make sure native listener is started.  if (this.numHandlers === 1 && handlers()=== 1) {      exec(battery._status, battery._error,"Battery", "start", []);  } else if (handlers() === 0) {      exec(null, null, "Battery","stop", []);  }};/** * Callback for battery status * * @param {Object} info            keys: level, isPlugged */Battery.prototype._status = function (info) {    if (info) {        if (battery._level !== info.level|| battery._isPlugged !== info.isPlugged) {                        if(info.level === null &&battery._level !== null) {                return; // special casewhere callback is called because we stopped listening to the native side.            }                        // Something changed. Firebatterystatus event            cordova.fireWindowEvent("batterystatus",info);            if (!info.isPlugged) { // donot fire low/critical if we are charging. issue: CB-4520                // note the following areNOT exact checks, as we want to catch a transition from                // above the threshold tobelow. issue: CB-4519                if (battery._level >STATUS_CRITICAL && info.level <= STATUS_CRITICAL) {                    // Fire criticalbattery event                    cordova.fireWindowEvent("batterycritical",info);                }                else if (battery._level >STATUS_LOW && info.level <= STATUS_LOW) {                    // Fire low battery event                    cordova.fireWindowEvent("batterylow",info);                }            }            battery._level = info.level;            battery._isPlugged = info.isPlugged;        }    }};/** * Error callback for battery start */Battery.prototype._error = function(e) {    console.log("Error initializingBattery: " + e);};var battery = new Battery(); // jshint ignore:linemodule.exports = battery;
 
其中，exec(battery._status, battery._error, "Battery","start", []);
用來讓插件的js代碼和硬件本地代碼聯系起來。
整個插件的工作原理是：
 
1． 定義自定義消息
2． 調用端增加對自定義消息的接聽（addEventListener）
3． 本地產生消息，通過exec函數指定的回調函數來響應。，然后在相應函數里，激發自定義消息。(fireWindowEvent)
4． 接聽者收到消息和參數
 
其中自定義消息部分是：
// Create newevent handlers on the window (returns a channel instance)    this.channels = {      batterystatus:cordova.addWindowEventHandler("batterystatus"),      batterylow:cordova.addWindowEventHandler("batterylow"),      batterycritical:cordova.addWindowEventHandler("batterycritical")    };    for (var key in this.channels) {        this.channels[key].onHasSubscribersChange= Battery.onHasSubscribersChange;    }};
 
這里共定義了三條消息。格式參考以上格式，
 
 
當有人注冊這個自定義消息的時候會響應以下函數：
/** * Event handlers for when callbacks getregistered for the battery. * Keep track of how many handlers wehave so we can start and stop the native battery listener * appropriately (and hopefully save onbattery life!). */Battery.onHasSubscribersChange = function() {  // If we just registered the firsthandler, make sure native listener is started.  if (this.numHandlers === 1 && handlers()=== 1) {      exec(battery._status, battery._error,"Battery", "start", []);  } else if (handlers() === 0) {      exec(null, null, "Battery","stop", []);  }};
在函數里執行exec函數來完成插件與本地原生代碼的連接。使用exec函數調用原生代碼，然后原始代碼返回結果，在這個例子中，原始代碼保持了返回數據的通道，使得，原生代碼一檢測到電量變化，就立馬使用這個保持的通道，來告知插件電量的變化，，然后插件激發自定義消息，。這樣用戶界面就會收到電量變化的事件。
 
本文僅提供思路與方法，具體定制自定義消息，需要深入閱讀并測試這個插件的源代碼。
插件源碼地址：https://github.com/apache/cordova-plugin-battery-status