[Android]PhoneGap源码分析——CallbackServer异步回调

PhoneGap的js回调有几种实现方式。其中一种是ajax。

我们先来看一下js端相关代码：

// file: lib/android/plugin/android/callback.jsdefine("cordova/plugin/android/callback", function(require, exports, module) {var port = null,    token = null,    xmlhttp;function startXhr() {    // cordova/exec depends on this module, so we can't require cordova/exec on the module level.    var exec = require('cordova/exec'),    xmlhttp = new XMLHttpRequest();    // Callback function when XMLHttpRequest is ready    xmlhttp.onreadystatechange=function(){        if (!xmlhttp) {            return;        }        if (xmlhttp.readyState === 4){            // If callback has JavaScript statement to execute            if (xmlhttp.status === 200) {                // Need to url decode the response                var msg = decodeURIComponent(xmlhttp.responseText);                setTimeout(startXhr, 1);                exec.processMessages(msg);            }            // If callback ping (used to keep XHR request from timing out)            else if (xmlhttp.status === 404) {                setTimeout(startXhr, 10);            }            // 0 == Page is unloading.            // 400 == Bad request.            // 403 == invalid token.            // 503 == server stopped.            else {                console.log("JSCallback Error: Request failed with status " + xmlhttp.status);                exec.setNativeToJsBridgeMode(exec.nativeToJsModes.POLLING);            }        }    };    if (port === null) {        port = prompt("getPort", "gap_callbackServer:");    }    if (token === null) {        token = prompt("getToken", "gap_callbackServer:");    }    xmlhttp.open("GET", "http://127.0.0.1:"+port+"/"+token , true);    xmlhttp.send();}module.exports = {    start: function() {        startXhr();    },    stop: function() {        if (xmlhttp) {            var tmp = xmlhttp;            xmlhttp = null;            tmp.abort();        }    },    isAvailable: function() {        return ("true" != prompt("usePolling", "gap_callbackServer:"));    }};});

主要的处理是startXhr函数。它向java的server端发起了ajax请求，在onreadystatechange这个回调函数中等待server端返回结果。如果server端返回的结果正确，则再通过setTimeout(startXhr, 1)函数，1豪秒后再重新向server端发起ajax请求。如果从server返回的结果状态是404，则每隔10豪秒，重新向server端发起ajax请求。

接下来看看server端是如何处理的。server端的处理代码是CallbackServer.java中的run()方法中。

先看server端的源码：

它通过serverSocket来模拟http的server端。其中，jsMessageQueue中是从java端发送的js消息，serverSocket在接收到客户端（js端）的请求后，会将jsMessageQueue中的js代码发送到客户端（js端）。

    /**     * Start running the server.       * This is called automatically when the server thread is started.     */    public void run() {        // Start server        try {            this.active = true;            String request;            waitSocket = new ServerSocket(0);            this.port = waitSocket.getLocalPort();            //Log.d(LOG_TAG, "CallbackServer -- using port " +this.port);            this.token = java.util.UUID.randomUUID().toString();            //Log.d(LOG_TAG, "CallbackServer -- using token "+this.token);            while (this.active) {                //Log.d(LOG_TAG, "CallbackServer: Waiting for data on socket");                Socket connection = waitSocket.accept();                BufferedReader xhrReader = new BufferedReader(new InputStreamReader(connection.getInputStream()), 40);                DataOutputStream output = new DataOutputStream(connection.getOutputStream());                request = xhrReader.readLine();                String response = "";                //Log.d(LOG_TAG, "CallbackServerRequest="+request);                if (this.active && (request != null)) {                    if (request.contains("GET")) {                        // Get requested file                        String[] requestParts = request.split(" ");                        // Must have security token                        if ((requestParts.length == 3) && (requestParts[1].substring(1).equals(this.token))) {                            //Log.d(LOG_TAG, "CallbackServer -- Processing GET request");                        String payload = null;                            // Wait until there is some data to send, or send empty data every 10 sec                             // to prevent XHR timeout on the client                             while (this.active) {                            if (jsMessageQueue != null) {                            payload = jsMessageQueue.popAndEncode();                                if (payload != null) {                                break;                                }                            }                            synchronized (this) {                                    try {                                        this.wait(10000); // prevent timeout from happening                                        //Log.d(LOG_TAG, "CallbackServer>>> break <<<");                                        break;                                    } catch (Exception e) {                                    }                                }                            }                            // If server is still running                            if (this.active) {                                // If no data, then send 404 back to client before it times out                                if (payload == null) {                                    //Log.d(LOG_TAG, "CallbackServer -- sending data 0");                                    response = "HTTP/1.1 404 NO DATA\r\n\r\n "; // need to send content otherwise some Android devices fail, so send space                                }                                else {                                    //Log.d(LOG_TAG, "CallbackServer -- sending item");                                    response = "HTTP/1.1 200 OK\r\n\r\n";                                    response += encode(payload, "UTF-8");                                }                            }                            else {                                response = "HTTP/1.1 503 Service Unavailable\r\n\r\n ";                            }                        }                        else {                            response = "HTTP/1.1 403 Forbidden\r\n\r\n ";                        }                    }                    else {                        response = "HTTP/1.1 400 Bad Request\r\n\r\n ";                    }                    //Log.d(LOG_TAG, "CallbackServer: response="+response);                    //Log.d(LOG_TAG, "CallbackServer: closing output");                    output.writeBytes(response);                    output.flush();                }                output.close();                xhrReader.close();            }        } catch (IOException e) {            e.printStackTrace();        }        this.active = false;        //Log.d(LOG_TAG, "CallbackServer.startServer() - EXIT");    }

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