Appium源码分析(5)-请求是如何执行的

在上一篇文章中，我将queue理解为了队列，但其实在nodejs中，这只是业务名词，代表的是流程控制的一种方式，使用queue方式时，事件按照先进先出的准则执行。

queue

我们先了解一下事件的集中处理方式，参考文章。从这篇文章可以知道，nodejs提供了一个Async库，该库就是用来处理事件的。里面就有这一个queue。
在上一篇文章我们讲到了initQueue,我们这次再来看看：

Android.prototype.initQueue = function () {  this.queue = async.queue(function (task, cb) {    var action = task.action,        params = task.params;    this.cbForCurrentCmd = cb;    if (this.adb && !this.shuttingDown) {      this.uiautomator.sendAction(action, params, function (response) {        this.cbForCurrentCmd = null;        if (typeof cb === 'function') {          this.respond(response, cb);        }      }.bind(this));    } else {      this.cbForCurrentCmd = null;      var msg = "Tried to send command to non-existent Android device, " +                 "maybe it shut down?";      if (this.shuttingDown) {        msg = "We're in the middle of shutting down the Android device, " +              "so your request won't be executed. Sorry!";      }      this.respond({        status: status.codes.UnknownError.code      , value: msg      }, cb);    }  }.bind(this), 1);};

Async库中queue的定义一般如下所示：

var q = async.queue(function(task, callback) {    log(‘worker is processing task: ‘, task.name);    task.run(callback);}, 1);

上面定义了一个工作线程为1的队列。当队列正在处理事件时，其他事件就一直等待。所以当我们使用queue的push方法时，queue会自己唤起然后执行，如果正在执行，push添加进来的事件会等到下一次执行。

uiautomator.sendAction

上面的请求，最终调用的是devices/android/uiautomator的sendAction的方法:

UiAutomator.prototype.sendAction = function (action, params, cb) {  if (typeof params === "function") {    cb = params;    params = {};  }  var extra = {action: action, params: params};  this.sendCommand('action', extra, cb);};

然后转向sendCommand方法

uiautomator.sendCommand

UiAutomator.prototype.sendCommand = function (type, extra, cb) {  if (this.cmdCb !== null) {    logger.warn("Trying to run a command when one is already in progress. " +                "Will spin a bit and try again");    var start = Date.now();    var timeoutMs = 10000;    var intMs = 200;    var waitForCmdCbNull = function () {      if (this.cmdCb === null) {        this.sendCommand(type, extra, cb);      } else if ((Date.now() - start) < timeoutMs) {        setTimeout(waitForCmdCbNull, intMs);      } else {        cb(new Error("Never became able to push strings since a command " +                     "was in process"));      }    }.bind(this);    waitForCmdCbNull();  } else if (this.socketClient) {    this.resendLastCommand = function () {      this.sendCommand(type, extra, cb);    }.bind(this);    if (typeof extra === "undefined" || extra === null) {      extra = {};    }    var cmd = {cmd: type};    cmd = _.extend(cmd, extra);    var cmdJson = JSON.stringify(cmd) + "\n";    this.cmdCb = cb;    var logCmd = cmdJson.trim();    if (logCmd.length > 1000) {      logCmd = logCmd.substr(0, 1000) + "...";    }    this.debug("Sending command to android: " + logCmd);    this.socketClient.write(cmdJson);  } else {    cb({      status: status.codes.UnknownError.code    , value: "Tried to send command to non-existent Android socket, " +               "maybe it's shutting down?"    });  }};

最终会向设备端的socket发送一条命令，关于设备端接受命令以及如何处理，请查看我之前写的关于appium中bootstrap源码分析相关文章。

Done!