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!