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http://greasemonkey.devjavu.com/browser/trunk/src/chrome/chromeFiles/content/xmlhttprequester.js

Constellation が教えてくれたけど GM_xmlhttpRequest ってたかだか118行しかないんだな。しかもコメントたくさんあるし。

function GM_xmlhttpRequester(unsafeContentWin, chromeWindow) {
  this.unsafeContentWin = unsafeContentWin;
  this.chromeWindow = chromeWindow;
}

// this function gets called by user scripts in content security scope to
// start a cross-domain xmlhttp request.
//
// details should look like:
// {method,url,onload,onerror,onreadystatechange,headers,data}
// headers should be in the form {name:value,name:value,etc}
// can't support mimetype because i think it's only used for forcing
// text/xml and we can't support that
GM_xmlhttpRequester.prototype.contentStartRequest = function(details) {
  if (!GM_apiLeakCheck("GM_xmlhttpRequest")) {
    return;
  }

  // don't actually need the timer functionality, but this pops it
  // out into chromeWindow's thread so that we get that security
  // context.
  GM_log("> GM_xmlhttpRequest.contentStartRequest");

  // important to store this locally so that content cannot trick us up with
  // a fancy getter that checks the number of times it has been accessed,
  // returning a dangerous URL the time that we actually use it.
  var url = details.url;

  // make sure that we have an actual string so that we can't be fooled with
  // tricky toString() implementations.
  if (typeof url != "string") {
    throw new Error("Invalid url: url must be of type string");
  }

  var ioService = Components.classes["@mozilla.org/network/io-service;1"]
                  .getService(Components.interfaces.nsIIOService);
  var scheme = ioService.extractScheme(url);

  // This is important - without it, GM_xmlhttpRequest can be used to get
  // access to things like files and chrome. Careful.
  switch (scheme) {
    case "http":
    case "https":
    case "ftp":
      this.chromeWindow.setTimeout(
        GM_hitch(this, "chromeStartRequest", url, details), 0);
      break;
    default:
      throw new Error("Invalid url: " + url);
  }

  GM_log("< GM_xmlhttpRequest.contentStartRequest");
};

// this function is intended to be called in chrome's security context, so
// that it can access other domains without security warning
GM_xmlhttpRequester.prototype.chromeStartRequest = function(safeUrl, details) {
  GM_log("> GM_xmlhttpRequest.chromeStartRequest");
  var req = new this.chromeWindow.XMLHttpRequest();

  this.setupRequestEvent(this.unsafeContentWin, req, "onload", details);
  this.setupRequestEvent(this.unsafeContentWin, req, "onerror", details);
  this.setupRequestEvent(this.unsafeContentWin, req, "onreadystatechange",
                         details);

  req.open(details.method, safeUrl);

  if (details.overrideMimeType) {
    req.overrideMimeType(details.overrideMimeType);
  }

  if (details.headers) {
    for (var prop in details.headers) {
      req.setRequestHeader(prop, details.headers[prop]);
    }
  }

  req.send((details.data) ? details.data : null);
  GM_log("< GM_xmlhttpRequest.chromeStartRequest");
}

// arranges for the specified 'event' on xmlhttprequest 'req' to call the
// method by the same name which is a property of 'details' in the content
// window's security context.
GM_xmlhttpRequester.prototype.setupRequestEvent =
function(unsafeContentWin, req, event, details) {
  GM_log("> GM_xmlhttpRequester.setupRequestEvent");

  if (details[event]) {
    req[event] = function() {
      GM_log("> GM_xmlhttpRequester -- callback for " + event);

      var responseState = {
        // can't support responseXML because security won't
        // let the browser call properties on it
        responseText:req.responseText,
        readyState:req.readyState,
        responseHeaders:(req.readyState == 4 ?
                         req.getAllResponseHeaders() :
                         ""),
        status:(req.readyState == 4 ? req.status : 0),
        statusText:(req.readyState == 4 ? req.statusText : ""),
        finalUrl:(req.readyState == 4 ? req.channel.URI.spec : "")
      }

      // Pop back onto browser thread and call event handler.
      // Have to use nested function here instead of GM_hitch because
      // otherwise details[event].apply can point to window.setTimeout, which
      // can be abused to get increased priveledges.
      new XPCNativeWrapper(unsafeContentWin, "setTimeout()")
        .setTimeout(function(){details[event](responseState);}, 0);

      GM_log("< GM_xmlhttpRequester -- callback for " + event);
    }
  }

  GM_log("< GM_xmlhttpRequester.setupRequestEvent");
};

GM的にはdetail.methodをそのままopenしてるだけみたい

http://d.hatena.ne.jp/taizooo/20090308#c1236441932