(FFOS Gecko & Gaia) OTA

　　UpdatePrompt接收Gaia返回的'update-available-result'消息，进行下一步处理。

1. UpdatePrompt.handleEvent

  handleEvent: function UP_handleEvent(evt) {
    if (evt.type !== "mozContentEvent") {
      return;
    }

    let detail = evt.detail;
    if (!detail) {
      return;
    }

    switch (detail.type) {
      case "force-update-check":
        this.forceUpdateCheck();
        break;
      case "update-available-result":
        this.handleAvailableResult(detail);
        // If we started the apply prompt timer, this means that we're waiting
        // for the user to press Later or Install Now. In this situation we
        // don't want to clear this._update, becuase handleApplyPromptResult
        // needs it.
        if (this._applyPromptTimer == null && !this._waitingForIdle) {
          this._update = null;
        }
        break;
      case "update-download-cancel":
        this.handleDownloadCancel();
        break;
      case "update-prompt-apply-result":
        this.handleApplyPromptResult(detail);
        break;
    }
  },

2. UpdatePrompt.handleAvailableResult

  handleAvailableResult: function UP_handleAvailableResult(aDetail) {
    // If the user doesn't choose "download", the updater will implicitly call
    // showUpdateAvailable again after a certain period of time
    switch (aDetail.result) {
      case "download":
　　　　// 这个this._update就是从之前分析过的Checker返回数据中得到的一个Update对象。
        this.downloadUpdate(this._update);
        break;
    }
  },

3. UpdatePrompt.downloadUpdate

  downloadUpdate: function UP_downloadUpdate(aUpdate) {
    if (!aUpdate) {
      aUpdate = Services.um.activeUpdate;
      if (!aUpdate) {
        log("No active update found to download");
        return;
      }
    }

　　// 调用UpdateService的downloadUpdate方法，true表示background
    let status = Services.aus.downloadUpdate(aUpdate, true);
    if (status == STATE_DOWNLOADING) {
　　　　// 为UpdateService设置DownloadListener
      Services.aus.addDownloadListener(this);
      return;
    }

    // If the update has already been downloaded and applied, then
    // Services.aus.downloadUpdate will return immediately and not
    // call showUpdateDownloaded, so we detect this.
    if (aUpdate.state == "applied" && aUpdate.errorCode == 0) {
      this.showUpdateDownloaded(aUpdate, true);
      return;
    }

    log("Error downloading update " + aUpdate.name + ": " + aUpdate.errorCode);
    let errorCode = aUpdate.errorCode >>> 0;
    if (errorCode == Cr.NS_ERROR_FILE_TOO_BIG) {
      aUpdate.statusText = "file-too-big";
    }
    this.showUpdateError(aUpdate);
  },