/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

const EXPORTED_SYMBOLS = ["SessionStoreManager"];

const { JSONFile } = ChromeUtils.importESModule(
  "resource://gre/modules/JSONFile.sys.mjs"
);

/**
 * asuth arbitrarily chose this value to trade-off powersaving,
 * processor usage, and recency of state in the face of the impossibility of
 * our crashing; he also worded this.
 */
var SESSION_AUTO_SAVE_DEFAULT_MS = 300000; // 5 minutes

var SessionStoreManager = {
  _initialized: false,

  /**
   * Session restored successfully on startup; use this to test for an early
   * failed startup which does not restore user tab state to ensure a session
   * save on close will not overwrite the last good session state.
   */
  _restored: false,

  _sessionAutoSaveTimer: null,

  _sessionAutoSaveTimerIntervalMS: SESSION_AUTO_SAVE_DEFAULT_MS,

  /**
   * The persisted state of the previous session. This is resurrected
   * from disk when the module is initialized and cleared when all
   * required windows have been restored.
   */
  _initialState: null,

  /**
   * A flag indicating whether the state "just before shutdown" of the current
   * session has been persisted to disk. See |observe| and |unloadingWindow|
   * for justification on why we need this.
   */
  _shutdownStateSaved: false,

  /**
   * The JSONFile store object.
   */
  get store() {
    if (this._store) {
      return this._store;
    }

    return (this._store = new JSONFile({
      path: this.sessionFile.path,
      backupTo: this.sessionFile.path + ".backup",
    }));
  },

  /**
   * Gets the nsIFile used for session storage.
   */
  get sessionFile() {
    let sessionFile = Services.dirsvc.get("ProfD", Ci.nsIFile);
    sessionFile.append("session.json");
    return sessionFile;
  },

  /**
   * This is called on startup, and when a new 3 pane window is opened after
   * the last 3 pane window was closed (e.g., on the mac, closing the last
   * window doesn't shut down the app).
   */
  async _init() {
    await this._loadSessionFile();

    // we listen for "quit-application-granted" instead of
    // "quit-application-requested" because other observers of the
    // latter can cancel the shutdown.
    Services.obs.addObserver(this, "quit-application-granted");

    this.startPeriodicSave();

    this._initialized = true;
  },

  /**
   * Loads the session file into _initialState. This should only be called by
   * _init and a unit test.
   */
  async _loadSessionFile() {
    if (!this.sessionFile.exists()) {
      return;
    }

    // Read the session state data from file, asynchronously.
    // An error on the json file returns an empty object which corresponds
    // to a null |_initialState|.
    await this.store.load();
    this._initialState =
      this.store.data.toSource() == {}.toSource() ? null : this.store.data;
  },

  /**
   * Opens the windows that were open in the previous session.
   */
  _openOtherRequiredWindows(aWindow) {
    // XXX we might want to display a restore page and let the user decide
    // whether to restore the other windows, just like Firefox does.

    if (!this._initialState || !this._initialState.windows || !aWindow) {
      return;
    }

    for (var i = 0; i < this._initialState.windows.length; ++i) {
      aWindow.open(
        "chrome://messenger/content/messenger.xhtml",
        "_blank",
        "chrome,extrachrome,menubar,resizable,scrollbars,status,toolbar"
      );
    }
  },

  /**
   * Writes the state object to disk.
   */
  _saveStateObject(aStateObj) {
    if (!this.store) {
      console.error(
        "SessionStoreManager: could not create data store from file"
      );
      return;
    }

    let currentStateString = JSON.stringify(aStateObj);
    let storedStateString =
      this.store.dataReady && this.store.data
        ? JSON.stringify(this.store.data)
        : null;

    // Do not save state (overwrite last good state) in case of a failed startup.
    // Write async to disk only if state changed since last write.
    if (!this._restored || currentStateString == storedStateString) {
      return;
    }

    this.store.data = aStateObj;
    this.store.saveSoon();
  },

  /**
   * @returns an empty state object that can be populated with window states.
   */
  _createStateObject() {
    return {
      rev: 0,
      windows: [],
    };
  },

  /**
   * Writes the state of all currently open 3pane windows to disk.
   */
  _saveState() {
    let state = this._createStateObject();

    // XXX we'd like to support other window types in future, but for now
    // only get the 3pane windows.
    for (let win of Services.wm.getEnumerator("mail:3pane")) {
      if (
        win &&
        "complete" == win.document.readyState &&
        win.getWindowStateForSessionPersistence
      ) {
        state.windows.push(win.getWindowStateForSessionPersistence());
      }
    }

    this._saveStateObject(state);
  },

  // Timer Callback
  _sessionAutoSaveTimerCallback() {
    SessionStoreManager._saveState();
  },

  // Observer Notification Handler
  observe(aSubject, aTopic, aData) {
    switch (aTopic) {
      // This is observed before any windows start unloading if something other
      // than the last 3pane window closing requested the application be
      // shutdown. For example, when the user quits via the file menu.
      case "quit-application-granted":
        if (!this._shutdownStateSaved) {
          this.stopPeriodicSave();
          this._saveState();

          // this is to ensure we don't clobber the saved state when the
          // 3pane windows unload.
          this._shutdownStateSaved = true;
        }
        break;
    }
  },

  // Public API

  /**
   * Called by each 3pane window instance when it loads.
   *
   * @returns a window state object if aWindow was opened as a result of a
   *         session restoration, null otherwise.
   */
  async loadingWindow(aWindow) {
    let firstWindow = !this._initialized || this._shutdownStateSaved;
    if (firstWindow) {
      await this._init();
    }

    // If we are seeing a new 3-pane, we are obviously not in a shutdown
    // state anymore.  (This would happen if all the 3panes got closed but
    // we did not quit because another window was open and then a 3pane showed
    // up again.  This can happen in both unit tests and real life.)
    // We treat this case like the first window case, and do a session restore.
    this._shutdownStateSaved = false;

    let windowState = null;
    if (this._initialState && this._initialState.windows) {
      windowState = this._initialState.windows.pop();
      if (0 == this._initialState.windows.length) {
        this._initialState = null;
      }
    }

    if (firstWindow) {
      this._openOtherRequiredWindows(aWindow);
    }

    return windowState;
  },

  /**
   * Called by each 3pane window instance when it unloads. If aWindow is the
   * last 3pane window, its state is persisted. The last 3pane window unloads
   * first before the "quit-application-granted" event is generated.
   */
  unloadingWindow(aWindow) {
    if (!this._shutdownStateSaved) {
      // determine whether aWindow is the last open window
      let lastWindow = true;
      for (let win of Services.wm.getEnumerator("mail:3pane")) {
        if (win != aWindow) {
          lastWindow = false;
        }
      }

      if (lastWindow) {
        // last chance to save any state for the current session since
        // aWindow is the last 3pane window and the "quit-application-granted"
        // event is observed AFTER this.
        this.stopPeriodicSave();

        let state = this._createStateObject();
        state.windows.push(aWindow.getWindowStateForSessionPersistence());
        this._saveStateObject(state);

        // XXX this is to ensure we don't clobber the saved state when we
        // observe the "quit-application-granted" event.
        this._shutdownStateSaved = true;
      }
    }
  },

  /**
   * Stops periodic session persistence.
   */
  stopPeriodicSave() {
    if (this._sessionAutoSaveTimer) {
      this._sessionAutoSaveTimer.cancel();

      delete this._sessionAutoSaveTimer;
      this._sessionAutoSaveTimer = null;
    }
  },

  /**
   * Starts periodic session persistence.
   */
  startPeriodicSave() {
    if (!this._sessionAutoSaveTimer) {
      this._sessionAutoSaveTimer = Cc["@mozilla.org/timer;1"].createInstance(
        Ci.nsITimer
      );

      this._sessionAutoSaveTimer.initWithCallback(
        this._sessionAutoSaveTimerCallback,
        this._sessionAutoSaveTimerIntervalMS,
        Ci.nsITimer.TYPE_REPEATING_SLACK
      );
    }
  },
};