racket-wiki/static/cmap/controller/cmap-history.js

/**
 * Undo/redo history for an editor whose state can be represented as JSON.
 *
 * The history owns snapshots, stack limits and the asynchronous commit boundary.
 * It does not know how a document is rendered or restored: those concerns are
 * supplied by the editor through the constructor callbacks.
 */
export class CmapHistory {
  /**
  * goal   : Create a history coordinator for one editor state.
  * pre    : snapshot and restore are functions for the same serialized state.
  * post   : The coordinator is ready to track state after reset() is called.
  * result : A CmapHistory instance with empty undo and redo stacks.
  * internals : The callbacks keep this controller independent of the editor's
  * model and view; the stacks contain only serialized snapshots.
  *
   * @param {object} options History callbacks and configuration.
   * @param {Function} options.snapshot Returns the current serialized state.
   * @param {Function} options.restore Restores one serialized state.
   * @param {Function} [options.onChange] Receives undo/redo availability changes.
   * @param {number} [options.limit=100] Maximum number of undo snapshots.
   */
  constructor({ snapshot, restore, onChange = null, limit = 100 }) {
    if (typeof snapshot !== "function") throw new TypeError("A snapshot function is required");
    if (typeof restore !== "function") throw new TypeError("A restore function is required");
    this.snapshot = snapshot;
    this.restoreDocument = restore;
    this.onChange = onChange;
    this.limit = Math.max(1, Number(limit) || 100);
    this.undoStack = [];
    this.redoStack = [];
    this.currentSnapshot = null;
    this.timer = null;
    this.ready = false;
    this.isRestoring = false;
  }

  /** Notify the host about the current availability of undo and redo. */
  notify() {
    if (this.onChange) {
      this.onChange({
        canUndo: this.canUndo(),
        canRedo: this.canRedo()
      });
    }
  }

  /**
   * goal   : Start a new history session at the current editor state.
   * pre    : The snapshot callback returns the current serialized state.
   * post   : Both stacks are empty and the current state is the history baseline.
   * result : Undefined; the host is notified of the empty stacks.
   * internals : A pending timer is cancelled before the baseline is captured.
   */
  reset() {
    this.cancelScheduledCommit();
    this.undoStack = [];
    this.redoStack = [];
    this.ready = true;
    this.currentSnapshot = this.snapshot();
    this.notify();
  }

  /** Cancel a pending asynchronous history commit. */
  cancelScheduledCommit() {
    if (this.timer !== null) {
      window.clearTimeout(this.timer);
      this.timer = null;
    }
  }

  /**
   * Schedule one commit for the current mutation transaction.
   * The zero-delay timer groups synchronous editor changes into one undo step.
   */
  scheduleCommit() {
    if (!this.ready || this.isRestoring) return;
    this.cancelScheduledCommit();
    this.timer = window.setTimeout(() => {
      this.timer = null;
      this.commit();
    }, 0);
  }

  /** Update the baseline after renderer-only normalization. */
  refreshSnapshot() {
    if (!this.ready || this.isRestoring || this.timer !== null) return;
    this.currentSnapshot = this.snapshot();
  }

  /**
   * Commit the current state when it differs from the baseline.
   * @returns {boolean} Whether a new undo step was recorded.
   */
  commit() {
    if (!this.ready || this.isRestoring) return false;
    this.cancelScheduledCommit();
    const nextSnapshot = this.snapshot();
    if (nextSnapshot === this.currentSnapshot) return false;
    if (this.currentSnapshot !== null) {
      this.undoStack.push(this.currentSnapshot);
      if (this.undoStack.length > this.limit) this.undoStack.shift();
    }
    this.currentSnapshot = nextSnapshot;
    this.redoStack = [];
    this.notify();
    return true;
  }

  /** Return whether an undo operation is available. */
  canUndo() {
    return this.undoStack.length > 0;
  }

  /** Return whether a redo operation is available. */
  canRedo() {
    return this.redoStack.length > 0;
  }

  get undoCount() {
    return this.undoStack.length;
  }

  get redoCount() {
    return this.redoStack.length;
  }

  /**
   * Restore one snapshot while suppressing history commits caused by loading.
   * The editor callback performs the actual model and view reconstruction.
   */
  restoreSnapshot(snapshot) {
    this.isRestoring = true;
    try {
      this.restoreDocument(snapshot);
    } finally {
      this.isRestoring = false;
    }
    this.currentSnapshot = snapshot;
    this.notify();
  }

  /** Restore the previous committed state, if one exists. */
  undo() {
    this.commit();
    if (!this.canUndo()) return false;
    this.redoStack.push(this.currentSnapshot);
    const snapshot = this.undoStack.pop();
    this.restoreSnapshot(snapshot);
    return true;
  }

  /** Restore the most recently undone state, if one exists. */
  redo() {
    this.commit();
    if (!this.canRedo()) return false;
    this.undoStack.push(this.currentSnapshot);
    const snapshot = this.redoStack.pop();
    this.restoreSnapshot(snapshot);
    return true;
  }

  /**
   * goal   : Replace the current document as one undoable operation.
   * pre    : replaceDocument performs the complete document replacement.
   * post   : The replacement is current and redo history has been discarded.
   * result : Undefined; the host receives the new undo/redo availability.
   * internals : The old baseline is pushed before the callback runs, while
   * isRestoring prevents loading callbacks from creating nested history steps.
   */
  replace(replaceDocument) {
    this.commit();
    const previousSnapshot = this.currentSnapshot || this.snapshot();
    this.isRestoring = true;
    try {
      replaceDocument();
    } finally {
      this.isRestoring = false;
    }
    const nextSnapshot = this.snapshot();
    if (previousSnapshot !== nextSnapshot) {
      this.undoStack.push(previousSnapshot);
      if (this.undoStack.length > this.limit) this.undoStack.shift();
    }
    this.currentSnapshot = nextSnapshot;
    this.redoStack = [];
    this.notify();
  }

  /** Release the timer when the owning editor is destroyed. */
  destroy() {
    this.cancelScheduledCommit();
  }
}