# Undo/Redo Task Pattern in Mailspring

This document explains how to implement undoable tasks in Mailspring following the established patterns.

## Overview

Mailspring uses a task-based architecture where operations are represented as `Task` objects that get queued and executed by the sync engine. The `UndoRedoStore` automatically tracks tasks that can be undone and provides undo/redo functionality.

## How It Works

### Automatic Registration

When a task is queued via `Actions.queueTask()`, the `UndoRedoStore._onQueue()` listener checks if the task has `canBeUndone = true`. If so, it automatically registers the task for undo:

```typescript
// In UndoRedoStore
_onQueue = (taskOrTasks: Task | Task[]) => {
  const tasks = taskOrTasks instanceof Array ? taskOrTasks : [taskOrTasks];

  if (tasks.every(t => t.canBeUndone)) {
    const block = {
      tasks: tasks,
      description: tasks.map(t => t.description()).join(', '),
      undo: () => {
        Actions.queueTasks(tasks.map(t => t.createUndoTask()));
      },
      redo: () => {
        Actions.queueTasks(tasks.map(t => t.createIdenticalTask()));
      },
    };
    this._onQueueBlock(block);
  }
};
```

### Task Requirements

For a task to be undoable, it must:

1. Have `canBeUndone` return `true`
2. Implement `createUndoTask()` that returns a task to reverse the operation
3. Implement `description()` for the undo toast message

## Patterns

### Pattern 1: Simple State Toggle (ChangeStarredTask)

For operations that toggle a boolean state:

```typescript
export class ChangeStarredTask extends ChangeMailTask {
  starred: boolean;

  // Inherited from ChangeMailTask: canBeUndone defaults to true

  createUndoTask() {
    const task = super.createUndoTask();
    task.starred = !this.starred;  // Simply flip the flag
    return task;
  }
}
```

### Pattern 2: State Snapshot (SyncbackMetadataTask, SyncbackEventTask)

For operations where the "inverse" isn't a simple toggle, store the original state:

```typescript
interface EventSnapshot {
  ics: string;
  recurrenceStart: number;
  recurrenceEnd: number;
}

export class SyncbackEventTask extends Task {
  event: Event;
  undoData?: EventSnapshot;  // Original state before modification
  taskDescription?: string;

  static forUpdating({ event, undoData, description }) {
    return new SyncbackEventTask({
      event,
      calendarId: event.calendarId,
      accountId: event.accountId,
      undoData,
      taskDescription: description,
    });
  }

  get canBeUndone(): boolean {
    return !!this.undoData;  // Only undoable if we have original state
  }

  description(): string | null {
    return this.taskDescription || null;
  }

  createUndoTask(): SyncbackEventTask {
    // Restore the original state
    const restoredEvent = this.event.clone();
    restoredEvent.ics = this.undoData.ics;
    restoredEvent.recurrenceStart = this.undoData.recurrenceStart;
    restoredEvent.recurrenceEnd = this.undoData.recurrenceEnd;

    // The undo task stores current state so redo works
    const undoTaskUndoData: EventSnapshot = {
      ics: this.event.ics,
      recurrenceStart: this.event.recurrenceStart,
      recurrenceEnd: this.event.recurrenceEnd,
    };

    return new SyncbackEventTask({
      event: restoredEvent,
      calendarId: this.calendarId,
      accountId: this.accountId,
      undoData: undoTaskUndoData,
      taskDescription: localized('Undo %@', this.taskDescription),
    });
  }
}
```

## Implementation Steps

### Step 1: Add Undo Data Attributes

Add attributes to store the original state:

```typescript
static attributes = {
  ...Task.attributes,

  undoData: Attributes.Obj({
    modelKey: 'undoData',
  }),
  taskDescription: Attributes.String({
    modelKey: 'taskDescription',
  }),
};
```

### Step 2: Capture State Before Modification

Before modifying the model, capture its current state:

```typescript
function modifyEvent(event: Event, newData: EventData) {
  // Capture BEFORE modifying
  const undoData = {
    ics: event.ics,
    recurrenceStart: event.recurrenceStart,
    recurrenceEnd: event.recurrenceEnd,
  };

  // Now modify
  event.ics = newData.ics;
  event.recurrenceStart = newData.start;
  event.recurrenceEnd = newData.end;

  // Queue with undo support
  Actions.queueTask(SyncbackEventTask.forUpdating({
    event,
    undoData,
    description: localized('Edit event'),
  }));
}
```

### Step 3: Implement canBeUndone and createUndoTask

```typescript
get canBeUndone(): boolean {
  return !!this.undoData;
}

createUndoTask(): YourTask {
  if (!this.undoData) {
    throw new Error('Cannot create undo task without undoData');
  }

  // Create task that restores original state
  // Store current state in the undo task's undoData for redo
}
```

## Anti-Patterns to Avoid

### Don't Use Custom UndoBlock

❌ **Wrong**: Manually registering undo callbacks

```typescript
// DON'T DO THIS
const undoBlock = {
  description: 'My action',
  undo: async () => { /* custom undo logic */ },
  redo: async () => { /* custom redo logic */ },
};
UndoRedoStore.queueUndoBlock(undoBlock);
```

✅ **Correct**: Use task-based undo

```typescript
// DO THIS
Actions.queueTask(MyTask.forUpdating({
  model,
  undoData: captureCurrentState(model),
  description: 'My action',
}));
```

### Don't Forget to Capture State First

❌ **Wrong**: Capturing after modification

```typescript
event.ics = newIcs;  // Modified first!
const undoData = { ics: event.ics };  // Too late - this is the new state
```

✅ **Correct**: Capture before modification

```typescript
const undoData = { ics: event.ics };  // Capture original
event.ics = newIcs;  // Now modify
```

## Testing Undo

1. Perform the action
2. Press Cmd/Ctrl+Z to undo
3. Verify the model is restored to original state
4. Press Cmd/Ctrl+Shift+Z to redo
5. Verify the model has the modified state again

## Related Files

- `app/src/flux/stores/undo-redo-store.ts` - UndoRedoStore implementation
- `app/src/flux/tasks/task.ts` - Base Task class
- `app/src/flux/tasks/change-mail-task.ts` - Base class for mail changes
- `app/src/flux/tasks/syncback-metadata-task.ts` - Example of snapshot pattern
- `app/src/flux/tasks/syncback-event-task.ts` - Calendar event undo implementation