Option Strict On Option Explicit On Imports System Imports System.Collections.Generic Imports System.Diagnostics Namespace Nexamas.UI.TextInput ' CONTRACT: ' MASTextUndoManager owns undo/redo history only. ' ' It stores undoable operations as units: ' - Undo restores BeforeSnapshot. ' - Redo restores AfterSnapshot. ' ' It must not know UI, renderer, layout, keyboard, mouse, or Skia. ' It may only capture and restore MASTextInputState snapshots. Friend NotInheritable Class MASTextUndoManager Private ReadOnly _undoStack As New Stack(Of MASTextUndoUnit)() Private ReadOnly _redoStack As New Stack(Of MASTextUndoUnit)() Private Const HISTORY_LIMIT As Integer = 256 Private Const MERGE_TIMEOUT_MS As Integer = 900 Private _mergeOpen As Boolean Private _lastEditKind As MASTextEditKind = MASTextEditKind.Programmatic Private _lastEditTick As Integer Friend ReadOnly Property CanUndo As Boolean Get Return _undoStack.Count > 0 End Get End Property Friend ReadOnly Property CanRedo As Boolean Get Return _redoStack.Count > 0 End Get End Property Friend ReadOnly Property UndoCount As Integer Get Return _undoStack.Count End Get End Property Friend ReadOnly Property RedoCount As Integer Get Return _redoStack.Count End Get End Property Friend Sub BreakMerge() _mergeOpen = False _lastEditKind = MASTextEditKind.Programmatic _lastEditTick = 0 End Sub Friend Sub Clear() _undoStack.Clear() _redoStack.Clear() BreakMerge() End Sub Friend Function Execute(state As MASTextInputState, kind As MASTextEditKind, action As Func(Of Boolean)) As Boolean If state Is Nothing Then Return False If action Is Nothing Then Return False Dim before As MASTextInputSnapshot = CreateSnapshot(state) Dim beforeVersion As Integer = state.Version Dim changed As Boolean = action.Invoke() If Not changed Then Return False Dim afterSnap As MASTextInputSnapshot = CreateSnapshot(state) If before.Equals(afterSnap) Then Return False Debug.Assert(state.Version <> beforeVersion, "Undo contract failed: undoable text operation changed snapshot without changing State.Version.") Dim merged As Boolean = TryMerge(kind, before, afterSnap) If Not merged Then PushUndoUnit(New MASTextUndoUnit(kind, before, afterSnap)) End If _redoStack.Clear() _mergeOpen = CanContinueMerge(kind) _lastEditKind = kind _lastEditTick = Nexamas.UI.Timing.MASInteractionClock.TickCount() ValidateStacks() Return True End Function Friend Function Undo(state As MASTextInputState) As Boolean If state Is Nothing Then Return False If _undoStack.Count <= 0 Then Return False BreakMerge() Dim unit As MASTextUndoUnit = _undoStack.Pop() RestoreSnapshot(state, unit.BeforeSnapshot) _redoStack.Push(unit) ValidateStacks() Return True End Function Friend Function Redo(state As MASTextInputState) As Boolean If state Is Nothing Then Return False If _redoStack.Count <= 0 Then Return False BreakMerge() Dim unit As MASTextUndoUnit = _redoStack.Pop() RestoreSnapshot(state, unit.AfterSnapshot) _undoStack.Push(unit) ValidateStacks() Return True End Function Friend Function ExecuteProgrammatic(state As MASTextInputState, action As Func(Of Boolean)) As Boolean BreakMerge() Return Execute(state, MASTextEditKind.Programmatic, action) End Function ' Compatibility helper for older controller code. ' Prefer ExecuteProgrammatic for new code. Friend Sub PushProgrammaticSnapshot(state As MASTextInputState) If state Is Nothing Then Return BreakMerge() PushUndoUnit(New MASTextUndoUnit( kind:=MASTextEditKind.Programmatic, beforeSnapshot:=CreateSnapshot(state), afterSnapshot:=CreateSnapshot(state))) _redoStack.Clear() End Sub Private Function TryMerge(kind As MASTextEditKind, before As MASTextInputSnapshot, afterSnap As MASTextInputSnapshot) As Boolean If _undoStack.Count <= 0 Then Return False If Not ShouldMerge(kind, before, afterSnap) Then Return False Dim top As MASTextUndoUnit = _undoStack.Peek() top.AfterSnapshot = afterSnap top.LastMergedTick = Nexamas.UI.Timing.MASInteractionClock.TickCount() Return True End Function Private Function ShouldMerge(kind As MASTextEditKind, before As MASTextInputSnapshot, afterSnap As MASTextInputSnapshot) As Boolean If kind <> MASTextEditKind.Typing Then Return False If Not _mergeOpen Then Return False If _lastEditKind <> MASTextEditKind.Typing Then Return False If _undoStack.Count <= 0 Then Return False Dim nowTick As Integer = Nexamas.UI.Timing.MASInteractionClock.TickCount() Dim dt As Integer = nowTick - _lastEditTick If dt < 0 OrElse dt > MERGE_TIMEOUT_MS Then Return False Dim top As MASTextUndoUnit = _undoStack.Peek() If top.Kind <> MASTextEditKind.Typing Then Return False If top.AfterSnapshot Is Nothing Then Return False ' Merge only continuous simple typing. ' Selection replacement, paste, delete, and caret jumps intentionally break merge. If before.SelectionLength <> 0 Then Return False If top.AfterSnapshot.SelectionLength <> 0 Then Return False If Not top.AfterSnapshot.Equals(before) Then Return False If afterSnap.Text.Length <= before.Text.Length Then Return False If afterSnap.CaretIndex < before.CaretIndex Then Return False Return True End Function Private Shared Function CanContinueMerge(kind As MASTextEditKind) As Boolean Return kind = MASTextEditKind.Typing End Function Private Sub PushUndoUnit(unit As MASTextUndoUnit) If unit Is Nothing Then Return If unit.BeforeSnapshot.Equals(unit.AfterSnapshot) Then Return If _undoStack.Count > 0 Then Dim top As MASTextUndoUnit = _undoStack.Peek() If top.BeforeSnapshot.Equals(unit.BeforeSnapshot) AndAlso top.AfterSnapshot.Equals(unit.AfterSnapshot) Then Return End If End If _undoStack.Push(unit) TrimUndoStackIfNeeded() End Sub Private Sub TrimUndoStackIfNeeded() If _undoStack.Count <= HISTORY_LIMIT Then Return Dim arr As MASTextUndoUnit() = _undoStack.ToArray() _undoStack.Clear() Dim keep As Integer = Math.Min(HISTORY_LIMIT, arr.Length) For i As Integer = keep - 1 To 0 Step -1 _undoStack.Push(arr(i)) Next End Sub Private Shared Function CreateSnapshot(state As MASTextInputState) As MASTextInputSnapshot Return New MASTextInputSnapshot( text:=state.Text, caretIndex:=state.CaretIndex, selectionStart:=state.SelectionStart, selectionLength:=state.SelectionLength, selectionAnchor:=state.SelectionAnchor) End Function Private Shared Sub RestoreSnapshot(state As MASTextInputState, snapshot As MASTextInputSnapshot) If state Is Nothing OrElse snapshot Is Nothing Then Return state.ApplySnapshot(snapshot) End Sub Private Sub ValidateStacks() Debug.Assert(_undoStack.Count <= HISTORY_LIMIT, "Undo contract failed: undo stack exceeded history limit.") For Each unit As MASTextUndoUnit In _undoStack Debug.Assert(unit IsNot Nothing, "Undo contract failed: undo unit is Nothing.") If unit Is Nothing Then Continue For Debug.Assert(unit.BeforeSnapshot IsNot Nothing, "Undo contract failed: before snapshot is Nothing.") Debug.Assert(unit.AfterSnapshot IsNot Nothing, "Undo contract failed: after snapshot is Nothing.") Debug.Assert(Not unit.BeforeSnapshot.Equals(unit.AfterSnapshot), "Undo contract failed: no-op undo unit was stored.") Next End Sub End Class End Namespace