I have a TextBox which I would like to implement undo/redo functionality for. I have read that it might have some slight undo functionality already, but that it is buggy? Anyways, I would like to implement both undo and redo functionality also just to learn how you would go ahead and do that.
I have read about the Memento Pattern and looked some on a Generic Undo/Redo example on CodeProject. And the pattern kiiind of makes sense. I just can't seem to wrap my head around how to implement it. And how to do it effeciently for the contents of a TextBox.
Of course I could just store textbox.Text when TextChanges, but that would hug up quite a lot of memory pretty fast, especially if the TextBox contained a lot of text.
So anyways, I'm looking for some advice on how to implement a good, clear and efficient way of implementing this functionality. Both in general and especially for a TextBox c",)
The .NET System.ComponentModel namespace comes with an IEditableObject interface, you could also use INotifyPropertyChanging and INotifyPropertyChanged. MVC Pattern would also make it that your interface responds to changes in the model through events thus updating or restoring the value of your textbox.
Effectively the Memento Pattern.
Have you had a look into these? Here is a how to.
A simple and quicker version would be to store the state of the textbox OnTextChanged. Each undo would return the last event in an Array. The C# Stack Type would be handy here. You could clear the state once you are off the interface also or after Apply.
Here's a way to achieve it with minimal code: (This is the code behind of a win form with a single textbox on it)
public partial class Form1 : Form
{
Stack<Func<object>> undoStack = new Stack<Func<object>>();
public Form1()
{
InitializeComponent();
}
private void textBox_KeyDown(object sender, KeyEventArgs e)
{
if (e.KeyCode == Keys.U && Control.ModifierKeys == Keys.Control && undoStack.Count > 0)
undoStack.Pop()();
}
private void textBox_KeyPress(object sender, KeyPressEventArgs e)
{
if (e.KeyChar != 'u' || Control.ModifierKeys != Keys.Control)
{
var textBox = (TextBox)sender;
undoStack.Push(textBox.Text(textBox.Text));
}
}
}
public static class Extensions
{
public static Func<TextBox> Text(this TextBox textBox, string text)
{
return () => { textBox.Text = text; return textBox; };
}
}
By implementing an extension method for other input types the undoStack can service the whole of your UI, undoing all UI actions in order.
A good solution can be found here:
Add Undo/Redo or Back/Forward Functionality to your Application
Undo/Redo Capable TextBox (winforms)
The code is in VB.NET, but you can easily convert it to C# without much efforts. Online converters are also available.
This is the most helpful page I found on the topic, more generic, suitable for different object types on the undo/redo stack.
When I got to implementing it, I was surprised how simple and elegant it ended up being. That makes it a win for me.
I need to reset the selection, too, into its original positions when undoing / redoing. Watch "class Extensions", at the bottom of my just basic and well working code, for a form with just one textbox "textBox1" to try:
public partial class Form1 : Form
{
Stack<Func<object>> undoStack = new Stack<Func<object>>();
Stack<Func<object>> redoStack = new Stack<Func<object>>();
public Form1()
{
InitializeComponent();
textBox1.KeyDown += TextBox1_KeyDown;
}
private void TextBox1_KeyDown(object sender, KeyEventArgs e)
{
if (e.KeyCode == Keys.ControlKey && ModifierKeys == Keys.Control) { }
else if (e.KeyCode == Keys.U && ModifierKeys == Keys.Control)
{
if(undoStack.Count > 0)
{
StackPush(sender, redoStack);
undoStack.Pop()();
}
}
else if (e.KeyCode == Keys.R && ModifierKeys == Keys.Control)
{
if(redoStack.Count > 0)
{
StackPush(sender, undoStack);
redoStack.Pop()();
}
}
else
{
redoStack.Clear();
StackPush(sender, undoStack);
}
}
private void StackPush(object sender, Stack<Func<object>> stack)
{
TextBox textBox = (TextBox)sender;
var tBT = textBox.Text(textBox.Text, textBox.SelectionStart);
stack.Push(tBT);
}
}
public static class Extensions
{
public static Func<TextBox> Text(this TextBox textBox, string text, int sel)
{
return () =>
{
textBox.Text = text;
textBox.SelectionStart = sel;
return textBox;
};
}
}
I would listen for a change event, and when it occurs push the diff of the previous state and present state onto a stack. The diffs should be much smaller than storing the entire text. Also, you might not want to push a new undo state onto the stack at every edit... I'd lump all typing together until the user changes the cursor position, for example.
The smartest way is with immutable persistent objects. Never make a change to an object only make new objects that change slightly from the old version. This can be done somewhat efficiently by only cloning parts of the tree on the hot path.
I have an example of an undo stack written with minimal code
[Fact]
public void UndoStackSpec()
{
var stack = new UndoStack<A>(new A(10, null));
stack.Current().B.Should().Be(null);
stack.Set(x => x.B, new B(20, null));
stack.Current().B.Should().NotBe(null);
stack.Current().B.P.Should().Be(20);
stack.Undo();
stack.Current().B.Should().Be(null);
}
where A and B as classes with private setters on all properties ie
immutable
class A : Immutable
{
public int P { get; private set; }
public B B { get; private set; }
public A(int p, B b)
{
P = p;
B = b;
}
}
class B : Immutable
{
public int P { get; private set; }
public C C { get; private set; }
public B(int p, C c)
{
P = p;
C = c;
}
}
class C : Immutable
{
public int P { get; private set; }
public C(int p)
{
P = p;
}
}
you can find the full source here https://gist.github.com/bradphelan/5395652
来源：https://stackoverflow.com/questions/597792/how-to-implement-good-and-efficient-undo-redo-functionality-for-a-textbox