I know immutable objects are preferred to mutable objects for reasoning and maintenance and... but in occasions making a class immutable have some costs, let me explain it with a simple example:
I have a class which draws a text in a box:
class TextDrawer
{
public string Text {set; get}
public RECT Box {set; get; }
public void DrawTextInBox()
{
}
}
It is probable that the `Text` overflows from the box and part of it wasn't written. Now I have two options:
1- Make the `TextDrawer` mutable and `DrawTextInBox` a non-pure function and on each call it draws the remaining part of text in the box.
class TextDrawer
{
public string Text {set {remaining = text = value;} get {return text; }}
public RECT Box {set; get; }
private string remaining;
public void DrawTextInBox()
{
...
update remaining part;
}
}
2- Keep the `TextDrawer` immutable and `DrawTextInBox` a pure function which returns the remaining part, then the caller should keep track of the remaining and pass it again to the `TextDrawer` or instantiate a new `TextDrawer` by the remaining text until there is nothing to write.
class TextDrawer
{
public string Text {set; get}
public RECT Box {set; get; }
public string DrawTextInBox()
{
draw as you can;
return remaining;
}
}
The second approach is immutable but actually shifts the responsibility of updating the remaining part outside of the class. The first approach while it is mutable but just need a call to `DrawTextInBox` to draw the remaining part.
The logic of the above example was simple but note this logic could be more complex, then I don't know to extract this logic from a class in order to make it immutable or let it be mutable and have the logic inside?