StackOverflowException in binding

[donandren]

StackOverflowException in binding when property is changed in setter before raise property changed.
That's probably the root reason for problem in #824

Example if you bind a listbox selected Item to property selected item and property is like:

            set
            {
                if (_selectedItem != value)
                {
                    int index = Items.IndexOf(value);
                    if (index > 1)//or some other logic that's changing the current value
                    {
                        _selectedItem = Items[index - 1];
                    }
                    else
                    {
                        _selectedItem = value;
                        
                    }
                    this.RaisePropertyChanged();
                }
            }

[donandren]

easy repros can be found in branch https://github.com/donandren/Avalonia/tree/issues/855

Repro steps:
1.Start Virtualization Application
2.Select Item after the second and StackOverflowException will occur

Another repro added (covering also #824):
1.Start Virtualization Application
2.Try change Slider value and StackOverflowException will occur

[jkoritzinsky]

I've finally figured out why this causes a stack overflow. Below is the order of events that cause it.

Objects required: A, B are AvaloniaObjects, and C is an object that implements INPC and changes the current value before setting setting it and raising PropertyChanged.

Two-way bind A.Prop <---> C.Prop. Then bind B.Prop <---> A.Prop.

Order of events:

1. Set a value on A.Prop that C.Prop will change before it sets. Lets call this value X.
2. A raises AvaloniaPropertyChanged event for Prop with value X
3. A sends an update to its binding to update to value X.
   a. Value X is set on C.
   b. C changes value X to value Y before raising PropertyChanged
   c. C raises PropertyChanged after setting Prop to Y
   d. Because A.Prop <--> C.Prop is two-way, A.Prop is set to Y
   e. Because B.Prop <--> A.Prop is two-way, B.Prop is set to Y
   f. At this point, A.Prop, B.Prop, C.Prop all equal Y.
4. Because B.Prop <--> A.Prop is two-way, B.Prop is set to X
5. Because B.Prop <--> A.Prop is two-way, A.Prop is set to X
6. Go to step 2.

Now I have no idea how to fix this, but at least we have the execution path to work with.

[jkoritzinsky]

In this case it looks like we can just make B update A before C, since that will lead us to a steady-state. However, that does not work in all cases. Let's look at the situation below:

Objects required: A, B are AvaloniaObjects, and C is an object that implements INPC and changes the current value before setting setting it and raising PropertyChanged.

Two-way bind A.Prop <---> C.Prop. Then bind B.Prop <---> A.Prop.

Order of events:

1. Set a value on A.Prop that C.Prop will change before it sets and B.Prop will change before it sets (via a validate function). Lets call this value X.
2. A raises AvaloniaPropertyChanged event for Prop with value X
3. A sends an update to its binding to update to value X.
4. Value X is set on C.
5. C changes the value to Y before raising PropertyChanged
6. C raises PropertyChanged after setting Prop to Y
7. Because A.Prop <--> C.Prop is two-way, A.Prop is set to Y
8. Because B.Prop <--> A.Prop is two-way, B.Prop transforms the value to Z and sets itself to Z.
9. Because B.Prop <--> A.Prop is two-way, A.Prop is set to Z.
10. Because A.Prop <---> C.Prop is two-way, Value Z is set on C.Prop.
11. Go to step 5

In this situation, we never hit a steady-state so there is no good way to resolve this stack overflow without making the bindings inconsistent.