When does Xamarin Forms trigger binding?

Question

I'm creating a custom control and created a bindable property. I wanted to setup the children based on these properties. What is the correct way of handling this scenario? I tried looking for anything that makes sense to override in a base control or events that I can hook up.

For example, I want to create the column/row definitions of a Grid when I set the ColumnCount and RowCount in XAML

public class HeatMap: Grid
{
        public HeatMap()
        {
             // Where should I move these?
              Enumerable.Range(1, RowCount)
                .ToList()
                .ForEach(x => RowDefinitions.Add(new RowDefinition() { Height = GridLength.Auto }));
              Enumerable.Range(1, ColumnCount)
                  .ToList()
                  .ForEach(x => ColumnDefinitions.Add(new ColumnDefinition() { Width = new GridLength(1, GridUnitType.Star) }));
        }


        public static readonly BindableProperty RowCountProperty =
            BindableProperty.Create<HeatMap, int>(p => p.RowCount, 0);

        public int RowCount
        {
            get { return (int)GetValue(RowCountProperty); }
            set { SetValue(RowCountProperty, value); }
        }

        public static readonly BindableProperty ColumnCountProperty =
            BindableProperty.Create<HeatMap, int>(p => p.ColumnCount, 0);

        public int ColumnCount
        {
            get { return (int)GetValue(ColumnCountProperty); }
            set { SetValue(ColumnCountProperty, value); }
        }
 }

Answer 1

Do you mean update columns/rows on when properties are updated? BindableProperty.Create has argument propertyChanged to handle value updates.

public class HeatMap : Grid
{
    public HeatMap()
    {
        // Where should I move these?
        UpdateRows ();
        UpdateColumns ();
    }

    void UpdateColumns ()
    {
        ColumnDefinitions.Clear ();
        Enumerable.Range (1, ColumnCount).ToList ().ForEach (x => ColumnDefinitions.Add (new ColumnDefinition () {
            Width = new GridLength (1, GridUnitType.Star)
        }));
    }

    void UpdateRows ()
    {
        RowDefinitions.Clear ();
        Enumerable.Range (1, RowCount).ToList ().ForEach (x => RowDefinitions.Add (new RowDefinition () {
            Height = GridLength.Auto
        }));
    }

    public static readonly BindableProperty RowCountProperty =
        BindableProperty.Create<HeatMap, int> (p => p.RowCount, 0,
        propertyChanged: (bindable, oldValue, newValue) => ((HeatMap)bindable).UpdateRows ());

    public int RowCount
    {
        get { return (int)GetValue(RowCountProperty); }
        set { SetValue(RowCountProperty, value); }
    }

    public static readonly BindableProperty ColumnCountProperty =
        BindableProperty.Create<HeatMap, int>(p => p.ColumnCount, 0,
        propertyChanged: (bindable, oldValue, newValue) => ((HeatMap)bindable).UpdateColumns ());

    public int ColumnCount
    {
        get { return (int)GetValue(ColumnCountProperty); }
        set { SetValue(ColumnCountProperty, value); }
    }
}