// -----------------------------------------------------------------------
// <copyright file="BoundsTracker.cs" company="Steven Kirk">
// Copyright 2015 MIT Licence. See licence.md for more information.
// </copyright>
// -----------------------------------------------------------------------

namespace Perspex.VisualTree
{
    using System;
    using System.Collections.Generic;
    using System.Linq;
    using System.Reactive.Linq;

    /// <summary>
    /// Tracks the bounds of a control.
    /// </summary>
    /// <remarks>
    /// This class is used by Adorners to track the control that the adorner is attached to.
    /// </remarks>
    public class BoundsTracker
    {
        /// <summary>
        /// Starts tracking the specified visual.
        /// </summary>
        /// <param name="visual">The visual.</param>
        /// <returns>An observable that returns the tracked bounds.</returns>
        public IObservable<TransformedBounds> Track(Visual visual)
        {
            return this.Track(visual, (Visual)visual.GetVisualRoot());
        }

        /// <summary>
        /// Starts tracking the specified visual relative to another control.
        /// </summary>
        /// <param name="visual">The visual.</param>
        /// <param name="relativeTo">The control that the tracking should be relative to.</param>
        /// <returns>An observable that returns the tracked bounds.</returns>
        public IObservable<TransformedBounds> Track(Visual visual, Visual relativeTo)
        {
            var visuals = visual.GetSelfAndVisualAncestors()
                .TakeWhile(x => x != relativeTo)
                .Reverse();
            var boundsSubscriptions = new List<IObservable<Rect>>();

            foreach (var v in visuals.Cast<Visual>())
            {
                boundsSubscriptions.Add(v.GetObservable(Visual.BoundsProperty));
            }

            var bounds = Observable.CombineLatest(boundsSubscriptions).Select(ExtractBounds);

            // TODO: Track transform and clip rectangle.
            return Observable.Select(bounds, x => new TransformedBounds((Rect)x, (Rect)new Rect(), (Matrix)Matrix.Identity));
        }

        /// <summary>
        /// Sums a collection of rectangles.
        /// </summary>
        /// <param name="rects">The collection of rectangles.</param>
        /// <returns>The summed rectangle.</returns>
        private static Rect ExtractBounds(IList<Rect> rects)
        {
            var position = rects.Select(x => x.Position).Aggregate((a, b) => a + b);
            return new Rect(position, rects.Last().Size);
        }
    }
}