// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
 
 
using System.Collections;
using System.Windows;
using System.Windows.Controls;
 
namespace MS.Internal.Controls
{
    internal abstract class ModelTreeEnumerator : IEnumerator
    {
        internal ModelTreeEnumerator(object content)
        {
            _content = content;
        }
 
        #region IEnumerator
 
        object IEnumerator.Current
        {
            get
            {
                return this.Current;
            }
        }
 
        bool IEnumerator.MoveNext()
        {
            return this.MoveNext();
        }
 
        void IEnumerator.Reset()
        {
            this.Reset();
        }
 
        #endregion
 
        #region Protected
 
        protected object Content
        {
            get
            {
                return _content;
            }
        }
 
        protected int Index
        {
            get
            {
                return _index;
            }
 
            set
            {
                _index = value;
            }
        }
 
        protected virtual object Current
        {
            get
            {
                // Don't VerifyUnchanged(); According to MSDN:
                //     If the collection is modified between MoveNext and Current,
                //     Current will return the element that it is set to, even if
                //     the enumerator is already invalidated.
 
                if (_index == 0)
                {
                    return _content;
                }
                // Exception is part of the IEnumerator.Current contract when moving beyond begin/end
                throw new InvalidOperationException(SR.EnumeratorInvalidOperation);
            }
        }
 
        protected virtual bool MoveNext()
        {
            if (_index < 1)
            {
                // Singular content, can move next to 0 and that's it.
                _index++;
 
                if (_index == 0)
                {
                    // don't call VerifyUnchanged if we're returning false anyway.
                    // This permits users to change the Content after enumerating
                    // the content (e.g. in the invalidation callback of an inherited
                    // property).  See bug 955389.
 
                    VerifyUnchanged();
                    return true;
                }
            }
 
            return false;
        }
 
        protected virtual void Reset()
        {
            VerifyUnchanged();
            _index = -1;
        }
 
        protected abstract bool IsUnchanged
        {
            get;
        }
 
        protected void VerifyUnchanged()
        {
            // If the content has changed, then throw an exception
            if (!IsUnchanged)
            {
                throw new InvalidOperationException(SR.EnumeratorVersionChanged);
            }
        }
 
        #endregion
 
        #region Data
 
        private int _index = -1;
        private object _content;
 
        #endregion
    }
 
    internal class ContentModelTreeEnumerator : ModelTreeEnumerator
    {
        internal ContentModelTreeEnumerator(ContentControl contentControl, object content) : base(content)
        {
            Debug.Assert(contentControl != null, "contentControl should be non-null.");
 
            _owner = contentControl;
        }
 
        protected override bool IsUnchanged
        {
            get
            {
                return Object.ReferenceEquals(Content, _owner.Content);
            }
        }
 
        private ContentControl _owner;
    }
 
    internal class HeaderedContentModelTreeEnumerator : ModelTreeEnumerator
    {
        internal HeaderedContentModelTreeEnumerator(HeaderedContentControl headeredContentControl, object content, object header) : base(header)
        {
            Debug.Assert(headeredContentControl != null, "headeredContentControl should be non-null.");
            Debug.Assert(header != null, "Header should be non-null. If Header was null, the base ContentControl enumerator should have been used.");
 
            _owner = headeredContentControl;
            _content = content;
        }
 
        protected override object Current
        {
            get
            {
                if ((Index == 1) && (_content != null))
                {
                    return _content;
                }
 
                return base.Current;
            }
        }
 
        protected override bool MoveNext()
        {
            if (_content != null)
            {
                if (Index == 0)
                {
                    // Moving from the header to content
                    Index++;
                    VerifyUnchanged();
                    return true;
                }
                else if (Index == 1)
                {
                    // Going from content to the end
                    Index++;
                    return false;
                }
            }
 
            return base.MoveNext();
        }
 
        protected override bool IsUnchanged
        {
            get
            {
                object header = Content;    // Header was passed to the base so that it would appear in index 0
                return Object.ReferenceEquals(header, _owner.Header) &&
                       Object.ReferenceEquals(_content, _owner.Content);
            }
        }
 
        private HeaderedContentControl _owner;
        private object _content;
    }
 
    internal class HeaderedItemsModelTreeEnumerator : ModelTreeEnumerator
    {
        internal HeaderedItemsModelTreeEnumerator(HeaderedItemsControl headeredItemsControl, IEnumerator items, object header) : base(header)
        {
            Debug.Assert(headeredItemsControl != null, "headeredItemsControl should be non-null.");
            Debug.Assert(items != null, "items should be non-null.");
            Debug.Assert(header != null, "header should be non-null. If Header was null, the base ItemsControl enumerator should have been used.");
 
            _owner = headeredItemsControl;
            _items = items;
        }
 
        protected override object Current
        {
            get
            {
                if (Index > 0)
                {
                    return _items.Current;
                }
 
                return base.Current;
            }
        }
 
        protected override bool MoveNext()
        {
            if (Index >= 0)
            {
                Index++;
                return _items.MoveNext();
            }
 
            return base.MoveNext();
        }
 
        protected override void Reset()
        {
            base.Reset();
            _items.Reset();
        }
 
        protected override bool IsUnchanged
        {
            get
            {
                object header = Content;    // Header was passed to the base so that it would appear in index 0
                return Object.ReferenceEquals(header, _owner.Header);
            }
        }
 
        private HeaderedItemsControl _owner;
        private IEnumerator _items;
    }
}