// Licensed to the .NET Foundation under one or more agreements.
// The .NET Foundation licenses this file to you under the MIT license.
// See the LICENSE file in the project root for more information.
 
using System.Diagnostics;
using Microsoft.CodeAnalysis.Editor.Shared.Extensions;
using Microsoft.CodeAnalysis.Editor.Shared.Utilities;
using Microsoft.VisualStudio.Text;
using Microsoft.VisualStudio.Text.Editor;
 
namespace Microsoft.CodeAnalysis.Editor.Shared.Tagging;
 
internal partial class TaggerEventSources
{
    private class ViewSpanChangedEventSource : AbstractTaggerEventSource
    {
        private readonly IThreadingContext _threadingContext;
        private readonly ITextView _textView;
 
        private Span? _span;
 
        public ViewSpanChangedEventSource(IThreadingContext threadingContext, ITextView textView)
        {
            Debug.Assert(textView != null);
            _threadingContext = threadingContext;
            _textView = textView;
        }
 
        public override void Connect()
        {
            _threadingContext.ThrowIfNotOnUIThread();
            _textView.LayoutChanged += OnLayoutChanged;
        }
 
        public override void Disconnect()
        {
            _threadingContext.ThrowIfNotOnUIThread();
            _textView.LayoutChanged -= OnLayoutChanged;
        }
 
        private void OnLayoutChanged(object? sender, TextViewLayoutChangedEventArgs e)
        {
            _threadingContext.ThrowIfNotOnUIThread();
            // The formatted span refers to the span of the textview's buffer that is visible.
            // If it changes, then we want to reclassify.  Note: the span might not change if
            // text were overwritten.  However, in the case of text-edits, we'll hear about 
            // through other means as we have an EventSource for that purpose.  This event
            // source is for knowing if the user moves the view around.  This handles direct
            // moves using the caret/scrollbar, as well as moves that happen because someone
            // jumped directly to a location using goto-def.  It also handles view changes
            // caused by the user collapsing an outlining region.
 
            var lastSpan = _span;
            _span = _textView.TextViewLines.FormattedSpan.Span;
 
            if (_span != lastSpan)
            {
                // The span changed.  This could have happened for a few different reasons.  
                // If none of the view's text snapshots changed, then it was because of scrolling.
                RaiseChanged();
            }
        }
    }
}