76 lines
3.5 KiB
C#
76 lines
3.5 KiB
C#
using System;
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namespace AcDream.App.UI;
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/// <summary>
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/// Pixel-based vertical scroll model. Port of retail <c>UIElement_Scrollable</c>:
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/// the scroll offset is an integer pixel value (<c>m_iScrollableY</c>) clamped to
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/// [0, ContentHeight - ViewHeight]; the thumb ratio is view/content; the position
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/// ratio is scroll/(content-view). Pure (no GL) so it is fully unit-tested and
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/// shared by the transcript (UiText) and the scrollbar (UiScrollbar).
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/// Decomp anchors: SetScrollableXY @0x4740c0, UpdateScrollbarSize_ @0x4741a0,
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/// UpdateScrollbarPosition_ @0x473f20, UIElement_Text::InqScrollDelta @0x4689b0.
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/// </summary>
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public sealed class UiScrollable
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{
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/// <summary>Total wrapped content height in px (m_iScrollableHeight).</summary>
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public int ContentHeight { get; set; }
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/// <summary>Visible viewport height in px.</summary>
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public int ViewHeight { get; set; }
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/// <summary>Pixels per text line (scroll quantum). InqScrollDelta line case.</summary>
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public int LineHeight { get; set; } = 16;
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private int _scrollY;
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/// <summary>Current scroll offset in px from the top of the content.</summary>
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public int ScrollY => _scrollY;
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/// <summary>Max scroll = max(0, content - view).</summary>
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public int MaxScroll => Math.Max(0, ContentHeight - ViewHeight);
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/// <summary>True when content exceeds the view (a scrollbar is warranted).</summary>
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public bool HasOverflow => ContentHeight > ViewHeight;
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/// <summary>True when the offset is at (or past) the bottom — used for bottom-pin.</summary>
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public bool AtEnd => _scrollY >= MaxScroll;
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/// <summary>
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/// Update the content/view extents and clamp the current offset. When
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/// <paramref name="preserveEnd"/> is true, a view that was at the end before
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/// the geometry change remains at the end afterward. User-driven scrollbar
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/// changes move <see cref="ScrollY"/> off the end, so later layout passes
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/// preserve that selected position instead of snapping it back.
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/// </summary>
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public void SetExtents(int contentHeight, int viewHeight, bool preserveEnd = false)
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{
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bool wasAtEnd = AtEnd;
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ContentHeight = Math.Max(0, contentHeight);
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ViewHeight = Math.Max(0, viewHeight);
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if (preserveEnd && wasAtEnd)
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ScrollToEnd();
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else
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SetScrollY(_scrollY);
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}
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/// <summary>Set the offset, clamped to [0, MaxScroll] (SetScrollableXY clamp).</summary>
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public void SetScrollY(int y) => _scrollY = Math.Clamp(y, 0, MaxScroll);
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/// <summary>Pin to the bottom (newest content visible).</summary>
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public void ScrollToEnd() => _scrollY = MaxScroll;
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/// <summary>Thumb size ratio = view/content, clamped to 1 (UpdateScrollbarSize_).</summary>
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public float ThumbRatio => ContentHeight <= 0 ? 1f : Math.Min(1f, (float)ViewHeight / ContentHeight);
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/// <summary>Position ratio = scroll/(content-view) in [0,1] (UpdateScrollbarPosition_).</summary>
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public float PositionRatio => MaxScroll <= 0 ? 0f : (float)_scrollY / MaxScroll;
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/// <summary>Inverse of PositionRatio — used when the user drags the thumb.</summary>
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public void SetPositionRatio(float ratio)
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=> SetScrollY((int)MathF.Round(Math.Clamp(ratio, 0f, 1f) * MaxScroll));
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/// <summary>Scroll by whole lines (sign: +down/newer, -up/older).</summary>
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public void ScrollByLines(int lines) => SetScrollY(_scrollY + lines * LineHeight);
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/// <summary>Scroll by a page = one view height (InqScrollDelta page case).</summary>
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public void ScrollByPage(int pages) => SetScrollY(_scrollY + pages * ViewHeight);
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}
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