DodoSSH/src/DodoSSH.Client.App/Views/HostsScreen.axaml.cs

using Avalonia;
using Avalonia.Controls;
using Avalonia.Input;
using Avalonia.Interactivity;
using Avalonia.VisualTree;
using DodoSSH.Client.Shell.ViewModels;

namespace DodoSSH.Client.App.Views;

/// <summary>
/// The grid of groups and hosts, and everything on it that is a gesture rather than a binding.
/// </summary>
/// <remarks>
/// <para>
/// All of this was <c>HostSidebar</c>'s until the host list became a grid of cards. It moved with the list
/// rather than staying with the editor: every handler here is about the thing that was clicked, dragged or
/// right-clicked, and the drawer beside the grid has none of those. See <see cref="HostDrawer"/>.
/// </para>
/// <para>
/// Its data context is the <c>VaultViewModel</c>, as <see cref="KeychainScreen"/>'s is, so every binding in the
/// markup is a property of the vault. The window hands it over; see <see cref="MainWindow"/>. The drawer
/// beside the grid inherits the same one.
/// </para>
/// </remarks>
internal sealed partial class HostsScreen : UserControl
{
    /// <summary>
    /// How a host travels from the card it was picked up on to the group card it is dropped on.
    /// </summary>
    /// <remarks>
    /// An in-process format carrying the row itself, rather than text carrying an id. The drag never leaves
    /// this window — there is nothing outside it that could accept a host — and the row is what the drop
    /// needs: it knows which vault the edit has to return to, which an id on its own does not.
    /// </remarks>
    private static readonly DataFormat<HostRowViewModel> HostFormat =
        DataFormat.CreateInProcessFormat<HostRowViewModel>("dodossh-host-row");

    /// <summary>How far the pointer has to travel before a press becomes a drag.</summary>
    /// <remarks>
    /// A threshold, because a press on this grid is nearly always a click: selecting a host, or the first
    /// half of the double-click that connects. Starting a drag on the press itself would turn every one of
    /// those into a drag gesture the user never asked for.
    /// </remarks>
    private const double DragThreshold = 5;

    /// <summary>How close to the top or bottom of the grid a drag has to be held to scroll it.</summary>
    /// <remarks>
    /// Deeper than a card's own margin, because the band has to be reachable while the pointer is still
    /// carrying something the user is looking at — a band the width of a hairline would only be found by
    /// accident, and only by somebody who did not need it.
    /// </remarks>
    private const double EdgeBand = 48;

    /// <summary>How far one drag event inside that band moves the grid.</summary>
    /// <remarks>
    /// Roughly a third of a card, so a pointer moving inside the band travels the grid at about the speed it
    /// is moving. A step of a whole card would jump the target out from under the pointer between two events.
    /// </remarks>
    private const double EdgeStep = 24;

    /// <summary>The press a drag would start from, or null once it has become one or been let go of.</summary>
    /// <remarks>
    /// Held because <see cref="DragDrop.DoDragDropAsync"/> takes the press rather than the movement: the
    /// gesture belongs to the pointer that went down, and the platform needs that event to hand the drag
    /// over to the operating system.
    /// </remarks>
    private PointerPressedEventArgs? press;

    private HostRowViewModel? pickedUp;

    private Point origin;

    /// <summary>The group card the pointer is currently over, while a drag is in flight.</summary>
    private ListBoxItem? marked;

    public HostsScreen()
    {
        InitializeComponent();

        // Wired here rather than in the markup because it is a gesture rather than a binding, which is how
        // the transfers screen opens a directory too. Double-clicking a machine to get a shell on it is what
        // every other client of this kind does, and CONNECT stays: it is the one in the drawer with the
        // password box above it, and a host that asks for a password still needs it typed first.
        HostGrid.DoubleTapped += OnHostActivated;

        // And a group card opens the group, on the same gesture, for the same reason: going inside
        // something by double-clicking it is what the transfers screen's directories do and what every file
        // manager does. One click used to open a group, which made the card that names a group and the
        // control that narrows the grid to it the same press — so there was no way to select a group in
        // order to rename it without also losing sight of every host outside it.
        GroupGrid.DoubleTapped += OnGroupActivated;

        // Tunnelled, so the card under the pointer is read before the ListBox has answered the press itself.
        // Bubbling would work for the drag but not for the menu: by then the control has already decided
        // what is selected, and the menu is about to open against it.
        HostGrid.AddHandler(PointerPressedEvent, OnPointerPressed, RoutingStrategies.Tunnel);
        HostGrid.AddHandler(ContextRequestedEvent, OnContextRequested, RoutingStrategies.Tunnel);

        // And the group cards answer a right click the same way, for the same reason: their menu's commands
        // read the vault's group selection, and without this they would act on whichever card was selected
        // before — or, with none, on the group the trail ends with, which is not on screen at all.
        GroupGrid.AddHandler(ContextRequestedEvent, OnGroupContextRequested, RoutingStrategies.Tunnel);

        HostGrid.PointerMoved += OnPointerMoved;
        HostGrid.PointerReleased += OnPointerReleased;
        HostGrid.PointerCaptureLost += OnPointerCaptureLost;

        // The host grid is where a drag starts and the group cards are where it lands. They used to be the
        // same control: the target was a heading among the cards, and with the headings gone the group cards
        // are the only thing on this screen that names a group. A card dropped onto another card is refused
        // rather than filed beside it — in a grid with no headings there is nothing to say which group that
        // would be, and a gesture whose result you cannot see before you let go is one that files machines
        // somewhere the user did not intend.
        DragDrop.AddDragOverHandler(GroupGrid, OnDragOver);
        DragDrop.AddDragLeaveHandler(GroupGrid, OnDragLeave);
        DragDrop.AddDropHandler(GroupGrid, OnDrop);

        // Everywhere else the pointer can be during a drag, and it is a handler rather than the absence of
        // one because AllowDrop is an inherited property: it is set on the scroller so that a drag anywhere
        // over the grid is reported at all, and that makes every card inside it a drop target as far as the
        // platform is concerned. This is where all of them but a group card are turned down — and where a
        // drag held at the top or bottom edge pulls the grid towards the target.
        DragDrop.AddDragOverHandler(Scroll, OnDragOverScroll);
    }

    /// <remarks>
    /// Null before the window has handed one over, and while the previewer is showing this control with no
    /// data context at all. Every handler below checks rather than assuming.
    /// </remarks>
    private VaultViewModel? Vault => DataContext as VaultViewModel;

    /// <summary>
    /// Where the keyboard should land when the terminal hands it back.
    /// </summary>
    /// <remarks>
    /// <para>
    /// Exposed as a property rather than left for the window to find by name, because the name is inside
    /// this control's template and the window cannot see it.
    /// </para>
    /// <para>
    /// It has to be a control the keyboard can actually go to. <c>Focus()</c> on a collapsed control is
    /// measurably a no-op and is not replayed when the control is revealed, so handing the keyboard to
    /// something that is not there would swallow it: the terminal would let go and nothing would take it.
    /// The grid no longer folds away as the sidebar's list could, but an empty grid is still a
    /// <c>ListBox</c> with no item to take focus — and an empty grid is exactly what a filter that matches
    /// nothing produces, which is a state somebody typing is very likely to be in. The find box is the
    /// answer then, and it is a good one: it is where they were typing.
    /// </para>
    /// </remarks>
    internal IInputElement KeyboardTarget =>
        Vault is { HasVisibleHosts: true } ? HostGrid : HostFilter;

    /// <remarks>
    /// Fire-and-forget, as the transfers screen's is: the command reports its own failures onto the status
    /// line — an unknown host key, a refused password — and awaiting it here would mean an event handler
    /// returning a task nothing observes.
    /// </remarks>
    private void OnHostActivated(object? sender, TappedEventArgs e)
    {
        // Only over a card. A double-tap on the space around them must not connect to whichever host was
        // selected before — which is what an unguarded handler would do, on a machine the user is not even
        // pointing at.
        if (Vault is { } vault && RowUnder(e.Source) is HostRowViewModel)
        {
            _ = vault.ConnectCommand.ExecuteAsync(null);
        }
    }

    /// <summary>
    /// Opens the group card that was double-clicked.
    /// </summary>
    /// <remarks>
    /// Guarded over the space around the cards exactly as the host grid's is, and it is the same mistake
    /// being guarded against: an unguarded handler would open whichever group happened to be selected when
    /// somebody double-clicked the gap beside it, throwing every host outside that group off the screen.
    /// </remarks>
    private void OnGroupActivated(object? sender, TappedEventArgs e)
    {
        if (Vault is { } vault && RowUnder(e.Source) is HostGroupRowViewModel group)
        {
            vault.OpenGroupCommand.Execute(group);
        }
    }

    /// <summary>
    /// Points the menu at whatever was right-clicked.
    /// </summary>
    /// <remarks>
    /// <para>
    /// The menu's three commands all read the vault's host selection, and a right click does not move it —
    /// which would mean a menu that quietly acted on whichever host happened to be selected instead of the
    /// one under the pointer. Deleting the wrong machine is the version of that mistake worth designing
    /// against.
    /// </para>
    /// <para>
    /// Cancelled outright over the space around the cards. That is not a host, and a menu offering Connect,
    /// Edit and Delete over it would be three buttons that either do nothing or act on something else
    /// entirely.
    /// </para>
    /// </remarks>
    private void OnContextRequested(object? sender, ContextRequestedEventArgs e)
    {
        if (Vault is not { } vault || RowUnder(e.Source) is not HostRowViewModel row)
        {
            e.Handled = true;
            return;
        }

        vault.SelectedSidebarRow = row;
    }

    /// <summary>
    /// Points the group menu at whatever was right-clicked.
    /// </summary>
    /// <remarks>
    /// <para>
    /// The host grid's rule, applied to the cards above it — see <see cref="OnContextRequested"/>. What is
    /// different is what an unaimed menu would have done: <c>GroupTarget</c> falls back to the open group
    /// when no card is selected, so Edit and Delete over a card would have been offered about the group whose
    /// contents are showing rather than the one the pointer is on. This menu is the only way to either of
    /// them now, so aiming it is the whole of aiming them.
    /// </para>
    /// <para>
    /// Cancelled outright over the space around the cards, as the host grid's is. That is not a group, and
    /// the fallback is exactly what would make the menu look like it worked there.
    /// </para>
    /// </remarks>
    private void OnGroupContextRequested(object? sender, ContextRequestedEventArgs e)
    {
        if (Vault is not { } vault || RowUnder(e.Source) is not HostGroupRowViewModel row)
        {
            e.Handled = true;
            return;
        }

        vault.SelectedGroup = row;
    }

    /// <remarks>
    /// Remembered rather than acted on. Whether this press is a click or the start of a drag is not known
    /// until the pointer moves, so this is the point at which both are still possible.
    /// </remarks>
    private void OnPointerPressed(object? sender, PointerPressedEventArgs e)
    {
        press = null;
        pickedUp = null;

        if (!e.GetCurrentPoint(HostGrid).Properties.IsLeftButtonPressed
            || RowUnder(e.Source) is not HostRowViewModel row)
        {
            return;
        }

        press = e;
        pickedUp = row;
        origin = e.GetPosition(HostGrid);
    }

    /// <remarks>
    /// The drag is started from the remembered press once the pointer has travelled far enough — see
    /// <see cref="DragThreshold"/>. Fire-and-forget, because the drag loop runs for as long as the user holds
    /// the button and an event handler cannot wait on that; what happens after it is only clearing the mark.
    /// </remarks>
    private void OnPointerMoved(object? sender, PointerEventArgs e)
    {
        if (press is not { } pressed || pickedUp is not { } row)
        {
            return;
        }

        if (!e.GetCurrentPoint(HostGrid).Properties.IsLeftButtonPressed)
        {
            Forget();
            return;
        }

        var moved = e.GetPosition(HostGrid) - origin;

        if (Math.Abs(moved.X) < DragThreshold && Math.Abs(moved.Y) < DragThreshold)
        {
            return;
        }

        Forget();

        _ = DragAsync(pressed, row);
    }

    private void OnPointerReleased(object? sender, PointerReleasedEventArgs e) => Forget();

    private void OnPointerCaptureLost(object? sender, PointerCaptureLostEventArgs e) => Forget();

    /// <summary>Carries one host for as long as the user holds it.</summary>
    private async Task DragAsync(PointerPressedEventArgs pressed, HostRowViewModel row)
    {
        var carried = new DataTransfer();
        carried.Add(DataTransferItem.Create(HostFormat, row));

        try
        {
            // ConfigureAwait(true): what follows touches the grid's own containers, and those are the UI
            // thread's.
            await DragDrop
                .DoDragDropAsync(pressed, carried, DragDropEffects.Move)
                .ConfigureAwait(true);
        }
        finally
        {
            // Whatever the drop did or did not do. A mark left behind would be a card that looks like a
            // target for a drag that ended somewhere else entirely.
            Unmark();
        }
    }

    /// <summary>
    /// Says whether what is under the pointer would take this host, and marks it if it would.
    /// </summary>
    /// <remarks>
    /// A host over the card of the group it is already in is refused, which is not pedantry:
    /// <c>DragDropEffects.None</c> is what turns the cursor into the "no" one, and a drag that looks like it
    /// would do something and then does nothing is worse than one that says so while it is still in the air.
    /// </remarks>
    private void OnDragOver(object? sender, DragEventArgs e)
    {
        e.Handled = true;

        if (Target(e) is not { } target)
        {
            e.DragEffects = DragDropEffects.None;
            Unmark();
            return;
        }

        e.DragEffects = DragDropEffects.Move;
        Mark(target.Container);
    }

    private void OnDragLeave(object? sender, DragEventArgs e) => Unmark();

    /// <summary>
    /// Carries the grid under a drag that is over the cards rather than over a group.
    /// </summary>
    /// <remarks>
    /// <para>
    /// <b>Without this the gesture is only available to whoever can see both ends of it.</b> The group cards
    /// are the first thing in the scrolling stack and the host being filed may be the fortieth card down, and
    /// a drag cannot use the wheel — the pointer button is held. So a drag held near the top edge pulls the
    /// grid down towards the target, which is what every file manager does with a drag near the edge of a
    /// list.
    /// </para>
    /// <para>
    /// A step per event rather than a timer, and that is a real limit rather than a simplification: a
    /// stationary pointer receives no drag events on any platform this runs on, so the scroll follows the
    /// pointer moving inside the band and stops when it stops. A timer would scroll on its own and would then
    /// need cancelling on the drop, on the leave, and on the drag that ends outside the window entirely.
    /// </para>
    /// <para>
    /// Reached only when the group cards did not handle the event first, which is what makes refusing the
    /// drop here correct: the space around the cards is not a target, and saying so keeps the "no" cursor on
    /// everything that is not a group.
    /// </para>
    /// </remarks>
    private void OnDragOverScroll(object? sender, DragEventArgs e)
    {
        if (e.DataTransfer.TryGetValue(HostFormat) is null)
        {
            return;
        }

        e.Handled = true;
        e.DragEffects = DragDropEffects.None;
        Unmark();

        var at = e.GetPosition(Scroll).Y;
        var height = Scroll.Bounds.Height;

        var step = at switch
        {
            _ when at < EdgeBand => -EdgeStep,
            _ when at > height - EdgeBand => EdgeStep,
            _ => 0,
        };

        if (step == 0)
        {
            return;
        }

        var furthest = Math.Max(0, Scroll.Extent.Height - Scroll.Viewport.Height);

        Scroll.Offset = Scroll.Offset.WithY(Math.Clamp(Scroll.Offset.Y + step, 0, furthest));
    }

    /// <remarks>
    /// Fire-and-forget, like every other command this control runs: the move writes to the vault and reports
    /// itself onto the status line, and a drop handler that awaited it would be an event handler returning a
    /// task nothing observes.
    /// </remarks>
    private void OnDrop(object? sender, DragEventArgs e)
    {
        e.Handled = true;
        Unmark();

        if (Vault is not { } vault || Target(e) is not { } target)
        {
            e.DragEffects = DragDropEffects.None;
            return;
        }

        e.DragEffects = DragDropEffects.Move;
        vault.MoveHostToGroupCommand.Execute(new HostGroupMove(target.Host, target.GroupId));
    }

    /// <summary>
    /// Where a drag currently is, or null if it is over nothing that would take it.
    /// </summary>
    /// <remarks>
    /// <para>
    /// One kind of target: a group card. It is the control that already answers "which group", it is drawn
    /// at the top of the screen where a drag can reach it from anywhere in the grid, and what it does when
    /// dropped on is what it says on it. The space around the cards takes nothing.
    /// </para>
    /// <para>
    /// A group the vault no longer has is read as no group at all, which is what the list already does with
    /// a dangling reference — see <c>VaultViewModel.RebuildSidebarRows</c>. That is decided in the command
    /// rather than here, so the rule has one home.
    /// </para>
    /// </remarks>
    private static DropTarget? Target(DragEventArgs e)
    {
        if (e.DataTransfer.TryGetValue(HostFormat) is not { } dragged
            || Container(e.Source) is not { DataContext: HostGroupRowViewModel group } container
            || dragged.Host.GroupId == group.EntityId)
        {
            return null;
        }

        return new DropTarget(dragged, group.EntityId, container);
    }

    private void Mark(ListBoxItem container)
    {
        if (ReferenceEquals(marked, container))
        {
            return;
        }

        Unmark();

        marked = container;
        marked.Classes.Add("droptarget");
    }

    private void Unmark()
    {
        marked?.Classes.Remove("droptarget");
        marked = null;
    }

    /// <summary>Lets go of a press that turned out not to be a drag, or has become one.</summary>
    private void Forget()
    {
        press = null;
        pickedUp = null;
    }

    /// <summary>The view model of the grid item an event happened on, if it happened on one.</summary>
    private static object? RowUnder(object? source) => Container(source)?.DataContext;

    /// <remarks>
    /// Walks up from whatever was actually hit — a text block, a border, the card's own grid — because that
    /// is what an event's source is. Anything not inside an item, which is the space around the cards,
    /// yields null.
    /// </remarks>
    private static ListBoxItem? Container(object? source) => source is Visual visual
        ? visual.FindAncestorOfType<ListBoxItem>(includeSelf: true)
        : null;

    /// <summary>A drag in flight, and where it would land.</summary>
    /// <remarks>
    /// The group is a <see cref="Guid"/> rather than a nullable one, which it was while the ungrouped
    /// heading was also a target. Every target is now a group card and every group card has an id; the way
    /// out of a group is the host's own editor, which is the one place "no group" can be said in words.
    /// </remarks>
    private sealed record DropTarget(HostRowViewModel Host, Guid GroupId, ListBoxItem Container);
}