feat(TreeView): support Darg and Drop function

[syminomega]

Issues

#6216

New Feature

Add Draggable TreeView

Summary By Copilot

Regression?

• [ ] Yes
• [x] No

Risk

• [ ] High
• [ ] Medium
• [x] Low

Verification

• [x] Manual (required)
• [ ] Automated

Packaging changes reviewed?

• [ ] Yes
• [ ] No
• [x] N/A

☑️ Self Check before Merge

⚠️ Please check all items below before review. ⚠️

• [ ] Doc is updated/provided or not needed
• [ ] Demo is updated/provided or not needed
• [ ] Merge the latest code from the main branch

Summary by Sourcery

Implement drag-and-drop support for TreeView nodes with customizable drop behaviors and visual previews.

New Features:

• Add ItemDraggable, Draggable, and PreviewDrop parameters to enable node dragging and drop preview.
• Introduce OnItemDragStart, OnItemDragEnd, OnItemDrop callbacks and an OnDrop handler for drop event customization.
• Define TreeDropEventArgs class and TreeDropType enum to represent drop operations.

Enhancements:

• Add SCSS styling and CSS classes for drop zones and preview indicators on tree nodes.

Documentation:

• Update sample TreeViews component to demonstrate draggable TreeView usage and OnDrop event logging.

[bb-auto[bot]]

Thanks for your PR, @syminomega. Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[sourcery-ai[bot]]

Reviewer's Guide

This PR adds full drag-and-drop reordering support to the TreeView component by introducing draggable parameters and callbacks, rendering drop zones with preview indicators, and updating the component logic to handle item reordering on drop.

Sequence Diagram for TreeView Item Drag-and-Drop

sequenceDiagram
    actor User
    participant SourceRow as TreeViewRow (Source)
    participant TargetRow as TreeViewRow (Target)
    participant TreeViewComp as TreeView

    User->>SourceRow: Initiates drag (ondragstart)
    SourceRow->>SourceRow: DragStart(eventArgs)
    SourceRow->>TreeViewComp: OnItemDragStart(SourceItem)
    TreeViewComp->>TreeViewComp: OnItemDragStart(item): sets _draggingItem, _previewDrop, StateHasChanged()

    User->>TargetRow: Drags over drop zone (ondragenter)
    TargetRow->>TargetRow: DragEnterChildInside() / DragEnterChildBelow()
    TargetRow->>TargetRow: Sets preview flags (e.g., _previewChildLast), Renders drop preview

    User->>TargetRow: Drops item (ondrop)
    TargetRow->>TargetRow: DropChildInside() / DropChildBelow()
    TargetRow->>TreeViewComp: OnItemDrop(TreeDropEventArgs)
    TreeViewComp->>TreeViewComp: OnItemDrop(args): sets args.Source
    TreeViewComp->>TreeViewComp: Invokes user OnDrop(args) callback
    alt User OnDrop returns true
        TreeViewComp->>TreeViewComp: Reorders items (updates TreeViewItem parents/collections)
        TreeViewComp->>TreeViewComp: Resets _draggingItem, _previewDrop, StateHasChanged()
    end

    User->>SourceRow: Drag operation ends (ondragend)
    SourceRow->>SourceRow: DragEnd(eventArgs)
    SourceRow->>TreeViewComp: OnItemDragEnd()
    TreeViewComp->>TreeViewComp: OnItemDragEnd(): resets _previewDrop, _draggingItem, StateHasChanged()

File-Level Changes

Change	Details	Files
Implement draggable behavior in TreeViewRow	Add Draggable and PreviewDrop parameters Define OnItemDragStart, OnItemDragEnd and OnItemDrop callbacks Implement drag event handlers (DragStart, DragEnd, DragEnter/Leave, DropChildInside, DropChildBelow) Update markup to render draggable attribute and drop zones with preview elements Add CSS class for drop-pass state	Components/TreeView/TreeViewRow.razor.cs Components/TreeView/TreeViewRow.razor
Add drop preview and styling	Define CSS variable for preview color Add drop-zone layout and preview classes (.tree-drop-zone, .tree-preview-*) Disable user-select and pointer events during drag Add .tree-drop-pass class to suppress pointer events	Components/TreeView/TreeView.razor.scss
Extend TreeView component to handle drag-and-drop	Introduce ItemDraggable and OnDrop parameters Maintain preview state and current dragging item Implement OnItemDragStart, OnItemDragEnd, OnItemDrop to reorder items based on drop type Pass draggable settings and callbacks into each TreeViewRow	Components/TreeView/TreeView.razor.cs Components/TreeView/TreeView.razor
Define drop event arguments and types	Add TreeDropEventArgs with Source, Target, DropType and ExpandAfterDrop Introduce TreeDropType enum (AsFirstChild, AsLastChild, AsSiblingBelow)	Components/TreeView/TreeView.razor.cs Enums/TreeDropType.cs
Update demo to showcase draggable TreeView	Enable ItemDraggable and bind OnDrop in sample markup Implement OnDrop handler in sample code to log moves	Server/Components/Samples/TreeViews.razor Server/Components/Samples/TreeViews.razor.cs

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[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 100.00%. Comparing base (1ae8579) to head (0cd9ec8).

Additional details and impacted files

@@             Coverage Diff             @@
##             main     #6121      +/-   ##
===========================================
+ Coverage   99.98%   100.00%   +0.01%     
===========================================
  Files         718       718              
  Lines       31686     31679       -7     
  Branches     4471      4472       +1     
===========================================
- Hits        31681     31679       -2     
+ Misses          4         0       -4     
+ Partials        1         0       -1     

Flag	Coverage Δ
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[syminomega]

DraggableTree examples WIP