Constraints Design

Constraints determine whether a drag operation may complete.
They enforce structural rules and prevent invalid mutations.

Mermaid Diagram: Constraint Evaluation

flowchart TD
    A[User drops node] --> B["checkConstraints(dragged, target)"]
    B --> C{Valid?}
    C -->|yes| D[Allow drop]
    C -->|no| E[Reject drop]
    E --> F[Rollback via restoreSnapshot]

Built-in Constraints (since v0.1)

Self-Drop Handling

if (dragged === target) {
  return { allowed: true }; // Self-drops are allowed as no-ops
}

Selector Validation

if (!target.matches(options.selectors.node)) {
  return { allowed: false, reason: "invalid-target" };
}

Extending Constraints (v0.2+)

You may chain constraints:

const result = allConstraints.map(fn => fn(...args))
  .find(r => r.allowed === false);

Future features will allow:

• hierarchical constraints
• group-based constraints
• restricted-parent rules
• custom constraint registration

Guarantees

• Constraint evaluation is deterministic
• Rejecting a drop always invokes rollback

See Also

• Constraint Scoping Across Groups and Containers — how the group-boundary built-in check, user-registered group-level constraints, and cross-container constraints layer on top of each other, with the full evaluation order and real registration examples