Connection admission

Loading interactive example…Open in new tab ↗

Opening the live canvas…

Connect the sender to the listener using the toolbar or ports. The quiet node rejects connections. Rejected ports turn red; releasing cancels the connection. Denied commands show a reason and leave history unchanged. Try undo and redo on an accepted connection.

First, Run checks in the review panel. The missing-route finding selects both nodes when clicked. Create the connection, then run checks again to clear it. Undo restores the incomplete draft; another check finds the missing route again. Draft diagnostics are explicit and do not prevent editing. Admission rules reject invalid connections immediately. See domain diagnostics.

The example uses one endpoint rule at two complementary boundaries:

API	Responsibility
DrawingSession(connectionPolicy: ...)	Evaluate hover, preview, creation and reconnection intent. targetIsStart preserves direction when moving the source endpoint.
DrawingSession(documentValidator: ...)	Enforce the same invariant on initial content and every candidate document, including imports and history. It is fixed for the source session's lifetime.

A port hover without an opposite endpoint is allowed until a pair can be checked. This example requires bound endpoints, so a rejected target cannot leave a free connection behind. Application permissions can additionally use DrawingConfiguration.hooks.beforeChange.

The validator scans connections in this small fixture. It demonstrates admission semantics, not a large-document performance policy or remote authorization.

See Session and Events and hooks.

Source

DiagramPolicyDecision admitExampleEndpoints(
  ConnectorEndpoint? start,
  ConnectorEndpoint? end,
) =>
    start is BoundEndpoint &&
        start.elementId == 'sender' &&
        start.portId == 'out' &&
        end is BoundEndpoint &&
        end.elementId == 'listener' &&
        end.portId == 'in'
    ? const DiagramPolicyDecision.allow()
    : const DiagramPolicyDecision.deny('Only sender → listener is allowed.');