AUR9995

Error Message

AUR9995: The binding already has a target subscriber.

Description

This is an internal error indicating that Aurelia's binding system attempted to establish a connection (subscribe) between a binding instance and its target observer (the mechanism that updates the DOM when the view model changes), but such a connection was already in place for that binding instance.

Each binding should only have one primary subscriber managing the flow of data from the view model to the view target.

Cause

This error typically signifies an unexpected internal state within the binding system and should not occur during normal application development. Potential, though rare, causes might include:

1. Internal Race Condition: An unlikely race condition during the binding setup phase, potentially in highly complex or asynchronous scenarios.

2. Manual Binding Manipulation (Advanced): Incorrectly interacting with the internal APIs of a PropertyBinding instance manually, such as calling internal lifecycle methods like useTargetSubscriber inappropriately.

3. Memory Leaks or Incorrect Cleanup: If previous instances of bindings or observers were not cleaned up correctly, it might lead to inconsistent states, although this is speculative.

Solution

As this points to an internal issue, solutions involve ensuring standard practices and simplifying potential triggers:

1. Review Advanced Scenarios: If you are manually creating components, manipulating renderers, or interacting deeply with Aurelia's internal APIs, review that code carefully for correct usage of binding lifecycles.

2. Simplify Component: If the error occurs in a specific component, try simplifying its template and logic to isolate the cause. Pay attention to complex bindings, repeaters, conditional rendering, and asynchronous operations.

3. Check for External Interference: Ensure no external scripts are manipulating the DOM or Aurelia internals in unexpected ways.

4. Report Issue: If you consistently encounter this error in scenarios that seem like standard usage, it could indicate a bug in the framework. Report it to the Aurelia team with a reproducible example.

Example

This error is not typically triggered by specific template syntax but by internal state issues. A standard binding like this should not cause it:

<input value.bind="userName">
<p>${userMessage}</p>

The error would occur if, during the internal setup of the value.bind="userName" binding, the useTargetSubscriber method was somehow called twice or when the connection was already established.

Debugging Tips

• This error is difficult to debug from application code due to its internal nature.

• Use browser developer tools to set breakpoints within Aurelia's internal binding files (like property-binding.ts) if you suspect a specific area, but this requires familiarity with the framework internals.

• Focus on simplifying the component where the error occurs. Comment out bindings one by one to see if a particular binding is related to the issue.

• Look for patterns: Does it happen only during initial load? Only after certain actions? Only in specific browsers? This information is valuable if reporting a bug.