AUR0203

Error Message

AUR0203: Trying to retrieve a property or build a scope from a null/undefined scope

Description

This error indicates an attempt was made to perform an operation that requires a valid binding scope (like looking up a property or creating a child scope), but the operation was initiated on a scope object that was null or undefined.

Cause

This error typically points to an issue within the component's structure or lifecycle, where a part of the component tree or binding system has become detached or wasn't properly initialized within the Aurelia scope hierarchy. Possible causes include:

1. Detached Elements/Components: Attempting to bind or interact with elements or components that have been manually removed from the DOM or are otherwise not part of a connected Aurelia component tree with a valid root scope.

2. Incorrect Component Structure: Issues with custom element composition, template controllers (if.bind, repeat.for), or view creation/caching logic that might lead to orphaned scopes.

3. Lifecycle Timing: Attempting operations that require a scope (like accessing binding contexts) too early or too late in the component lifecycle, when the scope might not be available or has already been disposed.

4. Manual Scope Manipulation: Incorrectly manipulating scope objects manually, leading to null or undefined references.

5. Internal Errors: In rare cases, this could indicate an internal error within the Aurelia runtime's scope management.

Solution

Resolving this error often involves investigating the component structure and lifecycle interactions:

1. Verify Component Hierarchy: Ensure that the element or component involved in the error is correctly nested within an Aurelia application or component tree and hasn't been improperly manipulated outside of Aurelia's control.

2. Check Template Controller Logic: If using if.bind, repeat.for, au-compose, portal, etc., review the bindings and associated view models to ensure they are correctly managing scope creation and destruction.

3. Review Lifecycle Hooks: Ensure that operations requiring scope access are performed within appropriate lifecycle hooks (e.g., bound, attaching) and not during stages where the scope might be unavailable (dispose, unbinding).

4. Avoid Manual Scope Manipulation: Refrain from directly manipulating Scope objects unless you have a deep understanding of Aurelia's internal mechanisms.

5. Simplify and Isolate: Temporarily simplify the component's template and view model involved in the error to isolate the problematic section. Check if bindings involving $parent, $host, or complex context lookups are involved.

Example

While a direct, simple reproduction is difficult as it often depends on complex interactions, consider scenarios like:

<!-- Potentially problematic if <my-child> somehow detaches -->
<!-- or its internal logic tries to access parent scope after detachment -->
<my-parent>
  <my-child if.bind="isAttached"></my-child>
</my-parent>

<!-- Using view references incorrectly -->
<template with.bind="getViewReference()">
  <!-- If getViewReference() returns a view not properly scoped -->
  <au-compose view.bind="someView"></au-compose>
</template>

// View Model example potentially leading to scope issues if managed incorrectly
import { customElement, IViewFactory, Controller, Scope } from 'aurelia';

@customElement({ /* ... */ })
export class MyParent {
  isAttached = true;
}

// Assume MyChild tries to access parent scope in a problematic way
@customElement({ /* ... */ })
export class MyChild {
  private scope: Scope | null = null;

  binding(initiator: Controller, parent: Controller) {
    // Storing scope might be okay, but using it after detachment can cause issues
    this.scope = parent.scope;
  }

  someActionAfterDetached() {
    // If this method is called after the element is detached (isAttached=false)
    // and tries to use this.scope, it might lead to AUR0203 if the scope chain is broken.
    if (this.scope) {
      // This call could potentially trigger the error if the scope is invalid
      // Scope.create(this.scope, {});
      // this.scope.find(...)
    }
  }
}

Debugging Tips

• Identify the component or binding expression involved by examining the stack trace.

• Use browser developer tools to place breakpoints within the lifecycle hooks (binding, bound, attaching, detaching, unbinding) of the relevant components to observe the state of the scope (this.$controller.scope).

• Analyze interactions between parent and child components, especially those involving template controllers or dynamic composition.

• Check for any manual DOM manipulation that might interfere with Aurelia's element and scope tracking.

• Look for bindings that traverse the scope hierarchy ($parent, $host) and verify their validity in the context where the error occurs.