Error Handling - Detailed Behavior

See also:

Philosophy of error handling

  • Upon initial page load:
    • It is acceptable if not desirable to immediately show a static error, as the user hasn't yet interacted with the page.
    • For dynamic XForms, it is better not to show an error immediately (case of Form Builder and xxf:dynamic).
  • Upon subsequent interactions:
    • The XForms engine attempts to recover from errors occurring with XPath expressions, bindings, and actions.

How the XForms engine recovers from errors

  1. For control bindings, model bindings, values, variables
    • Upon XPath errors, a default value is chosen:
      • the empty sequence for bindings and variable values
      • the empty string for string values
    • Upon binding errors with the bind attribute
      • the binding resolves to the empty sequence
    • Upon binding errors with the model attribute
      • the model doesn't change, as if the model attribute was missing
  2. For XPath MIP values
    • Upon XPath errors for calculate and xxforms:default
      • the destination value is set to blank
      • if xxforms:expose-xpath-types="true" and there is an error accessing a typed value
        • the error is logged at debug level
      • else if xxforms:expose-xpath-types="false" or there is any other dynamic error
        • xxforms-xpath-error is dispatched to the model
    • Upon XPath errors for other MIPs
      • the MIP is not modified, as if the attribute specifying the property was missing
      • xxforms-xpath-error is dispatched to the model
    • Upon binding errors with complex or readonly content (calculate or xxforms:default only)
      • the instance value is not modified
      • xxforms-binding-error is dispatched to the model
  3. For the submission instance and xxforms:instance attributes
    • Upon incorrect instance id
      • xforms-submit-error is dispatched (as in the case of a target error with the targetref attribute)
    • Upon binding errors with complex or readonly content
      • xforms-submit-error is dispatched
  4. For actions
    • Any error taking place during action processing stops the outermost action handler, including:
      • XPath errors
      • binding errors with the bind or model attribute
      • binding errors with complex or readonly content
      • missing attributes or unsupported attribute values on action elements
    • xxforms-action-error event is dispatched to observer of the action
    • NOTE: Some actions silently ignore some error conditions, including:
      • <setvalue> pointing to an empty sequence or to an atomic item (such as a string) instead of a node
      • <delete> with an empty sequence or an empty overridden context
      • <insert> with an empty or non-element insert context, an empty overridden context, or an empty origin
      • actions with AVTs evaluating to the empty sequence
    • <dispatch>, <send>, <setfocus>, <setindex>, <toggle> when the target element is not found

In all cases except typed value access XPath errors on MIPs:

  • errors are logged at debug level [warning level until Orbeon Forms 4.8]
  • errors are added to a list of errors to send to the client
  • the client shows an error dialog which the user can discard (when enabled via properties)

In the case of typed value access XPath errors on MIPs:

  • errors are logged at debug level

Events dispatched

The xxforms-xpath-error event is dispatched:

  • to the model, upon encountering an XPath error during processing of an XPath model item property (MIP)

The xxforms-binding-error event is dispatched:

  • to the model, if a calculate or xxforms:default MIP points to complex or readonly content
  • to the control, if an attempt to store an external value (control value, filename, metadata or size) on a control to a node with complex or readonly content takes place NOTE: This should ideally not occur as the value control binding should not point to complex content, and readonly is disallowed.

The xxforms-action-error event is dispatched:

  • to the observer of the action, upon encountering an error during processing of an action
  • this includes: controls, model, instance, and submission

NOTE: The fatal, non-cancelable XForms xforms-compute-exception and xforms-binding-exception are no longer dispatched by Orbeon Forms.

Reference: kinds of errors that can occur

  • XForms errors
    • static XPath errors
      • some are detected during the static analysis phase (PE)
      • some expressions are not analyzed during static analysis of the page, and so can occur at runtime
    • dynamic XPath errors
    • binding errors with the bind or model attributes, e.g.: bind="foobar" where id foobar doesn't exist
      • some are detected during the static analysis phase
      • some are not analyzed during static analysis of the page, and so can occur at runtime
    • binding errors with complex or readonly content
      • XForms disallows setting the value of an element with complex content
      • XForms disallows setting the value of a readonly element or attribute
  • errors outside of XForms, for example:
    • in the XPL (XML pipeline) engine
    • in the controller
  • other server errors:
    • major errors such as the server running out of resources
    • server bugs!

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