error_handling.md

🦉 Error Handling 🦉

Content

• Overview
• Example
• Reference

Overview

By default, whenever an error occurs in the rendering of an Owl application, we destroy the whole application. Otherwise, we cannot offer any guarantee on the state of the resulting component tree. It might be hopelessly corrupted, but without any user-visible state.

Clearly, it sometimes is a little bit extreme to destroy the application. This is why we have a builtin mechanism to handle rendering errors (and errors coming from lifecycle hooks): the catchError hook.

Example

For example, here is how we could implement an ErrorBoundary component:

<div t-name="ErrorBoundary">
    <t t-if="state.error">
        Error handled
    </t>
    <t t-else="">
        <t t-slot="default" />
    </t>
</div>

class ErrorBoundary extends Component {
  state = useState({ error: false });

  catchError() {
    this.state.error = true;
  }
}

Using the ErrorBoundary is then extremely simple:

<ErrorBoundary><SomeOtherComponent/></ErrorBoundary>

Note that we need to be careful here: the fallback UI should not throw any error, otherwise we risk going into an infinite loop (also, see the page on slots for more information on the t-slot directive).

Reference

Whenever the catchError lifecycle hook is implemented, all errors coming from sub components rendering and/or lifecycle method calls will be caught and given to the catchError method. This allows us to properly handle the error, and to not break the application.

There are important things to know:

• If an error that occured in the internal rendering cycle is not caught, then Owl will destroy the full application. This is done on purpose, because Owl cannot guarantee that the state is not corrupted from this point on.

• errors coming from event handlers are NOT managed by catchError or any other owl mechanism. This is up to the application developer to properly recover from an error

Also, it may be useful to know that whenever an error is caught, it is then broadcasted to the application by an event on the qweb instance. It may be useful, for example, to log the error somewhere.

env.qweb.on("error", null, function (error) {
  // do something
  // react to the error
});