Lifecycle hooks

New in version 0.96

Intercept the rendering lifecycle with Component hooks.

Unlike the extension hooks, these are defined directly on the Component class.

Available hooks¤

`on_render_before`¤

def on_render_before(
    self: Component,
    context: Context,
    template: Optional[Template],
) -> None:

Component.on_render_before runs just before the component's template is rendered.

It is called for every component, including nested ones, as part of the component render lifecycle.

It receives the Context and the Template as arguments.

The template argument is None if the component has no template.

Example:

You can use this hook to access the context or the template:

from django.template import Context, Template
from django_components import Component

class MyTable(Component):
    def on_render_before(self, context: Context, template: Optional[Template]) -> None:
        # Insert value into the Context
        context["from_on_before"] = ":)"

        assert isinstance(template, Template)

Warning

If you want to pass data to the template, prefer using get_template_data() instead of this hook.

Warning

Do NOT modify the template in this hook. The template is reused across renders.

Since this hook is called for every component, this means that the template would be modified every time a component is rendered.

`on_render`¤

New in version 0.140

def on_render(
    self: Component,
    context: Context,
    template: Optional[Template],
) -> Union[str, SafeString, OnRenderGenerator, None]:

Component.on_render does the actual rendering.

You can override this method to:

Change what template gets rendered
Modify the context
Modify the rendered output after it has been rendered
Handle errors

The default implementation renders the component's Template with the given Context.

class MyTable(Component):
    def on_render(self, context, template):
        if template is None:
            return None
        else:
            return template.render(context)

The template argument is None if the component has no template.

Modifying rendered template¤

To change what gets rendered, you can:

Render a different template
Render a component
Return a different string or SafeString

class MyTable(Component):
    def on_render(self, context, template):
        return "Hello"

You can also use on_render() as a router, rendering other components based on the parent component's arguments:

class MyTable(Component):
    def on_render(self, context, template):
        # Select different component based on `feature_new_table` kwarg
        if self.kwargs.get("feature_new_table"):
            comp_cls = NewTable
        else:
            comp_cls = OldTable

        # Render the selected component
        return comp_cls.render(
            args=self.args,
            kwargs=self.kwargs,
            slots=self.slots,
            context=context,
        )

Post-processing rendered template¤

When you render the original template in on_render() as:

template.render(context)

The result is NOT the final output, but an intermediate result. Nested components are not rendered yet.

Instead, django-components needs to take this result and process it to actually render the child components.

To access the final output, you can yield the result instead of returning it.

This will return a tuple of (rendered HTML, error). The error is None if the rendering succeeded.

class MyTable(Component):
    def on_render(self, context, template):
        html, error = yield template.render(context)

        if error is None:
            # The rendering succeeded
            return html
        else:
            # The rendering failed
            print(f"Error: {error}")

At this point you can do 3 things:

Return a new HTML

The new HTML will be used as the final output.

If the original template raised an error, it will be ignored.

class MyTable(Component):
    def on_render(self, context, template):
        html, error = yield template.render(context)

        return "NEW HTML"

Raise a new exception

The new exception is what will bubble up from the component.

The original HTML and original error will be ignored.

class MyTable(Component):
    def on_render(self, context, template):
        html, error = yield template.render(context)

        raise Exception("Error message")

Return nothing (or None) to handle the result as usual

If you don't raise an exception, and neither return a new HTML, then original HTML / error will be used:
- If rendering succeeded, the original HTML will be used as the final output.
- If rendering failed, the original error will be propagated.
```
class MyTable(Component):
    def on_render(self, context, template):
        html, error = yield template.render(context)

        if error is not None:
            # The rendering failed
            print(f"Error: {error}")
```

Example: ErrorBoundary¤

on_render() can be used to implement React's ErrorBoundary.

That is, a component that catches errors in nested components and displays a fallback UI instead:

{% component "error_boundary" %}
  {% fill "content" %}
    {% component "nested_component" %}
  {% endfill %}
  {% fill "fallback" %}
    Sorry, something went wrong.
  {% endfill %}
{% endcomponent %}

To implement this, we render the fallback slot in on_render() and return it if an error occured:

class ErrorFallback(Component):
    template = """
      {% slot "content" default / %}
    """

    def on_render(self, context, template):
        fallback = self.slots.fallback

        if fallback is None:
            raise ValueError("fallback slot is required")

        html, error = yield template.render(context)

        if error is not None:
            return fallback()
        else:
            return html

`on_render_after`¤

def on_render_after(
    self: Component,
    context: Context,
    template: Optional[Template],
    result: Optional[str | SafeString],
    error: Optional[Exception],
) -> Union[str, SafeString, None]:

on_render_after() runs when the component was fully rendered, including all its children.

It receives the same arguments as on_render_before(), plus the outcome of the rendering:

result: The rendered output of the component. None if the rendering failed.
error: The error that occurred during the rendering, or None if the rendering succeeded.

on_render_after() behaves the same way as the second part of on_render() (after the yield).

class MyTable(Component):
    def on_render_after(self, context, template, result, error):
        if error is None:
            # The rendering succeeded
            return result
        else:
            # The rendering failed
            print(f"Error: {error}")

Same as on_render(), you can return a new HTML, raise a new exception, or return nothing:

Return a new HTML

The new HTML will be used as the final output.

If the original template raised an error, it will be ignored.

class MyTable(Component):
    def on_render_after(self, context, template, result, error):
        return "NEW HTML"

Raise a new exception

The new exception is what will bubble up from the component.

The original HTML and original error will be ignored.

class MyTable(Component):
    def on_render_after(self, context, template, result, error):
        raise Exception("Error message")

Return nothing (or None) to handle the result as usual

If you don't raise an exception, and neither return a new HTML, then original HTML / error will be used:
- If rendering succeeded, the original HTML will be used as the final output.
- If rendering failed, the original error will be propagated.
```
class MyTable(Component):
    def on_render_after(self, context, template, result, error):
        if error is not None:
            # The rendering failed
            print(f"Error: {error}")
```

Example¤

You can use hooks together with provide / inject to create components that accept a list of items via a slot.

In the example below, each tab_item component will be rendered on a separate tab page, but they are all defined in the default slot of the tabs component.

See here for how it was done

{% component "tabs" %}
  {% component "tab_item" header="Tab 1" %}
    <p>
      hello from tab 1
    </p>
    {% component "button" %}
      Click me!
    {% endcomponent %}
  {% endcomponent %}

  {% component "tab_item" header="Tab 2" %}
    Hello this is tab 2
  {% endcomponent %}
{% endcomponent %}