Apply widget_update_callback filter, before update() is called on a widget instance.

[jayarjo]

form_callback() in WP_Widget applies filter before it invokes form() method. Among other things this gives developer an opportunity to do some manipulations on the output:

	$instance = apply_filters( 'widget_form_callback', $instance, $this );

	$return = null;
	if ( false !== $instance ) {
		$return = $this->form($instance);

display_callback() has it right as well. But for the update_callback() the same is not true:

	$instance = $this->update( $new_instance, $old_instance );

	if ( $this->is_preview() ) {
		wp_suspend_cache_addition( $was_cache_addition_suspended );
	}

	$instance = apply_filters( 'widget_update_callback', $instance, $new_instance, $old_instance, $this );

So basically there's no way to manipulate the instance, before it gets itself into the update() method. Could we do it opposite instead?

	$instance = apply_filters( 'widget_update_callback', $instance, $new_instance, $old_instance, $this );
	
	$instance = $this->update( $new_instance, $old_instance );

	if ( $this->is_preview() ) {
		wp_suspend_cache_addition( $was_cache_addition_suspended );
	}

[Denis-de-Bernardy]

Methinks invalid. This is by design as I recollect when the filters got added. You want to use the update filter to do additional side effects in addition to the default update functionality, and that is how things are at the moment.

[westonruter]

Agreed that this is invalid. The update filter is specifically to filter the value that is returned by the widget's update callback. If a filter is desired for the value going into the update callback, then a new filter should be proposed. Also, we can't move the filter from where it exists right now without breaking existing plugins.