Composing producers

Often you will find that you need to call multiple producers in a sequence in order to get the data you actually want. Maybe because you call a producer that only returns an id and then you need an entity_load producer to use that id to return the actual entity, or maybe a route that returns a URL Object and then you want to take that URL can get the entity out of it using the route_entity data producer.
This can be accomplished using some of the built-in helpers inside the $builder object called compose. given our example for the previous example current_user, this is how it works :
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$registry->addFieldResolver('Query', 'currentUser', $builder->compose(
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  $builder->produce('current_user'),
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  $builder->produce('entity_load')
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    ->map('type', $builder->fromValue('user'))
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    ->map('id', $builder->fromParent())
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));
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We are chaining the two data producers together here, one after the other and calling fromParent will give us the result that was returned in the previous step.
Custom steps
What if we need to do some massaging but not necessarily using any data producer? The $builder object includes a callback property as well that we can use for this :
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$registry->addFieldResolver('Query', 'currentUser', $builder->compose(
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  $builder->produce('current_user'),
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  $builder->produce('entity_load')
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    ->map('type', $builder->fromValue('user'))
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    ->map('id', $builder->fromParent()),
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  $builder->callback(function ($entity) {
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    // Here we can do anything we want to the data. We get as a parameter anyting that was returned
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    // in the previous step.
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  })
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));
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Debugging producers
Note that you can always easily tap into the chain and e.g. use xdebug to debug the values:
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  $builder->compose(
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      $builder->tap($builder->callback(function ($parent, $args) {
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          // YOU CAN SET A XDEBUG BREAKPOINT IN THESE CALLBACKS TO CHECK THE VALUES.
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          echo("Compose step 0.");
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      })),
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      // Load the file object from the field.
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      $builder->produce('property_path')
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        ->map('type', $builder->fromValue('entity:YOUR_ENTITY_TYPE'))
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        ->map('value', $builder->fromParent())
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        ->map('path', $builder->fromValue('YOUR_FIELD_NAME.YOUR_FIELD_PROPERTY')),
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      $builder->tap($builder->callback(function ($parent, $args) {
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          // YOU CAN SET A XDEBUG BREAKPOINT IN THESE CALLBACKS TO CHECK THE VALUES.
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          echo("Compose step 1.");
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      })),
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      // Load the image style derivative of the file.
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      $builder->produce('image_derivative')
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        ->map('entity', $builder->fromParent())
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        ->map('style', $builder->fromValue('YOUR_IMAGE_STYLE')),
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      $builder->tap($builder->callback(function ($parent, $args) {
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          // YOU CAN SET A XDEBUG BREAKPOINT IN THESE CALLBACKS TO CHECK THE VALUES.
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          echo("Compose step 2.");
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      })),
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      // Retrieve the url of the generated image.
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      $builder->produce('image_style_url')
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        ->map('derivative', $builder->fromParent()),
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      $builder->tap($builder->callback(function ($parent, $args) {
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          // YOU CAN SET A XDEBUG BREAKPOINT IN THESE CALLBACKS TO CHECK THE VALUES.
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          echo("Compose step 3.");
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      }))
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    )
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  );
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