Add note about debugging produers. (#970)

Author: dasjo

doc/producers/composing.md

@@ -31,3 +31,41 @@ $registry->addFieldResolver('Query', 'currentUser', $builder->compose(
   })
 ));
 ```
+
+## Debugging producers
+
+Note that you can always easily tap into the chain and e.g. use xdebug to debug the values:
+
+```
+  $builder->compose(
+        $builder->tap($builder->callback(function ($parent, $args) {
+           // YOU CAN SET A XDEBUG BREAKPOINT IN THESE CALLBACKS TO CHECK THE VALUES.
+           echo("Compose step 0.");
+        })),
+        // Load the file object from the field.
+        $builder->produce('property_path')
+          ->map('type', $builder->fromValue('entity:YOUR_ENTITY_TYPE'))
+          ->map('value', $builder->fromParent())
+          ->map('path', $builder->fromValue('YOUR_FIELD_NAME.YOUR_FIELD_PROPERTY')),
+        $builder->tap($builder->callback(function ($parent, $args) {
+           // YOU CAN SET A XDEBUG BREAKPOINT IN THESE CALLBACKS TO CHECK THE VALUES.
+           echo("Compose step 1.");
+        })),
+        // Load the image style derivative of the file.
+        $builder->produce('image_derivative')
+          ->map('entity', $builder->fromParent())
+          ->map('style', $builder->fromValue('YOUR_IMAGE_STYLE')),
+        $builder->tap($builder->callback(function ($parent, $args) {
+           // YOU CAN SET A XDEBUG BREAKPOINT IN THESE CALLBACKS TO CHECK THE VALUES.
+           echo("Compose step 2.");
+        })),
+        // Retrieve the url of the generated image.
+        $builder->produce('image_style_url')
+          ->map('derivative', $builder->fromParent()),
+        $builder->tap($builder->callback(function ($parent, $args) {
+           // YOU CAN SET A XDEBUG BREAKPOINT IN THESE CALLBACKS TO CHECK THE VALUES.
+           echo("Compose step 3.");
+        }))
+      )
+  );
+```