TASK: documentation for interface, union, enum and scalar

Readme.md

@@ -253,6 +253,138 @@ All resolver methods share the same signature:
 method($source, $args, $context, $info)
 ```
 
+#### Interface Types
+
+When working with interfaces, you need Resolvers for your interfaces as well. Given this schema:
+
+```graphql schema
+interface Person {
+    firstName: String!
+    lastName: String!
+}
+
+type Customer implements Person {
+    firstName: String!
+    lastName: String!
+    email: String!
+}
+
+type Supplier implements Person {
+    firstName: String!
+    lastName: String!
+    phone: String
+}
+```
+
+You need to configure a `Person` Resolver as well as Resolvers for the concrete implementations.
+
+While the concrete Type Resolvers do not need any special attention, the `Person` Resolver implements the 
+`__resolveType` function returning the type names:
+
+```php
+<?php
+
+namespace Your\Package\GraphQL\Resolver\Type;
+
+use t3n\GraphQL\ResolverInterface;
+
+class PersonResolver implements ResolverInterface
+{
+    public function __resolveType($source): ?string
+    {
+        if ($source instanceof Customer) {
+            return 'Customer';
+        } elseif ($source instanceof Supplier) {
+            return 'Supplier';
+        }
+        return null;
+    }
+}
+```
+
+#### Union Types
+
+Unions are resolved the same way as interfaces. For the example above, the corresponding Schema looks like this:
+
+```graphql schema
+union Person = Customer | Supplier
+```
+
+It is resolved again with a `PersonResolver` implementing the `__resolveType` function.
+
+#### Enum Types
+
+Enums are resolved automatically. The Resolver method simply returns a string matching the Enum value. For the Schema:
+```graphql schema
+enum Status {
+  ACTIVE
+  INACTIVE
+}
+
+type Customer {
+    status: Status!
+}
+```
+
+The `CustomerResolver` method returns the value as string:
+```php
+public function status(Customer $customer): string
+{
+    return $customer->isActive() ? 'ACTIVE' : 'INACTIVE';
+}
+```
+
+#### Scalar Types
+
+Types at the leaves of a Json tree are defined by Scalars. Own Scalars are implemented by a Resolver 
+implementing the functions `serialize()`, `parseLiteral()` and `parseValue()`.
+
+This example shows the implementation of a `DateTime` Scalar Type. For the given Schema definition:
+
+```graphql schema
+scalar DateTime
+```
+
+The `DateTimeResolver` looks the following when working with Unix timestamps:
+
+```php
+<?php
+
+namespace Your\Package\GraphQL\Resolver\Type;
+
+use DateTime;
+use GraphQL\Language\AST\IntValueNode;
+use GraphQL\Language\AST\Node;
+use t3n\GraphQL\ResolverInterface;
+
+class DateTimeResolver implements ResolverInterface
+{
+    public function serialize(DateTime $value): ?int
+    {
+        return $value->getTimestamp();
+    }
+
+    public function parseLiteral(Node $ast): ?DateTime
+    {
+        if ($ast instanceof IntValueNode) {
+            $dateTime = new DateTime();
+            $dateTime->setTimestamp((int)$ast->value);
+            return $dateTime;
+        }
+        return null;
+    }
+
+    public function parseValue(int $value): DateTime
+    {
+        $dateTime = new DateTime();
+        $dateTime->setTimestamp($value);
+        return $dateTime;
+    }
+}
+```
+
+You have to make the `DateTimeResolver` available again through one of the configuration options in the Settings.yaml.
+
 ### Context
 
 The third argument in your Resolver method signature is the Context.