silverstripe-framework/docs/en/02_Developer_Guides/19_GraphQL/05_plugins/02_writing_a_simple_plugin.md

---
title: Writing a simple plugin
summary: In this tutorial, we add a simple plugin for string fields
---

# Plugins

[CHILDREN asList]

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## Writing a simple plugin

For this example, we want all `String` fields to have a `truncate` argument that will limit the length of the string
in the response.

Because it applies to fields, we'll want `FieldPlugin` for this.

```php
class Truncator implements FieldPlugin
{
    public function getIdentifier(): string
    {
        return 'truncate';
    }

    public function apply(Field $field, Schema $schema, array $config = [])
    {
        $field->addArg('truncate', 'Int');
    }
}
```

Now we've added an argument to any field that implements the `truncate` plugin. This is good, but it really
doesn't save us a whole lot of time. The real value here is that the field will automatically apply the truncation.

For that, we'll need to augment the resolver with some _afterware_.

```php
public function apply(Field $field, Schema $schema, array $config = [])
{
    // Sanity check
    Schema::invariant(
        $field->getType() === 'String',
        'Field %s is not a string. Cannot truncate.',
        $field->getName()
    );

    $field->addArg('truncate', 'Int');
    $field->addResolverAfterware([static::class, 'truncate']);
}

public static function truncate(string $result, array $args): string
{
    $limit = $args['truncate'] ?? null;
    if ($limit) {
        return substr($result, 0, $limit);
    }

    return $result;
}
```

Let's register the plugin:

```yaml
SilverStripe\Core\Injector\Injector:
  SilverStripe\GraphQL\Schema\Registry\PluginRegistry:
    constructor:
      truncate: '%$MyProject\Plugins\Truncator'
```

And now we can apply it to any string field we want:

**app/_graphql/types.yml**
```yaml
Country:
  name:
    type: String
    plugins:
      truncate: true
```

### Further reading

[CHILDREN]