[6.x] Default values can be computed

src/Fields/Field.php

@@ -297,9 +297,26 @@ public function value()
 
     public function defaultValue()
     {
+        if ($this->hasComputedDefault()) {
+            return Fields::resolveDefault(Str::chopStart($this->config['default'], 'computed:'));
+        }
+
         return $this->config['default'] ?? $this->fieldtype()->defaultValue();
     }
 
+    public function hasComputedDefault(): bool
+    {
+        if (! isset($this->config['default'])) {
+            return false;
+        }
+
+        if (! is_string($this->config['default'])) {
+            return false;
+        }
+
+        return Str::startsWith($this->config['default'], 'computed:');
+    }
+
     public function validationValue()
     {
         return $this->fieldtype()->validationValue($this->value);

src/Fields/Fields.php

@@ -2,6 +2,7 @@
 
 namespace Statamic\Fields;
 
+use Closure;
 use Facades\Statamic\Fields\FieldRepository;
 use Facades\Statamic\Fields\Validator;
 use Illuminate\Support\Collection;
@@ -21,6 +22,19 @@ class Fields
     protected $withValidatableValues = false;
     protected $withComputedValues = false;
 
+    /** @var array<string,Closure> */
+    protected static array $computedFieldDefaultCallbacks = [];
+
+    public static function default(string $key, Closure $callback): void
+    {
+        static::$computedFieldDefaultCallbacks[$key] = $callback;
+    }
+
+    public static function resolveDefault(string $key): mixed
+    {
+        return static::$computedFieldDefaultCallbacks[$key]();
+    }
+
     public function __construct($items = [], $parent = null, $parentField = null, $parentIndex = null)
     {
         $this

tests/Fields/FieldTest.php

@@ -6,6 +6,7 @@
 use PHPUnit\Framework\Attributes\Group;
 use PHPUnit\Framework\Attributes\Test;
 use Statamic\Fields\Field;
+use Statamic\Fields\Fields;
 use Statamic\Fields\Fieldtype;
 use Statamic\Fields\Value;
 use Statamic\Forms\Form;
@@ -294,6 +295,7 @@ public function converts_to_array_suitable_for_rendering_fields_in_publish_compo
             ->andReturn(new class extends Fieldtype
             {
                 protected $component = 'example';
+
                 protected $configFields = [
                     'a_config_field_with_pre_processing' => ['type' => 'with_processing'],
                     'a_config_field_without_pre_processing' => ['type' => 'without_processing'],
@@ -474,6 +476,31 @@ public function defaultValue()
         $this->assertEquals('fieldtype defined default preprocessed', $field->preProcess()->value());
     }
 
+    #[Test]
+    public function preprocessing_a_field_with_no_value_and_computed_default_value_will_use_the_computed_function()
+    {
+        FieldtypeRepository::shouldReceive('find')
+            ->with('fieldtype')
+            ->andReturn(new class extends Fieldtype
+            {
+                public function preProcess($data)
+                {
+                    return $data.' preprocessed';
+                }
+
+                public function defaultValue()
+                {
+                    return 'fieldtype defined default';
+                }
+            });
+
+        Fields::default('computed-value', fn () => 'computed defined default');
+
+        $field = (new Field('test', ['default' => 'computed:computed-value', 'type' => 'fieldtype']));
+
+        $this->assertEquals('computed defined default preprocessed', $field->preProcess()->value());
+    }
+
     #[Test]
     public function converting_to_an_array_will_inline_the_handle()
     {