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Metadata factories control how component configuration is transformed into JSON metadata that gets sent to the client. The framework provides a default implementation, but you can create custom factories for advanced scenarios.

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Understanding IMetadataFactory

The IMetadataFactory interface (source:UiMetadataFramework.Core/Binding/Component/IMetadataFactory.cs:8) defines a single method: 
public interface IMetadataFactory
{
    object? CreateMetadata(
        Type type,                                          // Component's type
        Type? derivedType,                                  // Derived component type (if any)
        IComponentBinding binding,                          // Component binding
        MetadataBinder binder,                              // Metadata binder instance
        params ComponentConfigurationAttribute[] configurations // Configuration attributes
    );
}

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DefaultMetadataFactory

The framework provides DefaultMetadataFactory (source:UiMetadataFramework.Core/Binding/Component/DefaultMetadataFactory.cs:11) which handles most scenarios. It:
  1. Iterates over all configuration attributes
  2. Finds properties marked with [ConfigurationProperty]
  3. Builds a dictionary of configuration values
  4. Validates mandatory configurations
  5. Returns the dictionary as JSON-serializable metadata

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How DefaultMetadataFactory Works

public class DefaultMetadataFactory : IMetadataFactory
{
    public object? CreateMetadata(
        Type type,
        Type? derivedType,
        IComponentBinding binding,
        MetadataBinder binder,
        params ComponentConfigurationAttribute[] configurations)
    {
        var result = new Dictionary<string, object?>();
        
        // Process each configuration attribute
        foreach (var configData in configurations)
        {
            var configType = configData.GetType();
            
            // Find properties marked with [ConfigurationProperty]
            var configProperties = configType
                .GetProperties()
                .Where(t => t.CanRead)
                .Select(t => new {
                    Property = t,
                    Info = t.GetCustomAttribute<ConfigurationPropertyAttribute>()
                })
                .Where(t => t.Info != null);
            
            // Add each property value to result dictionary
            foreach (var property in configProperties)
            {
                var value = property.Property.GetValue(configData);
                result[property.Info.Name] = value;
            }
        }
        
        // Call AugmentConfiguration for customization
        this.AugmentConfiguration(type, derivedType, binder, configurations, result);
        
        return result.Count == 0 ? null : result;
    }
    
    protected virtual void AugmentConfiguration(
        Type type,
        Type? derivedType,
        MetadataBinder binder,
        ComponentConfigurationAttribute[] configurations,
        Dictionary<string, object?> result)
    {
        // Override this to customize the configuration
    }
}

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Creating Custom Factories

Custom factories are useful when you need to:
  • Transform configuration data before sending to client
  • Generate metadata from external sources
  • Add computed properties to metadata
  • Validate complex configuration rules

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Example: DropdownMetadataFactory

Here’s a real example from UiMetadataFramework.Basic (source:UiMetadataFramework.Basic/Inputs/Dropdown/DropdownMetadataFactory.cs:16): 
public class DropdownMetadataFactory : DefaultMetadataFactory
{
    protected override void AugmentConfiguration(
        Type type,
        Type? derivedType,
        MetadataBinder binder,
        ComponentConfigurationAttribute[] configurations,
        Dictionary<string, object?> result)
    {
        var sourceType = configurations
            .OfType<DropdownAttribute>()
            .SingleOrDefault()?.Source;
        
        // Get the T in DropdownValue<T>
        var innerType = type.GenericTypeArguments[0];
        result["Subtype"] = Nullable.GetUnderlyingType(innerType)?.Name ?? innerType.Name;
        
        if (sourceType == null)
        {
            // No explicit source - check if T is an enum
            var enumType = type.GenericTypeArguments[0].GetEnumType();
            
            if (enumType != null)
            {
                // Generate dropdown items from enum values
                var items = Enum.GetValues(enumType)
                    .Cast<object>()
                    .Select(t => new DropdownItem(
                        label: t.ToString().Humanize(LetterCasing.Sentence),
                        value: t.ToString()))
                    .ToList();
                
                result["Items"] = items;
                result["Source"] = enumType.FullName;
                return;
            }
        }
        else
        {
            // Check if source is an inline source
            var inlineSource = sourceType
                .GetInterfaces(typeof(IDropdownInlineSource))
                .SingleOrDefault();
            
            if (inlineSource != null)
            {
                // Get items from inline source using DI
                var source = binder.Container.GetService(sourceType);
                var items = sourceType.GetTypeInfo()
                    .GetMethod(nameof(IDropdownInlineSource.GetItems))
                    .Invoke(source, null);
                
                result["Items"] = items;
                result["Source"] = sourceType.FullName;
                return;
            }
            
            // Check if source is a remote source (typeahead)
            if (sourceType.GetInterfaces(typeof(ITypeaheadRemoteSource)).Any())
            {
                result["Source"] = sourceType.GetFormId();
                return;
            }
        }
        
        throw new BindingException("Field defines an invalid dropdown source.");
    }
}
This factory:
  1. Extends DefaultMetadataFactory to leverage existing functionality
  2. Overrides AugmentConfiguration to add custom logic
  3. Determines the dropdown’s data source (enum, inline, or remote)
  4. Generates appropriate metadata for each source type
  5. Uses the DI container to instantiate inline sources

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Field Metadata Factories

For customizing how individual fields are processed, implement IFieldMetadataFactory (source:UiMetadataFramework.Core/Binding/Field/IFieldMetadataFactory.cs:8): 
public interface IFieldMetadataFactory
{
    FieldMetadata GetMetadata(
        FieldAttribute? attribute,      // Field attribute (InputField/OutputField)
        PropertyInfo property,           // The property being processed
        ComponentBinding binding,        // Component binding for the field's type
        MetadataBinder binder           // Metadata binder instance
    );
}

​
DefaultFieldMetadataFactory

The default implementation (source:UiMetadataFramework.Core/Binding/Field/DefaultFieldMetadataFactory.cs:7): 
public class DefaultFieldMetadataFactory : IFieldMetadataFactory
{
    public virtual FieldMetadata GetMetadata(
        FieldAttribute? attribute,
        PropertyInfo property,
        ComponentBinding binding,
        MetadataBinder binder)
    {
        // Validate event handler attributes
        var eventHandlerAttributes = property
            .GetCustomAttributesImplementingInterface<IFieldEventHandlerAttribute>()
            .ToList();
            
        var illegalAttributes = eventHandlerAttributes
            .Where(t => !t.ApplicableToFieldCategory(binding.Category))
            .ToList();
            
        if (illegalAttributes.Any())
        {
            throw new BindingException(
                $"Field '{property.DeclaringType.FullName}.{property.Name}' cannot use " +
                $"'{illegalAttributes[0].GetType().FullName}', because the attribute is not " +
                $"applicable for this field category.");
        }
        
        // Build component metadata
        var component = binder.GetFieldCollection(binding.Category).BuildComponent(property);
        
        return new FieldMetadata(component)
        {
            Id = property.Name,
            Hidden = attribute?.Hidden ?? false,
            Label = attribute?.Label ?? property.Name,
            OrderIndex = attribute?.OrderIndex ?? 0,
            CustomProperties = property.GetCustomProperties(binder),
            EventHandlers = eventHandlerAttributes
                .Select(t => t.ToMetadata(property, binder))
                .ToList()
        };
    }
}

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When to Use Custom Factories

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Use DefaultMetadataFactory when:

  • Configuration is simple key-value pairs
  • No data transformation is needed
  • Standard validation is sufficient

​
Create a custom factory when:

  • You need to compute metadata from multiple sources
  • Configuration requires transformation (e.g., enum to dropdown items)
  • You need to validate complex business rules
  • You want to integrate with external services via DI
  • You need to generate metadata dynamically based on types

​
Override AugmentConfiguration when:

  • You want default behavior plus custom additions
  • You need to modify the result dictionary
  • You want to add computed properties

​
Implement IMetadataFactory from scratch when:

  • You need completely different metadata structure
  • Default behavior doesn’t fit your use case
  • You want full control over the metadata generation

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Best Practices

  1. Extend DefaultMetadataFactory: Start with the default and override AugmentConfiguration rather than implementing IMetadataFactory from scratch
  2. Use the DI container: Access services through binder.Container.GetService() for testability
  3. Validate early: Throw BindingException with clear messages when configuration is invalid
  4. Keep it pure: Avoid side effects in metadata generation
  5. Cache expensive operations: Use static caches for reflection-heavy operations
  6. Document metadata structure: Clearly document what metadata your factory produces for frontend developers

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Next Steps

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