c# - GoF Factory的命名约定？

Question

此模式使用抽象工厂，然后使用工厂的实现。

我确定这两个类有一个标准的命名约定，但我不知道它是什么。

例如：

public abstract class ChocolateFactory { };

public class MyChocolateFactory { } : ChocolateFactory

这里的标准约定是什么？

我在考虑ChocolateFactoryBase或ConcreteChocolateFactory，但也许还有其他东西（就像Enums常​​常在Enum后缀，例如PetTypeEnum以便您可以执行PetTypeEnum PetType;

希望这不是主观的。

Answer 1

问题与答案

好的，这个问题从抽象工厂的命名问题开始。根据经验，请始终使用您正在做的“正式”名称（例如Factory，Decorator等）和具体实现的描述（例如Snickers，Mars，MotifWidget等）。

因此，基本上，您创建的MSSQLConnection是您要描述的具体内容，而Factory则意味着它遵循了工厂模式的特征。

好的，到目前为止，是命名和原始问题。现在为酷的东西。讨论是在C＃中实现抽象工厂的最佳方法，这是一个不同的主题。我在用C＃实现所有设计模式方面做了很多工作，在这里我将分享有关工厂的一些详细信息。开始：

抽象工厂和工厂

抽象工厂基本上是基类或接口与具体实现的组合。如果共享大量代码，则需要基类；否则，则需要接口。

我通常区分“工厂”和“抽象工厂”。工厂是创建（某种类型的）对象的事物，“抽象工厂”是创建任意类型的对象的事物。这样，可以得出结论，抽象工厂的实现就是工厂。这与下一条信息有关。

工厂模式

支持RTTI的语言能够实现工厂模式。工厂模式是创建对象的事物。最简单的实现是仅包含创建对象的方法的类，例如：

// ...

public void CreateConnection()
{
    return new SqlConnection();
}

// ...

通常，您可以使用它来抽象事物。例如，HTML解析器中生成XML节点的事物会基于HTML标记创建某种类型的节点。

工厂经常根据运行时信息做出决策。因此，可以泛化Factory模式以实现以下内容：

public T Create(string name) 
{
    // lookup constructor, invoke.
}

使用RTTI创建通用工厂模式非常容易，该模式为每个名称存储一个 Type。查找名称，使用反射创建对象。做完了

哦，此外，与手工制造所有工厂相比，您必须编写更少的代码。因为所有实现都是相同的，所以您最好将其放在基类中，然后将Dictionary填充到静态构造函数中。

概括抽象工厂

抽象工厂基本上是工厂的集合，它们以与工厂模式相同的方式创建对象。唯一共享的是接口（例如，创建或您可以使用继承来创建抽象）。

该实现非常琐碎，因此我只保留它。

解耦工厂和类型

让我们回到GoF示例。他们谈论 MotifFactory和 PMFactory。将来我们还会遇到另一个UI东西，我们需要一个 ASPNETFactory或 SilverlightFactory。但是，未来是未知的，如果我们不需要，则不要发布旧的DLL-毕竟这不灵活。

如果我们想向工厂添加新方法，则会出现第二个问题。因此，这样做将涉及更换所有工厂。您可能已经猜到了，我不想在多个地方进行更改。

幸运的是，我们可以解决这两个问题。接口是相同的（甚至可以泛化），因此我们可以在运行时简单地向工厂添加新功能。

无需告诉工厂要创建什么对象，我们可以使用属性告诉类某个工厂应该实现它。我们还可以在加载程序集时扫描所有类型，因此，如果加载了程序集，我们可以简单地动态建立新工厂。

我为此牺牲的是编译时检查，但是由于Factory模式通常使用运行时信息，所以这不一定是问题。

总结一下，这是我工厂的代码：

/// <summary>
/// This attribute is used to tag classes, enabling them to be constructed by a Factory class. See the <see cref="Factory{Key,Intf}"/> 
/// class for details.
/// </summary>
/// <remarks>
/// <para>
/// It is okay to mark classes with multiple FactoryClass attributes, even when using different keys or different factories.
/// </para>
/// </remarks>
/// <seealso cref="Factory{Key,Intf}"/>
[AttributeUsage(AttributeTargets.Class, AllowMultiple = true, Inherited = false)]
public class FactoryClassAttribute : Attribute
{
    /// <summary>
    /// This marks a class as eligible for construction by the specified factory type.
    /// </summary>
    /// <example>
    /// [FactoryClass("ScrollBar", typeof(MotifFactory))]
    /// public class MotifScrollBar : IControl { }
    /// </example>
    /// <param name="key">The key used to construct the object</param>
    /// <param name="factoryType">The type of the factory class</param>
    public FactoryClassAttribute(object key, Type factoryType)
    {
        if ((factoryType.IsGenericType &&
             factoryType.GetGenericTypeDefinition() == typeof(Factory<,>)) ||
            factoryType.IsAbstract || 
            factoryType.IsInterface)
        {
            throw new NotSupportedException("Incorrect factory type: you cannot use GenericFactory or an abstract type as factory.");
        }
        this.Key = key;
        this.FactoryType = factoryType;
    }

    /// <summary>
    /// The key used to construct the object when calling the <see cref="Factory{Key,Intf}.Create(Key)"/> method.
    /// </summary>
    public object Key { get; private set; }

    /// <summary>
    /// The type of the factory class
    /// </summary>
    public Type FactoryType { get; private set; }
}

/// <summary>
/// Provides an interface for creating related or dependent objects.
/// </summary>
/// <remarks>
/// <para>
/// This class is an implementation of the Factory pattern. Your factory class should inherit this Factory class and 
/// you should use the [<see cref="FactoryClassAttribute"/>] attribute on the objects that are created by the factory.
/// The implementation assumes all created objects share the same constructor signature (which is not checked by the Factory). 
/// All implementations also share the same <typeparamref name="Intf"/> type and are stored by key. During runtime, you can 
/// use the Factory class implementation to build objects of the correct type.
/// </para>
/// <para>
/// The Abstract Factory pattern can be implemented by adding a base Factory class with multiple factory classes that inherit from 
/// the base class and are used for registration. (See below for a complete code example).
/// </para>
/// <para>
/// Implementation of the Strategy pattern can be done by using the Factory pattern and making the <typeparamref name="Intf"/>
/// implementations algorithms. When using the Strategy pattern, you still need to have some logic that picks when to use which key.
/// In some cases it can be useful to use the Factory overload with the type conversion to map keys on other keys. When implementing 
/// the strategy pattern, it is possible to use this overload to determine which algorithm to use.
/// </para>
/// </remarks>
/// <typeparam name="Key">The type of the key to use for looking up the correct object type</typeparam>
/// <typeparam name="Intf">The base interface that all classes created by the Factory share</typeparam>
/// <remarks>
/// The factory class automatically hooks to all loaded assemblies by the current AppDomain. All classes tagged with the FactoryClass
/// are automatically registered.
/// </remarks>
/// <example>
/// <code lang="c#">
/// // Create the scrollbar and register it to the factory of the Motif system
/// [FactoryClass("ScrollBar", typeof(MotifFactory))]
/// public class MotifScrollBar : IControl { }
/// 
/// // [...] add other classes tagged with the FactoryClass attribute here...
///
/// public abstract class WidgetFactory : Factory&lt;string, IControl&gt;
/// {
///     public IControl CreateScrollBar() { return Create("ScrollBar") as IScrollBar; }
/// }
///
/// public class MotifFactory : WidgetFactory { }
/// public class PMFactory : WidgetFactory { }
///
/// // [...] use the factory to create a scrollbar
/// 
/// WidgetFactory widgetFactory = new MotifFactory();
/// var scrollbar = widgetFactory.CreateScrollBar(); // this is a MotifScrollbar intance
/// </code>
/// </example>
public abstract class Factory<Key, Intf> : IFactory<Key, Intf>
    where Intf : class
{
    /// <summary>
    /// Creates a factory by mapping the keys of the create method to the keys in the FactoryClass attributes.
    /// </summary>
    protected Factory() : this((a) => (a)) { }

    /// <summary>
    /// Creates a factory by using a custom mapping function that defines the mapping of keys from the Create 
    /// method, to the keys in the FactoryClass attributes.
    /// </summary>
    /// <param name="typeConversion">A function that maps keys passed to <see cref="Create(Key)"/> to keys used with [<see cref="FactoryClassAttribute"/>]</param>
    protected Factory(Func<Key, object> typeConversion)
    {
        this.typeConversion = typeConversion;
    }

    private Func<Key, object> typeConversion;
    private static object lockObject = new object();
    private static Dictionary<Type, Dictionary<object, Type>> dict = null;

    /// <summary>
    /// Creates an instance a class registered with the <see cref="FactoryClassAttribute"/> attribute by looking up the key.
    /// </summary>
    /// <param name="key">The key used to lookup the attribute. The key is first converted using the typeConversion function passed 
    /// to the constructor if this was defined.</param>
    /// <returns>An instance of the factory class</returns>
    public virtual Intf Create(Key key)
    {
        Dictionary<Type, Dictionary<object, Type>> dict = Init();
        Dictionary<object, Type> factoryDict;
        if (dict.TryGetValue(this.GetType(), out factoryDict))
        {
            Type t;
            return (factoryDict.TryGetValue(typeConversion(key), out t)) ? (Intf)Activator.CreateInstance(t) : null;
        }
        return null;
    }

    /// <summary>
    /// Creates an instance a class registered with the <see cref="FactoryClassAttribute"/> attribute by looking up the key.
    /// </summary>
    /// <param name="key">The key used to lookup the attribute. The key is first converted using the typeConversion function passed 
    /// to the constructor if this was defined.</param>
    /// <param name="constructorParameters">Additional parameters that have to be passed to the constructor</param>
    /// <returns>An instance of the factory class</returns>
    public virtual Intf Create(Key key, params object[] constructorParameters)
    {
        Dictionary<Type, Dictionary<object, Type>> dict = Init();
        Dictionary<object, Type> factoryDict;
        if (dict.TryGetValue(this.GetType(), out factoryDict))
        {
            Type t;
            return (factoryDict.TryGetValue(typeConversion(key), out t)) ? (Intf)Activator.CreateInstance(t, constructorParameters) : null;
        }
        return null;
    }

    /// <summary>
    /// Enumerates all registered attribute keys. No transformation is done here.
    /// </summary>
    /// <returns>All keys currently known to this factory</returns>
    public virtual IEnumerable<Key> EnumerateKeys()
    {
        Dictionary<Type, Dictionary<object, Type>> dict = Init();
        Dictionary<object, Type> factoryDict;
        if (dict.TryGetValue(this.GetType(), out factoryDict))
        {
            foreach (object key in factoryDict.Keys)
            {
                yield return (Key)key;
            }
        }
    }

    private void TryHook()
    {
        AppDomain.CurrentDomain.AssemblyLoad += new AssemblyLoadEventHandler(NewAssemblyLoaded);
    }

    private Dictionary<Type, Dictionary<object, Type>> Init()
    {
        Dictionary<Type, Dictionary<object, Type>> d = dict;
        if (d == null)
        {
            lock (lockObject)
            {
                if (dict == null)
                {
                    try
                    {
                        TryHook();
                    }
                    catch (Exception) { } // Not available in this security mode. You're probably using shared hosting

                    ScanTypes();
                }
                d = dict;
            }
        }
        return d;
    }

    private void ScanTypes()
    {
        Dictionary<Type, Dictionary<object, Type>> classDict = new Dictionary<Type, Dictionary<object, Type>>();
        foreach (Assembly ass in AppDomain.CurrentDomain.GetAssemblies())
        {
            AddAssemblyTypes(classDict, ass);
        }
        dict = classDict;
    }

    private void AddAssemblyTypes(Dictionary<Type, Dictionary<object, Type>> classDict, Assembly ass)
    {
        try
        {
            foreach (Type t in ass.GetTypes())
            {
                if (t.IsClass && !t.IsAbstract &&
                    typeof(Intf).IsAssignableFrom(t))
                {
                    object[] fca = t.GetCustomAttributes(typeof(FactoryClassAttribute), false);
                    foreach (FactoryClassAttribute f in fca)
                    {
                        if (!(f.Key is Key))
                        {
                            throw new InvalidCastException(string.Format("Cannot cast key of factory object {0} to {1}", t.FullName, typeof(Key).FullName));
                        }
                        Dictionary<object, Type> keyDict;
                        if (!classDict.TryGetValue(f.FactoryType, out keyDict))
                        {
                            keyDict = new Dictionary<object, Type>();
                            classDict.Add(f.FactoryType, keyDict);
                        }
                        keyDict.Add(f.Key, t);
                    }
                }
            }
        }
        catch (ReflectionTypeLoadException) { } // An assembly we cannot process. That also means we cannot use it.
    }

    private void NewAssemblyLoaded(object sender, AssemblyLoadEventArgs args)
    {
        lock (lockObject)
        {
            // Make sure new 'create' invokes wait till we're done updating the factory
            Dictionary<Type, Dictionary<object, Type>> classDict = new Dictionary<Type, Dictionary<object, Type>>(dict);
            dict = null;
            Thread.MemoryBarrier();

            AddAssemblyTypes(classDict, args.LoadedAssembly);
            dict = classDict;
        }
    }
}