浅谈BlogEngine扩展机制的实现

时间：2008-09-03 来源：david_shar

在Blogengine里，每一个扩展的Class都必须有Extension标记
ExtensionAttribute是一个密封类，继承自System.Attribute,System.Attribute没有任何实现，只是一个标记而已。
ExtensionAttribute类的代码:(完整代码可以从BlogEngine.NET.zip下载)

[AttributeUsage(AttributeTargets.Class)] //指明该Attrubite只能应用于Class上
public sealed class ExtensionAttribute : System.Attribute
{
/// <summary>
/// Creates an instance of the attribute and assigns a description.
/// </summary>
public ExtensionAttribute(string description, string version, string author)
{
_Description = description;
_Version = version;
_Author = author;
}
private string _Description;
/// <summary>
/// Gets the description of the extension.
/// </summary>
public string Description
{
get { return _Description; }
}
private string _Version;
/// <summary>
/// Gets the version number of the extension
/// </summary>
public string Version
{
get { return _Version; }
}
private string _Author;
/// <summary>
/// Gets the author of the extension
/// </summary>
public string Author
{
get { return _Author; }
}
}

[AttributeUsage(AttributeTargets.Class)] //指明该Attrubite只能应用于Class上 public sealed class ExtensionAttribute : System.Attribute { /// <summary> /// Creates an instance of the attribute and assigns a description. /// </summary> public ExtensionAttribute(string description, string version, string author) { _Description = description; _Version = version; _Author = author; } private string _Description; /// <summary> /// Gets the description of the extension. /// </summary> public string Description { get { return _Description; } } private string _Version; /// <summary> /// Gets the version number of the extension /// </summary> public string Version { get { return _Version; } } private string _Author; /// <summary> /// Gets the author of the extension /// </summary> public string Author { get { return _Author; } } }

这个类非常之简单，只是一些描述性的信息，如作者、版本、描述等。
到这里，ExtensionAttribute类就可以应用在任何一个Class上了
看看mp3player.cs

/// <summary>
/// 增加flashMp3播放器
/// </summary>
[Extension("mp3 player", "1.0.0.0", "lemongtree.com")] //此处给mp3player类打上Extension标记
public class mp3player
{
public mp3player()
{
Post.Serving += new EventHandler<ServingEventArgs>(Post_Serving);
}
private void Post_Serving(object sender, ServingEventArgs e)
{
//此处略过
}
}

/// <summary> /// 增加flashMp3播放器 /// </summary> [Extension("mp3 player", "1.0.0.0", "lemongtree.com")] //此处给mp3player类打上Extension标记 public class mp3player { public mp3player() { Post.Serving += new EventHandler<ServingEventArgs>(Post_Serving); } private void Post_Serving(object sender, ServingEventArgs e) { //此处略过 } }

mp3player就是一个针对于Post的扩展了。
有朋友可能会问了:mp3player这个在什么时候实例化的呢？或是我如何得知这个扩展是不是Enable的呢?
整个实例化的过程在global.asax的Application_Start事件中

Assembly a = Assembly.Load(assemblyName);
Type[] types = a.GetTypes(); //获取当前程序集中的所有类型
foreach (Type type in types)
{
object[] attributes = type.GetCustomAttributes(typeof(ExtensionAttribute), false); //获取含有ExtensionAttribute标记的类
foreach (object attribute in attributes)
{
if (ExtensionManager.ExtensionEnabled(type.Name))
{
a.CreateInstance(type.FullName); //创建实例
}
}
}

Assembly a = Assembly.Load(assemblyName); Type[] types = a.GetTypes(); //获取当前程序集中的所有类型 foreach (Type type in types) { object[] attributes = type.GetCustomAttributes(typeof(ExtensionAttribute), false); //获取含有ExtensionAttribute标记的类 foreach (object attribute in attributes) { if (ExtensionManager.ExtensionEnabled(type.Name)) { a.CreateInstance(type.FullName); //创建实例 } } }

到这里很明了了，扩展类决定于它有没有打Extension标记，是否创建实例取决于该扩展有没有被启用.
BlogEngine有很多地方是值得我们学习的.