突破 Silverlight 自身限制，做更好的动态加载导航机制（二）

在上一篇文章中，动态导航的思路已经比较完善了，现在来实现动态导航机制。

实现按需加载 Silverlight 组件

Silverlight 客户端加载一个程序集很容易，关键是如何分析并加载程序集引用的其它程序集，这些程序集又会引用另外的程序集，然后再加在这些程序集。

借助于 Mono.Cecil ，可以在客户端很容易的分析出程序集引用的其它程序集。

至于如何加载，我的实现思路是，做一个下载队列，每下载一个程序集，分析其引用的程序集列表，找出其没有加载过的程序集，添加到下载队列，从下载队列中删除下载过的程序集，如果队列不为空，则依次进行递归；否则，触发下载完成事件。实现代如下：

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public class AssemblyDownloader {
 
   private static readonly IDictionary<string assembly="" ,=""> LoadedAssemblies = new Dictionary<string assembly="" ,="">(StringComparer.OrdinalIgnoreCase);
   private readonly ISet<string> _loadingSet = new HashSet<string>();
   private static readonly object LoadingSetLock = new object();
    
   private static readonly string[] SilverlightRuntimeAssemblyNames = new[] {
      "Microsoft.VisualBasic.dll",
      "mscorlib.dll",
      "System.Core.dll",
      "System.dll",
      "System.Net.dll",
      "System.Runtime.Serialization.dll",
      "System.ServiceModel.dll",
      "System.ServiceModel.Web.dll",
      "System.Windows.Browser.dll",
      "System.Windows.dll",
      "System.Windows.RuntimeHost.dll",
      "System.Xml.dll"
   };
    
   private bool _isbusy;
   private string _loadingAssemblyName;
 
   public event EventHandler<downloadassemblycommpletedeventargs> DownloadAssemblyCommpleted;
   public event EventHandler<asynccompletedeventargs> DownloadFailed;
 
   public Assembly GetAssembly(string assemblyName) {
      return LoadedAssemblies.ContainsKey(assemblyName) ? LoadedAssemblies[assemblyName] : null;
   }
 
   public void OnDownloadFailed(Exception ex) {
      var handler = this.DownloadFailed;
      if (handler != null) {
         handler(this, new AsyncCompletedEventArgs(ex, true, null));
      }
   }
 
   private void OnDownloadAssemblyCommpleted(DownloadAssemblyCommpletedEventArgs e) {
      var handler = this.DownloadAssemblyCommpleted;
      if (handler != null) {
         handler(this, e);
      }
   }
 
   public void DownloadAssemblyAsync(string assemblyName) {
      if (this._isbusy) {
         throw new InvalidOperationException(string.Format("AssemblyDownloader is loading {0}, please waite ...", this._loadingAssemblyName));
      }
      assemblyName = EnsureEndWdithDll(assemblyName);
      this._loadingAssemblyName = assemblyName;
      if (IsAssemblyLoaded(assemblyName)) {
         this.DownloadCompleted();
      }
      else {
         DownloadAssemblyAsyncCore(assemblyName);
      }
   }
 
   private void DownloadAssemblyAsyncCore(string assemblyName) {
      this._isbusy = true;
      assemblyName = EnsureEndWdithDll(assemblyName);
      var name = assemblyName;
      var webClient = new WebClient();
      webClient.OpenReadCompleted += (sender, e) => this.OnReadOneAssembly(name, e);
      try {
         webClient.OpenReadAsync(new Uri(assemblyName, UriKind.Relative));
      }
      catch (Exception ex) {
         this.OnDownloadFailed(ex);
      }
   }
 
   private static string EnsureEndWdithDll(string assemblyName) {
      if (!assemblyName.EndsWith(".dll", StringComparison.OrdinalIgnoreCase)) {
         assemblyName += ".dll";
      }
      return assemblyName;
   }
 
   private void OnReadOneAssembly(string name, OpenReadCompletedEventArgs e) {
      if (e.Error != null) {
         this.OnDownloadFailed(e.Error);
         this._isbusy = false;
         return;
      }
      var assemblyStream = e.Result;
      var references = GetReferenceAssemblyNames(assemblyStream);
      AddNotLoadedReferenceAssemblyToLoadingSet(references);
 
      assemblyStream.Seek(0, SeekOrigin.Begin);
      LoadToAssemblyPart(assemblyStream, name);
 
      if (this._loadingSet.Count > 0) {
         var asm = this._loadingSet.First();
         lock (LoadingSetLock) {
            this._loadingSet.Remove(asm);
         }
         this.DownloadAssemblyAsyncCore(asm);
      }
      else {
         this.DownloadCompleted();
      }
   }
 
   private void DownloadCompleted() {
      this._isbusy = false;
      var assembly = this.GetAssembly(this._loadingAssemblyName);
      this.OnDownloadAssemblyCommpleted(new DownloadAssemblyCommpletedEventArgs(assembly));
   }
 
   private static void LoadToAssemblyPart(Stream assemblyStream, string name) {
      var part = new AssemblyPart {
         Source = name
      };
      var assembly = part.Load(assemblyStream);
      LoadedAssemblies.Add(name, assembly);
   }
 
   private void AddNotLoadedReferenceAssemblyToLoadingSet(IEnumerable<string> references) {
      var referencesNotLoaded = from reference in references
                                where !(IsAssemblyLoaded(reference))
                                select reference;
      foreach (var @ref in referencesNotLoaded) {
         lock (LoadingSetLock) {
            if (!this._loadingSet.Contains(@ref)) {
               this._loadingSet.Add(@ref);
            }
         }
      }
   }
 
   private static bool IsAssemblyLoaded(string assemblyName) {
      return SilverlightRuntimeAssemblyNames.Any(asmName => asmName.Equals(assemblyName, StringComparison.OrdinalIgnoreCase))
         || LoadedAssemblies.ContainsKey(assemblyName)
         || Deployment.Current.Parts.Any(ap => ap.Source.Equals(assemblyName, StringComparison.OrdinalIgnoreCase));
   }
 
   private static IEnumerable<string> GetReferenceAssemblyNames(Stream assemblyStream) {
      var asmDef = AssemblyDefinition.ReadAssembly(assemblyStream);
      return asmDef.MainModule.AssemblyReferences.Select(anr => anr.Name + ".dll");
   }
}</string></string></asynccompletedeventargs></downloadassemblycommpletedeventargs></string></string></string></string>

实现一个自定义的 ContentLoader

实现自定义的 ContentLoader 很容易，只要实现 INavigationContentLoader 接口即可，结合上面的AssemblyDownloader，实现代码如下：

public class MyContentLoader : INavigationContentLoader {
 
   private static AssemblyDownloader _assemblyDownloader;
 
   public IAsyncResult BeginLoad(Uri targetUri, Uri currentUri, AsyncCallback userCallback, object asyncState) {
      var typeFullName = targetUri.ToString();
      if (string.IsNullOrEmpty(typeFullName)) {
         return null;
      }
      var arr = typeFullName.Split(',');
      var typeName = arr[0];
      var assemblyName = arr[1];
      if (!assemblyName.EndsWith(".dll", StringComparison.OrdinalIgnoreCase)) {
         assemblyName += ".dll";
      }
      var asyncResult = new MyContentLoaderAsyncResult {
         AsyncState = asyncState,
         TypeName = typeName,
         AssemblyName = assemblyName
      };
      BeginLoadCore(userCallback, asyncResult);
      return asyncResult;
   }
 
   private static void BeginLoadCore(AsyncCallback userCallback, MyContentLoaderAsyncResult result) {
      if (_assemblyDownloader == null) {
         _assemblyDownloader = new AssemblyDownloader();
      }
      var handlers = new EventHandler<downloadassemblycommpletedeventargs>[1];
      handlers[0] = (sender, e) => {
         _assemblyDownloader.DownloadAssemblyCommpleted -= handlers[0];
         result.Assembly = e.Result;
         userCallback(result);
      };
      _assemblyDownloader.DownloadAssemblyCommpleted += handlers[0];
      _assemblyDownloader.DownloadAssemblyAsync(result.AssemblyName);
   }
 
   public void CancelLoad(IAsyncResult asyncResult) {
   }
 
   public LoadResult EndLoad(IAsyncResult asyncResult) {
      var result = asyncResult as MyContentLoaderAsyncResult;
      if (result == null) {
         throw new InvalidOperationException(string.Format("Wrong kind of {0} passed in.  The {0} passed in should only come from {1}.", "IAsyncResult", "MyContentLoader.BeginLoad"));
      }
      var loadResult = new LoadResult(result.GetResultInstance());
      return loadResult;
   }
 
   public bool CanLoad(Uri targetUri, Uri currentUri) {
      return targetUri.ToString().Split(',').Length == 2;
   }
}</downloadassemblycommpletedeventargs>

使用自定义的 ContentLoader

只要设置 Frame 控件的 ContentLoader 为自定义的 ContentLoader 即可，比如可以这样使用：

<hyperlinkbutton navigationuri="Widget1.HomeWidget, Widget1" navigationtarget="NavFrame">
<hyperlinkbutton navigationuri="Widget2.HomeWidget, Widget2" navigationtarget="NavFrame">
<sdk:frame name="NavFrame">
   <sdk:frame.contentloader>
      <my:mycontentloader>
   </SDK:FRAME.CONTENTLOADER />
</my:mycontentloader></sdk:frame.contentloader></sdk:frame></hyperlinkbutton></hyperlinkbutton>