volley3--Volley类

Volley这个类，Volley作为整个框架的入口，其实就是创建了一个RequestQueue队列

 public class Volley {  

     /**
      * Default on-disk cache directory.
      * 默认缓存目录名
      */
     private static final String DEFAULT_CACHE_DIR = "volley";  

     /**
      * Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it.
      * You may set a maximum size of the disk cache in bytes.
      * 创建一个默认工作池，并且启动请求队列
      * @param context A {@link Context} to use for creating the cache dir.用于创建缓存目录
      * @param stack An {@link HttpStack} to use for the network, or null for default.
      * @param maxDiskCacheBytes the maximum size of the disk cache, in bytes. Use -1 for default size.
      * 硬盘缓存最大值，-1则默认
      * @return A started {@link RequestQueue} instance.
      */
     public static RequestQueue newRequestQueue(Context context, HttpStack stack, int maxDiskCacheBytes) {
         File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);//缓存队列  

         String userAgent = "volley/0";
         try {
             String packageName = context.getPackageName();//包名
             PackageInfo info = context.getPackageManager().getPackageInfo(packageName, );
             userAgent = packageName + "/" + info.versionCode;
         } catch (NameNotFoundException e) {
         }  

         if (stack == null) {//如果没有限定stack
             if (Build.VERSION.SDK_INT >= ) {//adk版本在9或者以上
                 stack = new HurlStack();
             } else {
                 // Prior to Gingerbread, HttpUrlConnection was unreliable.
                 // See: http://android-developers.blogspot.com/2011/09/androids-http-clients.html
                 stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
             }
         }  

         Network network = new BasicNetwork(stack);  

         RequestQueue queue;
         if (maxDiskCacheBytes <= -)//小于等于-1,则默认值
         {
             // No maximum size specified
             queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
         }
         else
         {
             // Disk cache size specified
             queue = new RequestQueue(new DiskBasedCache(cacheDir, maxDiskCacheBytes), network);
         }  

         queue.start();  

         return queue;
     }  

可以看到，Volley的newRequestQueue()方法里面，根据版本创建了stack(分别是HurlStack和HttpClientStack)。至于不同adk版本会创建不同的stack，是由于android的版本原因，在9以上版本使用HurlStack比HttpClientStack更加好。

然后创建了网络类BasicNetwork，缓存类DiskBasedCache，然后使用这两个来创建RequestQueue对象。

或许现在大家还不明白这两个类的作用，大家只需要记得创建RequestQueue需要这两个类就可以了。