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, 0);  
            userAgent = packageName + "/" + info.versionCode;  
        } catch (NameNotFoundException e) {  
        }  
  
        if (stack == null) {//如果没有限定stack  
            if (Build.VERSION.SDK_INT >= 9) {//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 <= -1)//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。