android网络框架volley学习之Volley类

开题篇中最后提到了使用volley的三个步骤，这一篇就从步骤一的源码看起：建立RequestQueue对象。建立RequestQueue对象是同过Volley的静态方法建立的。

Volley类中，提供了三个生成RequestQueue的方法。两个利用默认的缓存目录生成RequestQueue的重载方法newRequestQueue，和一个用户自定义缓存目录的newRequestQueueInDisk方法。

下面看看源码

public class Volley {
    private static final String DEFAULT_CACHE_DIR = "volley";//默认的缓存目录
    //这里将context对象传入，保证了网络访问过程中activity销毁时，网络访问也会中断
    public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
        File cacheDir = new File(context.getCacheDir(), DEFAULT_CACHE_DIR);//建立缓存文件目录
        String userAgent = "volley/0";
        try {
            String packageName = context.getPackageName();//获得包名
            PackageInfo info = context.getPackageManager().getPackageInfo(packageName, 0);//获取包的信息
            userAgent = packageName + "/" + info.versionCode; //构造新的userAgent
        } catch (NameNotFoundException e) {
        }
//这里利用接口httpStack指向其实现此接口的类对象（HttpClientStack、HurlStack）
        if (stack == null) {
            if (Build.VERSION.SDK_INT >= 9) {
                stack = new HurlStack();//根据httpURLconnection建立一个httpStack
            } else {
                stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));//根据httpclient建立httpstack
            }
        }
//这里利用接口network指向实现此接口的类对象BasicNetwork，构建Network对象network。
        Network network = new BasicNetwork(stack);
        /*利用匿名对象建立cache对象（多态的又一次应用，实际上引用的是DiskBasedCache对象）
         *在这不难看出建立requestQueue对象的两个参数为cache缓存对象和network对象。其中缓存对象
         *负责从缓存文件中来获取对应请求的相应response，而network则是通过其内部封装的HttpStack
         *（HurlStack或者是HttpClientStack）来进行网络访问*/
        RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
        queue.start();//请求队列工作
        return queue;
    }

    /**
     * context 
     * dir sdcard's root dire
     * null
     * 用给定的文件目录建立缓存文件系统
     * */
    public static RequestQueue newRequestQueueInDisk(Context context, String dir, HttpStack stack) {
        File cacheDir = new File(dir, DEFAULT_CACHE_DIR);

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

        if (stack == null) {
            if (Build.VERSION.SDK_INT >= 9) {
                stack = new HurlStack();
            } else {
                stack = new HttpClientStack(AndroidHttpClient.newInstance(userAgent));
            }
        }

        Network network = new BasicNetwork(stack);

        RequestQueue queue = new RequestQueue(new DiskBasedCache(cacheDir), network);
        queue.start();

        return queue;
    }

    /**
     * Creates a default instance of the worker pool and calls {@link RequestQueue#start()} on it.
     *
     * @param context A {@link Context} to use for creating the cache dir.
     * @return A started {@link RequestQueue} instance.
     */
    public static RequestQueue newRequestQueue(Context context) {
        return newRequestQueue(context, null);
    }
}