Android Volley分析（二）——实现

在Android Volley分析（一）——结构中主要分析了Volley的基本组件和框架结构，组件主要是定义的接口，也就是说我们可以实现这些接口来定制自己的Volley版本，比如NetWork、Cache、Request等等。Android Volley在com.android.volley.toolbox下已经做了这些工作，下面就看看这些具体的实现内容

先看一个Volley使用的例子

final TextView mTextView = (TextView) findViewById(R.id.text);
...

// Instantiate the RequestQueue.
RequestQueue queue = Volley.newRequestQueue(this);
String url ="http://www.google.com";

// Request a string response from the provided URL.
StringRequest stringRequest = new StringRequest(Request.Method.GET, url,
            new Response.Listener() {
    @Override
    public void onResponse(String response) {
        // Display the first 500 characters of the response string.
        mTextView.setText("Response is: "+ response.substring(0,500));
    }
}, new Response.ErrorListener() {
    @Override
    public void onErrorResponse(VolleyError error) {
        mTextView.setText("That didn't work!");
    }
});
// Add the request to the RequestQueue.
queue.add(stringRequest);

这是官方volley教程的一个例子，使用Volley的步骤

1、Volley.newRequestQueue();

2、new Request();

3、queue.add(request);

在newRequestQueue的时候会初始化基本的组件

    public static RequestQueue newRequestQueue(Context context, HttpStack stack) {
        File cacheDir = new File(context.getCacheDir(), 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 {
                // 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 = new RequestQueue(new DiskBasedCache(cacheDir), network);
        queue.start();

        return queue;
    }

1、BasicNetwork

BasicNetwork是对Network接口的实现，同时还将联网操作部分提取出来形成HttpStack，原因是Android 2.3之前网络操作推荐使用HttpClient，2.3之后Android做了优化推荐使用HttpUrlConnection。

2、DiskBasedCache

DiskBasedCache是对Cache对实现，用于硬盘缓存，也就是将数据保存为特定的文件。

结合LruCache试着总结一下缓存的一般方法：

映射结构——LinkedHashMap，内部实现了Lru的算法排序，可以直接使用

put——存储，容量计数增加，判断是否超出最大容量，超出则删除最少使用的

get——key到entry的映射

remove——删除，容量计数减小

3、ImageLoader

ImageLoader是一个图片加载类，封装了请求，提供了内存缓存机制，批处理等，让图片加载更容易使用，下面看一下它的用法：

        mImageLoader = new ImageLoader(mRequestQueue,
                new ImageLoader.ImageCache() {
            private final LruCache<String, Bitmap>
                    cache = new LruCache<String, Bitmap>(20);

            @Override
            public Bitmap getBitmap(String url) {
                return cache.get(url);
            }

            @Override
            public void putBitmap(String url, Bitmap bitmap) {
                cache.put(url, bitmap);
            }

mImageLoader.get(IMAGE_URL, ImageLoader.getImageListener(mImageView,
         R.drawable.def_image, R.drawable.err_image));

（示例来源 Making a Standard Request ）

内存缓存推荐使用LruCache，需要实现ImageLoader.ImageCache接口。图片的加载通过imageLoader.get()方法实现，

    public ImageContainer get(String requestUrl, ImageListener imageListener,
            int maxWidth, int maxHeight) {
        // only fulfill requests that were initiated from the main thread.
        throwIfNotOnMainThread();

        final String cacheKey = getCacheKey(requestUrl, maxWidth, maxHeight);

        // Try to look up the request in the cache of remote images.
        Bitmap cachedBitmap = mCache.getBitmap(cacheKey);
        if (cachedBitmap != null) {
            // Return the cached bitmap.
            ImageContainer container = new ImageContainer(cachedBitmap, requestUrl, null, null);
            imageListener.onResponse(container, true);
            return container;
        }

        // The bitmap did not exist in the cache, fetch it!
        ImageContainer imageContainer =
                new ImageContainer(null, requestUrl, cacheKey, imageListener);

        // Update the caller to let them know that they should use the default bitmap.
        imageListener.onResponse(imageContainer, true);

        // Check to see if a request is already in-flight.
        BatchedImageRequest request = mInFlightRequests.get(cacheKey);
        if (request != null) {
            // If it is, add this request to the list of listeners.
            request.addContainer(imageContainer);
            return imageContainer;
        }

        // The request is not already in flight. Send the new request to the network and
        // track it.
        Request<?> newRequest =
            new ImageRequest(requestUrl, new Listener<Bitmap>() {
                @Override
                public void onResponse(Bitmap response) {
                    onGetImageSuccess(cacheKey, response);
                }
            }, maxWidth, maxHeight,
            Config.RGB_565, new ErrorListener() {
                @Override
                public void onErrorResponse(VolleyError error) {
                    onGetImageError(cacheKey, error);
                }
            });

        mRequestQueue.add(newRequest);
        mInFlightRequests.put(cacheKey,
                new BatchedImageRequest(newRequest, imageContainer));
        return imageContainer;
    }

首先得到一个cacheKey，这个是统一的规则

    private static String getCacheKey(String url, int maxWidth, int maxHeight) {
        return new StringBuilder(url.length() + 12).append("#W").append(maxWidth)
                .append("#H").append(maxHeight).append(url).toString();
    }

然后在缓存cache中查找，没找到就讲请求添加到批处理任务BitmapImageRequest中，这里有两个类，ImageContainer和BitmapImageRequest：

ImageContainer是一个包装类，里面包含了bitmap、url、cacheKey和listener，当得到bitmap后会通过listener将bitmap返回；

BitmapImageRequest是将相同url请求放在一起，这样就可以只向网络请求一次，返回结果后再分别设置到目标上，从而达到减少网络请求对目的。