Android中关于Volley的使用（十）对Request和Reponse的认识

标签：android   volley   request   response   源码   

我们知道，在网络Http通信中，一定会有一个Request，同样的，也一定会有一个Response，而我们在Volley中利用RequestQueue来添加请求之前，一定会先创建一个Request对象，比如StringRequest，JsonObjectRequest和ImageRequest等，如下分别是前面Demo中的JsonRequest和ImageRequest：

JsonObjectRequest：

    public JsonObjectRequest(String url, JSONObject jsonRequest, Listener<JSONObject> listener,
            ErrorListener errorListener) {

ImageRequest：

    public ImageRequest(String url, Response.Listener<Bitmap> listener, int maxWidth, int maxHeight,
            Config decodeConfig, Response.ErrorListener errorListener) 

public abstract class Request<T> implements Comparable<Request<T>> {

在这个类中，定义了一些请求中基本的参数变量，如

Method:

    /**
     * Request method of this request.  Currently supports GET, POST, PUT, DELETE, HEAD, OPTIONS,
     * TRACE, and PATCH.
     */
    private final int mMethod;

    /**
     * Supported request methods.
     */
    public interface Method {
        int DEPRECATED_GET_OR_POST = -1;
        int GET = 0;
        int POST = 1;
        int PUT = 2;
        int DELETE = 3;
        int HEAD = 4;
        int OPTIONS = 5;
        int TRACE = 6;
        int PATCH = 7;
    }

    /** URL of this request. */
    private final String mUrl;

    /** Listener interface for errors. */
    private final Response.ErrorListener mErrorListener;

    /**
     * Subclasses must implement this to parse the raw network response
     * and return an appropriate response type. This method will be
     * called from a worker thread.  The response will not be delivered
     * if you return null.
     * @param response Response from the network
     * @return The parsed response, or null in the case of an error
     */
    abstract protected Response<T> parseNetworkResponse(NetworkResponse response);

    /**
     * Subclasses must implement this to perform delivery of the parsed
     * response to their listeners.  The given response is guaranteed to
     * be non-null; responses that fail to parse are not delivered.
     * @param response The parsed response returned by
     * {@link #parseNetworkResponse(NetworkResponse)}
     */
    abstract protected void deliverResponse(T response);

1）parseNetworkResponse

当从网络中获取到Response的时候，怎么去解析对应的请求，这是由各个对应的Request去分析的，比如JsonObjectRequest中：

    @Override
    protected Response<JSONObject> parseNetworkResponse(NetworkResponse response) {
        try {
            String jsonString =
                new String(response.data, HttpHeaderParser.parseCharset(response.headers));
            return Response.success(new JSONObject(jsonString),
                    HttpHeaderParser.parseCacheHeaders(response));
        } catch (UnsupportedEncodingException e) {
            return Response.error(new ParseError(e));
        } catch (JSONException je) {
            return Response.error(new ParseError(je));
        }
    }

    @Override
    protected Response<Bitmap> parseNetworkResponse(NetworkResponse response) {
        // Serialize all decode on a global lock to reduce concurrent heap usage.
        synchronized (sDecodeLock) {
            try {
                return doParse(response);
            } catch (OutOfMemoryError e) {
                VolleyLog.e("Caught OOM for %d byte image, url=%s", response.data.length, getUrl());
                return Response.error(new ParseError(e));
            }
        }
    }

而在doParse中，其实是对图片进行处理，如下：

    private Response<Bitmap> doParse(NetworkResponse response) {
        byte[] data = response.data;
        BitmapFactory.Options decodeOptions = new BitmapFactory.Options();
        Bitmap bitmap = null;
        if (mMaxWidth == 0 && mMaxHeight == 0) {
            decodeOptions.inPreferredConfig = mDecodeConfig;
            bitmap = BitmapFactory.decodeByteArray(data, 0, data.length, decodeOptions);
        } else {
            // If we have to resize this image, first get the natural bounds.
            decodeOptions.inJustDecodeBounds = true;
            BitmapFactory.decodeByteArray(data, 0, data.length, decodeOptions);
            int actualWidth = decodeOptions.outWidth;
            int actualHeight = decodeOptions.outHeight;

            // Then compute the dimensions we would ideally like to decode to.
            int desiredWidth = getResizedDimension(mMaxWidth, mMaxHeight,
                    actualWidth, actualHeight);
            int desiredHeight = getResizedDimension(mMaxHeight, mMaxWidth,
                    actualHeight, actualWidth);

            // Decode to the nearest power of two scaling factor.
            decodeOptions.inJustDecodeBounds = false;
            // TODO(ficus): Do we need this or is it okay since API 8 doesn‘t support it?
            // decodeOptions.inPreferQualityOverSpeed = PREFER_QUALITY_OVER_SPEED;
            decodeOptions.inSampleSize =
                findBestSampleSize(actualWidth, actualHeight, desiredWidth, desiredHeight);
            Bitmap tempBitmap =
                BitmapFactory.decodeByteArray(data, 0, data.length, decodeOptions);

            // If necessary, scale down to the maximal acceptable size.
            if (tempBitmap != null && (tempBitmap.getWidth() > desiredWidth ||
                    tempBitmap.getHeight() > desiredHeight)) {
                bitmap = Bitmap.createScaledBitmap(tempBitmap,
                        desiredWidth, desiredHeight, true);
                tempBitmap.recycle();
            } else {
                bitmap = tempBitmap;
            }
        }

        if (bitmap == null) {
            return Response.error(new ParseError(response));
        } else {
            return Response.success(bitmap, HttpHeaderParser.parseCacheHeaders(response));
        }
    }

在上面的方法实现中，我们可以看到，最后都是通过Response.success方法返回一个Response对象，而这个Response对象是怎么用的呢，就要看下面deliverResponse方法了。

2）deliverResponse

在NetworkDispatcher线程中，当从网络中获取到数据，并通过请求的parseNetworkResponse方法解析之后，会返回一个Reponse对象，这个时候，就会调用Executor来将这个请求post回主线程，如下：

mDelivery.postResponse(request, response);

public void postResponse(Request<?> request, Response<?> response, Runnable runnable) {
        request.markDelivered();
        request.addMarker("post-response");
        mResponsePoster.execute(new ResponseDeliveryRunnable(request, response, runnable));
    }

ResponseDeliveryRunnable类的run方法中，我们可以看到：

            // Deliver a normal response or error, depending.
            if (mResponse.isSuccess()) {
                mRequest.deliverResponse(mResponse.result);
            } else {
                mRequest.deliverError(mResponse.error);
            }

private final Response.Listener<Bitmap> mListener;
...
@Override
    protected void deliverResponse(Bitmap response) {
        mListener.onResponse(response);
    }

ImageRequest imgRequest = new ImageRequest(imgUrl,
				new Response.Listener<Bitmap>() {
					@Override
					public void onResponse(Bitmap arg0) {
						// TODO Auto-generated method stub
						imageView.setImageBitmap(arg0);
					}
				}, 
				100, 
				100, 
				Config.ARGB_8888, 
				new ErrorListener() {
					@Override
					public void onErrorResponse(VolleyError arg0) {
						imageView.setImageResource(R.drawable.ic_launcher);
					}
				});

而从这里，我们也可以知道在Reponse类中，就会定义这么一个接口，如下，是Volley中Response类的定义：

public class Response<T> {

    /** Callback interface for delivering parsed responses. */
    public interface Listener<T> {
        /** Called when a response is received. */
        public void onResponse(T response);
    }

好了，到这里，总结一下：

1）创建一个Request的时候，会同时设置一个Response.Listener作为请求的一个参数变量，之后调用RequestQueue的add方法将其添加到Queue。

2）在Queue中的请求会由NetworkDispatcher去跟网络进行通信（如果有缓存的时候，就是CacheDispatcher）。

3）当请求结果回来的时候，Request会首先调用parseNetworkResponse方法根据不同的请求类型，如Json，Image等进行不同的处理。

4）当Request分析完之后，得到的Reponse对象，就会由ResponseDelivery类新起一个线程，调用1）步中的Listener来进行处理。

结束。

Android中关于Volley的使用（十）对Request和Reponse的认识,码迷,mamicode.com

Android中关于Volley的使用（十）对Request和Reponse的认识

标签：android   volley   request   response   源码   

原文地址：http://blog.csdn.net/linmiansheng/article/details/24742913