我们知道,在网络Http通信中。一定会有一个Request。相同的,也一定会有一个Response。而我们在Volley中利用RequestQueue来加入请求之前,一定会先创建一个Request对象。比方StringRequest,JsonObjectRequest和ImageRequest等,例如以下各自是前面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)

Volley中提供了一个基础的Request抽象类。例如以下:

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

在这个类中。定义了一些请求中主要的參数变量,如

Method:

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

它的值例如以下:

    /**
* 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;
}

请求中的Url:

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

一个ErroListener,

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

还有其他的一些參数,如shouldCache(是否须要缓存),tag(分类标签)等,而在Request中还提供了以下两个抽象方法,必须由子类实现:

    /**
* 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);

每个子类都必须实现两个方法,

1)parseNetworkResponse

当从网络中获取到Response的时候,怎么去解析相应的请求,这是由各个相应的Request去分析的,比方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));
}
}

再比方ImageRequest中的:

    @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,事实上是对图片进行处理,例如以下:

    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));
}
}

所以,假设我们自己定义一个Request的话。我们就要去实现我们自己的逻辑,比方是获取视频的话,就会去对数据进行解码等。

在上面的方法实现中,我们能够看到。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,就要看以下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。例如以下:

mDelivery.postResponse(request, response);

而mDelivery中的postResponse方法事实上是另起一个新线程来调用Request的deliverResponse方法,在ExecutorDelivery类中:

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,我们能够看到:

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

那我们看看StringRequest和ImageRequest中的deliverResponse方法:

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

我们能够看到,事实上都是调用一个Response.Listener类的onResponse方法。而事实上这个Listener。则是我们在创建请求的时候才实现,并传进来的,如前面Demo中创建JsonObjectRequest和ImageRequest的时候:

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);
}
});

如上面new Reponse.Listener方法中的实现,非常显然,之所以这么做的原因在于仅仅有调用者才知道怎么去处理Request解析过的数据。

而从这里,我们也能够知道在Reponse类中,就会定义这么一个接口。例如以下,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);
}

而除了这个接口的定义,另一个ErrorListener接口的定义就不列出来了。而除此之外,Response类中就存放了CacheEntry等信息,相对来说。由于定义了这种Listener接口,Response类是相对照较简单的。

好了,到这里,总结一下:

1)创建一个Request的时候,会同一时候设置一个Response.Listener作为请求的一个參数变量。之后调用RequestQueue的add方法将其加入到Queue。

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

3)当请求结果回来的时候,Request会首先调用parseNetworkResponse方法依据不同的请求类型。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。

05-11 09:38