Volley源码解读(下)

上一篇文章主要是分析了Volley框架的整个执行流程，我们可以了解到，在调用了Volley.newRequestQueue()方法之后首先会构建一个stack来对网络请求进行封装和提交执行，然后会构建一个真正执行网络请求的实例NetWork，最后是构建任务提交的一个队列，并通过start()方法来启动整个工作系统。

这一篇则主要看一下Volley中的各类的Request和Response类型

基类

首先看一下最基础的Request的基类

Request.java
这个基类中主要封装了一些公共的字段，包括请求方法，tag标志，请求头，请求体，请求参数等基础的信息，以及对应的set和get方法，Error的统一回调处理等。

//例如一下信息

    /**
     * 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;
    }


    /**
     * Creates a new request with the given method (one of the values from {@link Method}),
     * URL, and error listener.  Note that the normal response listener is not provided here as
     * delivery of responses is provided by subclasses, who have a better idea of how to deliver
     * an already-parsed response.
     */
    public Request(int method, String url, Response.ErrorListener listener) {
        mMethod = method;
        mUrl = url;
        mErrorListener = listener;
        setRetryPolicy(new DefaultRetryPolicy());

        mDefaultTrafficStatsTag = findDefaultTrafficStatsTag(url);
    }

其中最主要的有两个抽象方法需要注意，这也是其子类所必需的的两个方法

    /**
     * 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);

一个是用来将返回结果拼接封装，另一个是将封装好的结果通过listener来进行转发。

StringRequest

/**
 * A canned request for retrieving the response body at a given URL as a String.
 */
public class StringRequest extends Request<String> {
    private final Listener<String> mListener;

    /**
     * Creates a new request with the given method.
     *
     * @param method the request {@link Method} to use
     * @param url URL to fetch the string at
     * @param listener Listener to receive the String response
     * @param errorListener Error listener, or null to ignore errors
     */
    public StringRequest(int method, String url, Listener<String> listener,
            ErrorListener errorListener) {
        super(method, url, errorListener);
        mListener = listener;
    }

    /**
     * Creates a new GET request.
     *
     * @param url URL to fetch the string at
     * @param listener Listener to receive the String response
     * @param errorListener Error listener, or null to ignore errors
     */
    public StringRequest(String url, Listener<String> listener, ErrorListener errorListener) {
        this(Method.GET, url, listener, errorListener);
    }

    @Override
    protected void deliverResponse(String response) {
        mListener.onResponse(response);
    }

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

上边就是StringRequest，我们可以看到其重写了父类的两个抽象方法，并通过listener将最后response的拼装结果进行了返回。

JsonRequest

对于JsonArrayRequest和JsonObjectRequest而言其父类为JsonRequest。JsonRequest继承自Request，是针对Json而言做了进一步的封装。

/**
 * A request for retrieving a T type response body at a given URL that also
 * optionally sends along a JSON body in the request specified.
 *
 * @param <T> JSON type of response expected
 */
public abstract class JsonRequest<T> extends Request<T> {
    /** Default charset for JSON request. */
    protected static final String PROTOCOL_CHARSET = "utf-8";

    /** Content type for request. */
    private static final String PROTOCOL_CONTENT_TYPE =
        String.format("application/json; charset=%s", PROTOCOL_CHARSET);

    private final Listener<T> mListener;
    private final String mRequestBody;

    /**
     * Deprecated constructor for a JsonRequest which defaults to GET unless {@link #getPostBody()}
     * or {@link #getPostParams()} is overridden (which defaults to POST).
     *
     * @deprecated Use {@link #JsonRequest(int, String, String, Listener, ErrorListener)}.
     */
    public JsonRequest(String url, String requestBody, Listener<T> listener,
            ErrorListener errorListener) {
        this(Method.DEPRECATED_GET_OR_POST, url, requestBody, listener, errorListener);
    }

    public JsonRequest(int method, String url, String requestBody, Listener<T> listener,
            ErrorListener errorListener) {
        super(method, url, errorListener);
        mListener = listener;
        mRequestBody = requestBody;
    }

    @Override
    protected void deliverResponse(T response) {
        mListener.onResponse(response);
    }

    @Override
    abstract protected Response<T> parseNetworkResponse(NetworkResponse response);

    /**
     * @deprecated Use {@link #getBodyContentType()}.
     */
    @Override
    public String getPostBodyContentType() {
        return getBodyContentType();
    }

    /**
     * @deprecated Use {@link #getBody()}.
     */
    @Override
    public byte[] getPostBody() {
        return getBody();
    }

    @Override
    public String getBodyContentType() {
        return PROTOCOL_CONTENT_TYPE;
    }

    @Override
    public byte[] getBody() {
        try {
            return mRequestBody == null ? null : mRequestBody.getBytes(PROTOCOL_CHARSET);
        } catch (UnsupportedEncodingException uee) {
            VolleyLog.wtf("Unsupported Encoding while trying to get the bytes of %s using %s",
                    mRequestBody, PROTOCOL_CHARSET);
            return null;
        }
    }

可以看到JsonRequest的内容很简单，只是针对Json格式进行了数据字符集的转变，同时实现了response的转发这个抽象方法，这样其子类只需要实现response的拼接方法，并返回一个符合规范的Response即可，为实现不同类型的Request留下了接口。

JsonArrayRequest

继承自JsonRequest类，实现了其中的parseNetworkResponse方法，将返回结果封装成了一个JsonArray类型。

/**
 * A request for retrieving a {@link JSONArray} response body at a given URL.
 */
public class JsonArrayRequest extends JsonRequest<JSONArray> {

    /**
     * Creates a new request.
     * @param url URL to fetch the JSON from
     * @param listener Listener to receive the JSON response
     * @param errorListener Error listener, or null to ignore errors.
     */
    public JsonArrayRequest(String url, Listener<JSONArray> listener, ErrorListener errorListener) {
        super(Method.GET, url, null, listener, errorListener);
    }

    @Override
    protected Response<JSONArray> parseNetworkResponse(NetworkResponse response) {
        try {
             //获取到返回结果的string字符串
            String jsonString = new String(response.data,
                    HttpHeaderParser.parseCharset(response.headers, PROTOCOL_CHARSET));
        //通过string字符串来构造一个JsonArray对象，并返回
            return Response.success(new JSONArray(jsonString),
                    HttpHeaderParser.parseCacheHeaders(response));
        } catch (UnsupportedEncodingException e) {
            return Response.error(new ParseError(e));
        } catch (JSONException je) {
            return Response.error(new ParseError(je));
        }
    }
}

JsonObjectRequest

继承自JsonRequest对象，将返回结果封装成了一个JsonObject对象，其实内部原理和JsonArrayRequest一样，只是构造了一个JsonObject返回

/**
 * A request for retrieving a {@link JSONObject} response body at a given URL, allowing for an
 * optional {@link JSONObject} to be passed in as part of the request body.
 */
public class JsonObjectRequest extends JsonRequest<JSONObject> {

    /**
     * Creates a new request.
     * @param method the HTTP method to use
     * @param url URL to fetch the JSON from
     * @param jsonRequest A {@link JSONObject} to post with the request. Null is allowed and
     *   indicates no parameters will be posted along with request.
     * @param listener Listener to receive the JSON response
     * @param errorListener Error listener, or null to ignore errors.
     */
    public JsonObjectRequest(int method, String url, JSONObject jsonRequest,
            Listener<JSONObject> listener, ErrorListener errorListener) {
        super(method, url, (jsonRequest == null) ? null : jsonRequest.toString(), listener,
                    errorListener);
    }

    /**
     * Constructor which defaults to <code>GET</code> if <code>jsonRequest</code> is
     * <code>null</code>, <code>POST</code> otherwise.
     *
     * @see #JsonObjectRequest(int, String, JSONObject, Listener, ErrorListener)
     */
    public JsonObjectRequest(String url, JSONObject jsonRequest, Listener<JSONObject> listener,
            ErrorListener errorListener) {
        this(jsonRequest == null ? Method.GET : Method.POST, url, jsonRequest,
                listener, errorListener);
    }

    @Override
    protected Response<JSONObject> parseNetworkResponse(NetworkResponse response) {
        try {
            String jsonString = new String(response.data,
                    HttpHeaderParser.parseCharset(response.headers, PROTOCOL_CHARSET));
        //构造一个JsonObject对象返回
            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));
        }
    }

以上就是Volley中各种类型Request的源码解读，其实各种类型的Request都大同小异，主要是对于请求返回的结果的一次封装，在保证返回结果符合Response的规范下，都可以进行。

下边看一下Response中做了哪些规范

/**
 * Encapsulates a parsed response for delivery.
 *
 * @param <T> Parsed type of this response
 */
public class Response<T> {

    /** Callback interface for delivering parsed responses. */
    //定义的监听器，用于转发返回结果
    public interface Listener<T> {
        /** Called when a response is received. */
        public void onResponse(T response);
    }

    /** Callback interface for delivering error responses. */
    //发生错误时的监听器
    public interface ErrorListener {
        /**
         * Callback method that an error has been occurred with the
         * provided error code and optional user-readable message.
         */
        public void onErrorResponse(VolleyError error);
    }

    /** Returns a successful response containing the parsed result. */
    public static <T> Response<T> success(T result, Cache.Entry cacheEntry) {
        return new Response<T>(result, cacheEntry);
    }

    /**
     * Returns a failed response containing the given error code and an optional
     * localized message displayed to the user.
     */
    public static <T> Response<T> error(VolleyError error) {
        return new Response<T>(error);
    }

    /** Parsed response, or null in the case of error. */
    public final T result;

    /** Cache metadata for this response, or null in the case of error. */
    public final Cache.Entry cacheEntry;

    /** Detailed error information if <code>errorCode != OK</code>. */
    public final VolleyError error;

    /** True if this response was a soft-expired one and a second one MAY be coming. */
    public boolean intermediate = false;

    /**
     * Returns whether this response is considered successful.
     */
    public boolean isSuccess() {
        return error == null;
    }


    private Response(T result, Cache.Entry cacheEntry) {
        this.result = result;
        this.cacheEntry = cacheEntry;
        this.error = null;
    }

    private Response(VolleyError error) {
        this.result = null;
        this.cacheEntry = null;
        this.error = error;
    }
}