Volley源码解析及相关拓展

简述

关于Volley源码解析的文章很多，这里推荐一篇CodeKK的文章——《Volley 源码解析》
—— 注：文章链接已经找不到了，但网上转载很多。

总的来说，Volley的源码比较好理解，而且具有很强的拓展性，下面贴出两张图(流程图，类图)。建议直接看源码，然后再看这两张图，分分钟就懂了。

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Volley 请求流程图

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Volley 框架的主要类关系图

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关于Volley的拓展

根据类图，可以发现Volley支持多种类型的请求：

• StringRequest
• JsonRequest
  
  • JsonObjectRequest
  • JsonArrayRequest
• ImageRequest
• ClearCacheRequest

1. 自定义支持Gson解析的Request

1.1 先来看一下StringRequest

public class StringRequest extends Request<String> {
    private Listener mListener;

    public StringRequest(int method, String url, Listener listener, ErrorListener errorListener) {
        super(method, url, errorListener);
        mListener = listener;
    }

    public StringRequest(String url, Listener listener, ErrorListener errorListener) {
        this(Method.GET, url, listener, errorListener);
    }

    @Override
    protected void deliverResponse(String response) {
        if (mListener != null) {
            mListener.onResponse(response);
        }
    }

    @Override
    protected Response parseNetworkResponse(NetworkResponse response) {
        String parsed;
        try {
            // 主要是这里将网络获取的数据转成String类型
            parsed = new String(response.data, HttpHeaderParser.parseCharset(response.headers));
        } catch (UnsupportedEncodingException e) {
            parsed = new String(response.data);
        }
        // 这里只是将String类型的数据存放在Response对象的一个result字段里
        return Response.success(parsed, HttpHeaderParser.parseCacheHeaders(response));
    }
}

1.2 支持Gson解析的Request

前提：数据的返回类型必须是json格式的。

public class GsonRequest extends Request<T> {
    private Listener mListener;
    private final Class clazz;
    private final Gson gson = new Gson();

    // 这里要传一个实体对象进去
    public StringRequest(String url, Class clazz, Listener listener, ErrorListener errorListener) {
        super(Method.GET, url, listener, errorListener);
        mListener = listener;
        this.clazz = clazz; 
    }

    @Override
    protected void deliverResponse(T response) {
        if (mListener != null) {
            mListener.onResponse(response);
        }
    }

    @Override  
    protected Response parseNetworkResponse(NetworkResponse response) {  
        try {
            // 先转成String类型的json
            String json = new String(response.data, HttpHeaderParser.parseCharset(response.headers));  
            // 解析String (关键)
            return Response.success(gson.fromJson(json, clazz), HttpHeaderParser.parseCacheHeaders(response));  
        } catch (UnsupportedEncodingException e) {  
            return Response.error(new ParseError(e));  
        } catch (JsonSyntaxException e) {  
            return Response.error(new ParseError(e));  
        }  
    }  
}

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2. ImageRequest中图片处理的学习

public class ImageRequest extends Request<Bitmap> {
    /** Socket timeout in milliseconds for image requests */
    private static final int IMAGE_TIMEOUT_MS = 1000;

    /** Default number of retries for image requests */
    private static final int IMAGE_MAX_RETRIES = 2;

    /** Default backoff multiplier for image requests */
    private static final float IMAGE_BACKOFF_MULT = 2f;

    private final Response.Listener mListener;
    private final Config mDecodeConfig;
    private final int mMaxWidth;
    private final int mMaxHeight;
    private ScaleType mScaleType;

    /** Decoding lock so that we don't decode more than one image at a time (to avoid OOM's) */
    private static final Object sDecodeLock = new Object();

    /**
     * Creates a new image request.
     */
    public ImageRequest(String url, Response.Listener listener, int maxWidth, int maxHeight,
            ScaleType scaleType, Config decodeConfig, Response.ErrorListener errorListener) {
        super(Method.GET, url, errorListener); 
        setRetryPolicy(
                new DefaultRetryPolicy(IMAGE_TIMEOUT_MS, IMAGE_MAX_RETRIES, IMAGE_BACKOFF_MULT));
        mListener = listener;
        mDecodeConfig = decodeConfig;
        mMaxWidth = maxWidth;
        mMaxHeight = maxHeight;
        mScaleType = scaleType;
    }

    /**
     * Scales one side of a rectangle to fit aspect ratio.
     */
    private static int getResizedDimension(int maxPrimary, int maxSecondary, int actualPrimary, int actualSecondary, ScaleType scaleType) {

        // If no dominant value at all, just return the actual.
        if ((maxPrimary == 0) && (maxSecondary == 0)) {
            return actualPrimary;
        }

        // If ScaleType.FIT_XY fill the whole rectangle, ignore ratio.
        if (scaleType == ScaleType.FIT_XY) {
            if (maxPrimary == 0) {
                return actualPrimary;
            }
            return maxPrimary;
        }

        // If primary is unspecified, scale primary to match secondary's scaling ratio.
        if (maxPrimary == 0) {
            double ratio = (double) maxSecondary / (double) actualSecondary;
            return (int) (actualPrimary * ratio);
        }

        if (maxSecondary == 0) {
            return maxPrimary;
        }

        double ratio = (double) actualSecondary / (double) actualPrimary;
        int resized = maxPrimary;

        // If ScaleType.CENTER_CROP fill the whole rectangle, preserve aspect ratio.
        if (scaleType == ScaleType.CENTER_CROP) {
            if ((resized * ratio) < maxSecondary) {
                resized = (int) (maxSecondary / ratio);
            }
            return resized;
        }

        if ((resized * ratio) > maxSecondary) {
            resized = (int) (maxSecondary / ratio);
        }
        return resized;
    }

    @Override
    protected Response 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));
            }
        }
    }

    /**
     * 图片的主要处理逻辑(值得参考)
     * The real guts of parseNetworkResponse. Broken out for readability.
     */
    private Response 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, mScaleType);
            int desiredHeight = getResizedDimension(mMaxHeight, mMaxWidth,
                    actualHeight, actualWidth, mScaleType);

            // 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));
        }
    }

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

    /**
     * 返回一个inSimpleSize的最佳值
     */
    static int findBestSampleSize(
            int actualWidth, int actualHeight, int desiredWidth, int desiredHeight) {
        double wr = (double) actualWidth / desiredWidth;
        double hr = (double) actualHeight / desiredHeight;
        double ratio = Math.min(wr, hr);
        float n = 1.0f;
        while ((n * 2) <= ratio) {
            n *= 2;
        }

        return (int) n;
    }
}

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