点9图 NinePatch chunk解析

   最近在工作中需要解析点9图的头信息（chunk）的格式，读取拉伸坐标片段、padding信息，在网上找了一下没有相关信息，下面详解一下解析过程。

点9图科普

    点9图的定义见官方文档：http://developer.android.com/guide/topics/graphics/2d-graphics.html#nine-patch
    简单来说就是左边的点代表垂直拉伸的区域，上边的点代表水平拉伸的区域；右边的点代表文字等的垂直可可显示区域，下边的点代表文字等的水平可显示区域。所以，左上重合的区域就是拉伸区域；右下重合的区域就是显示区域。

    Android的编译工具aapt会把标记了拉伸区域的点9图处理成一个普通的png，并把相关的信息放入png头部的meta-data区域，即chunk。这个过程详见frameworks/base/tools/aapt/Images.cpp的static status_t do_9patch(const char* imageName, image_info* image)

PNG文件结构

    官方文档参见右边的链接：The Metadata in PNG files，摘抄部分如下

    A PNG always starts with an 8-byte signature: 137 80 78 71 13 10 26 10 (decimal values). The remainder of the file consists a series of chunks beginning with an IHDR chunk and ending with an IEND chunk.

    好了，了解了PNG的文件结构我们就可以取出chunk了，只需要按照文档的说明，按顺序取出Chunk data对应的信息即可。特别需要注意的是，读取字节流的时候需要采取大端方式。下面上代码

    /**     * PNG Chunk struct     * <a href="http://dev.exiv2.org/projects/exiv2/wiki/The_Metadata_in_PNG_files">The Metadata in PNG files</a>     *      *   +--------+---------+     *   | Length | 4 bytes |     *   +--------+---------+     *   | Chunk  | 4 bytes |     *   |  type  |         |     *   +--------+---------+     *   | Chunk  | Length  |     *   |  data  |  bytes  |     *   +--------+---------+     *   | CRC    | 4 bytes |     *   +--------+---------+     *        * @param pngName     * @return chunk     * @throws IOException     */    private byte[] loadNinePatchChunk(InputStream is) throws IOException {        IntReader reader = new IntReader(is, true);        // check PNG signature        // A PNG always starts with an 8-byte signature: 137 80 78 71 13 10 26 10 (decimal values).        if (reader.readInt() != 0x89504e47 || reader.readInt() != 0x0D0A1A0A) {            return null;        }        while (true) {            int length = reader.readInt();            int type = reader.readInt();            // check for nine patch chunk type (npTc)            if (type != 0x6E705463) {                reader.skip(length + 4/*crc*/);                continue;            }            return reader.readByteArray(length);        }    }

解析Chunk

    点9图的Chunk是按照struct Res_png_9patch的结构组织的，详见frameworks/base/include/androidfw/ResourceTypes.h。

struct Res_png_9patch{    Res_png_9patch() : wasDeserialized(false), xDivs(NULL),                       yDivs(NULL), colors(NULL) { }     int8_t wasDeserialized;    int8_t numXDivs;    int8_t numYDivs;    int8_t numColors;     // These tell where the next section of a patch starts.    // For example, the first patch includes the pixels from    // 0 to xDivs[0]-1 and the second patch includes the pixels    // from xDivs[0] to xDivs[1]-1.    // Note: allocation/free of these pointers is left to the caller.    int32_t* xDivs;    int32_t* yDivs;     int32_t paddingLeft, paddingRight;    int32_t paddingTop, paddingBottom;     enum {        // The 9 patch segment is not a solid color.        NO_COLOR = 0x00000001,         // The 9 patch segment is completely transparent.        TRANSPARENT_COLOR = 0x00000000    };       // Note: allocation/free of this pointer is left to the caller.    uint32_t* colors;     // Convert data from device representation to PNG file representation.    void deviceToFile();    // Convert data from PNG file representation to device representation.    void fileToDevice();    // Serialize/Marshall the patch data into a newly malloc-ed block    void* serialize();    // Serialize/Marshall the patch data    void serialize(void* outData);    // Deserialize/Unmarshall the patch data    static Res_png_9patch* deserialize(const void* data);    // Compute the size of the serialized data structure    size_t serializedSize();};

  了解了struct Res_png_9patch，我们只需根据struct Res_png_9patch的结构解析上面步骤得到的chunk data即可。代码如下：

    public static NinePatchChunk deserialize(byte[] data) {        ByteBuffer byteBuffer = ByteBuffer.wrap(data).order(ByteOrder.BIG_ENDIAN);        byte wasSerialized = byteBuffer.get();        //        if (wasSerialized == 0) return null;        NinePatchChunk chunk = new NinePatchChunk();        chunk.mDivX = new int[byteBuffer.get()];        chunk.mDivY = new int[byteBuffer.get()];        chunk.mColor = new int[byteBuffer.get()];        checkDivCount(chunk.mDivX.length);        checkDivCount(chunk.mDivY.length);        // skip 8 bytes        byteBuffer.getInt();        byteBuffer.getInt();        chunk.mPaddings.left = byteBuffer.getInt();        chunk.mPaddings.right = byteBuffer.getInt();        chunk.mPaddings.top = byteBuffer.getInt();        chunk.mPaddings.bottom = byteBuffer.getInt();        // skip 4 bytes        byteBuffer.getInt();        readIntArray(chunk.mDivX, byteBuffer);        readIntArray(chunk.mDivY, byteBuffer);        readIntArray(chunk.mColor, byteBuffer);        return chunk;    }

    NinePatchChunk.mDivX和NinePatchChunk.mDivY就是点9图的拉伸区域标识信息，NinePatchChunk.mPaddings是点9图的pading信息，这样点9图的Chunk信息就解析完毕了。

总结

    全部代码见https://gist.github.com/luxiaoyu/085135ff88d7c57a18c5

    调研这种SDK文档上没有详细信息的数据结构，需要从Android源码入手，耐心查看相关代码和注释，grep和find是好帮手。