源码见：ddxxll2008/gifdecoder_java
run()
public void run(){
if(in != null){
readStream();
}else if(gifData != null){
readByte();
}
}
private int readByte(){
in = new ByteArrayInputStream(gifData);
gifData = null;
return readStream();
}
GifDecoder的入口是run函数，里面包含了readStream()和readByte()两个方法，但是readByte()里也返回了一个readStream()，所以从readStream()方法进行分析。
readStream
private int readStream(){
init();
if(in != null){
readHeader();
if(!err()){
readContents();
if(frameCount < 0){
status = STATUS_FORMAT_ERROR;
action.parseOk(false,-1);
}else{
status = STATUS_FINISH;
action.parseOk(true,-1);
}
}
try {
in.close();
} catch (Exception e) {
e.printStackTrace();
}
}else {
status = STATUS_OPEN_ERROR;
action.parseOk(false,-1);
}
return status;
}
readStream()里先进行一些参数的初始化，然后读取Gif头文件。
private void readHeader() {
String id = "";
for (int i = 0; i < 6; i++) {
id += (char) read();
}
if (!id.startsWith("GIF")) {
status = STATUS_FORMAT_ERROR;
return;
}
readLSD();
if (gctFlag && !err()) {
gct = readColorTable(gctSize);
bgColor = gct[bgIndex];
}
}
private void readLSD() {
// logical screen size
width = readShort();
height = readShort();
// packed fields
int packed = read();
gctFlag = (packed & 0x80) != 0; // 1 : global color table flag
// 2-4 : color resolution
// 5 : gct sort flag
gctSize = 2 << (packed & 7); // 6-8 : gct size
bgIndex = read(); // background color index
pixelAspect = read(); // pixel aspect ratio
}
private int[] readColorTable(int ncolors) {
int nbytes = 3 * ncolors;
int[] tab = null;
byte[] c = new byte[nbytes];
int n = 0;
try {
n = in.read(c);
} catch (Exception e) {
e.printStackTrace();
}
if (n < nbytes) {
status = STATUS_FORMAT_ERROR;
} else {
tab = new int[256]; // max size to avoid bounds checks
int i = 0;
int j = 0;
while (i < ncolors) {
int r = ((int) c[j++]) & 0xff;
int g = ((int) c[j++]) & 0xff;
int b = ((int) c[j++]) & 0xff;
tab[i++] = 0xff000000 | (r << 16) | (g << 8) | b;
}
}
return tab;
}
readHeader()里主要是判断是否是Gif文件，如果是的话，则在readLSD()里获得Gif的一些基本信息。readLSD()读取的信息包括长宽，颜色和颜色索引等。之后通过readColorTable(int ncolors)读取颜色表单。之后就是通过readContents()来获取Gif的内容了。
readContents()
private void readContents() {
// read GIF file content blocks
boolean done = false;
while (!(done || err())) {
int code = read();
switch (code) {
case 0x2C: // image separator
readImage();
break;
case 0x21: // extension
code = read();
switch (code) {
case 0xf9: // graphics control extension
readGraphicControlExt();
break;
case 0xff: // application extension
readBlock();
String app = "";
for (int i = 0; i < 11; i++) {
app += (char) block[i];
}
if (app.equals("NETSCAPE2.0")) {
readNetscapeExt();
} else {
skip(); // don't care
}
break;
default: // uninteresting extension
skip();
}
break;
case 0x3b: // terminator
done = true;
break;
case 0x00: // bad byte, but keep going and see what happens
break;
default:
status = STATUS_FORMAT_ERROR;
}
}
}
在readContents()里，只要没有发生错误，就会一直读取下去，直到读取完成。这里面的readImage()方法是用来获取图片信息的。通过decodeImageData()方法来获取各个像素点的数据，之后新建一个bitmap，通过setPixels()来设置像素点的信息。然后根据此bitmap生成一个GifFrame的对象，并通过action.parseOk(true, frameCount)返回解析成功以及解析成功的帧数。全部解析完成后，帧数为-1。
private void readImage() {
ix = readShort(); // (sub)image position & size
iy = readShort();
iw = readShort();
ih = readShort();
int packed = read();
lctFlag = (packed & 0x80) != 0; // 1 - local color table flag
interlace = (packed & 0x40) != 0; // 2 - interlace flag
// 3 - sort flag
// 4-5 - reserved
lctSize = 2 << (packed & 7); // 6-8 - local color table size
if (lctFlag) {
lct = readColorTable(lctSize); // read table
act = lct; // make local table active
} else {
act = gct; // make global table active
if (bgIndex == transIndex) {
bgColor = 0;
}
}
int save = 0;
if (transparency) {
save = act[transIndex];
act[transIndex] = 0; // set transparent color if specified
}
if (act == null) {
status = STATUS_FORMAT_ERROR; // no color table defined
}
if (err()) {
return;
}
decodeImageData(); // decode pixel data
skip();
if (err()) {
return;
}
frameCount++;
// create new image to receive frame data
image = Bitmap.createBitmap(width, height, Bitmap.Config.ARGB_4444);
// createImage(width, height);
setPixels(); // transfer pixel data to image
if (gifFrame == null) {
gifFrame = new GifFrame(image, delay);
currentFrame = gifFrame;
} else {
GifFrame f = gifFrame;
while(f.nextFrame != null){
f = f.nextFrame;
}
f.nextFrame = new GifFrame(image, delay);
}
// frames.addElement(new GifFrame(image, delay)); // add image to frame
// list
if (transparency) {
act[transIndex] = save;
}
resetFrame();
action.parseOk(true, frameCount);
}
readContents()里还有读取其他信息的函数，比如readGraphicControlExt()，readBlock()和readNetscapeExt()。这些都是获取一些信息用的函数。
使用方法
GIFDecoder里有很多方法，可以获取Gif的信息。先构造一个GifDecoder，然后调用gifDecoder.run()方法，就可以得到一个完整的GIFDecoder对象，之后便能通过GIFDecoder里的各种方法去获得Gif图片的信息了。
//解析gif图片
gifDecoder = new GifDecoder(fileInputStream, new GifAction() {
@Override
public void parseOk(boolean parseStatus, int frameIndex) {
Logger.d(parseStatus + " " + frameIndex);
}
});
gifDecoder.run();