zxing二维码扫描的流程简析（Android版）

目前市面上二维码的扫描似乎用开源google的zxing比较多，接下去以2.2版本做一个简析吧，勿喷。。。

下载下来后定位两个文件夹，core和android，core是一些核心的库，android是针对android的一些代码。

我们先看核心库，在package com.google.zxing中的一些生成二维码的类关系

接口Writer里面有两个encode的重载函数，不同的格式的二维码有各自的类实现了Writer接口，MultiformatWriter类比较特殊，根据代码的注释可见其其实是个工厂类，根据BarcodeFormat实例化不同的Writer，然后最终调用各自的Encode.encode()方法

 public final class MultiFormatWriter implements Writer {

   @Override
   public BitMatrix encode(String contents,
                           BarcodeFormat format,
                           int width,
                           int height) throws WriterException {
     return encode(contents, format, width, height, null);
   }

   @Override
   public BitMatrix encode(String contents,
                           BarcodeFormat format,
                           int width, int height,
                           Map<EncodeHintType,?> hints) throws WriterException {

     Writer writer;
     switch (format) {
       case EAN_8:
         writer = new EAN8Writer();
         break;
       case EAN_13:
         writer = new EAN13Writer();
         break;
       case UPC_A:
         writer = new UPCAWriter();
         break;
       case QR_CODE:
         writer = new QRCodeWriter();
         break;
       case CODE_39:
         writer = new Code39Writer();
         break;
       case CODE_128:
         writer = new Code128Writer();
         break;
       case ITF:
         writer = new ITFWriter();
         break;
       case PDF_417:
         writer = new PDF417Writer();
         break;
       case CODABAR:
         writer = new CodaBarWriter();
         break;
       case DATA_MATRIX:
         writer = new DataMatrixWriter();
         break;
       case AZTEC:
         writer = new AztecWriter();
         break;
       default:
         throw new IllegalArgumentException("No encoder available for format " + format);
     }
     return writer.encode(contents, format, width, height, hints);
   }

 }

然后看解析二维码的类结构

关键就是这个MultiformatReader，里面聚合了多个reader,并且根据客户端设置的DecodeHintType值，确定添加reader以及添加reader的顺序，最后调用reader.decode方法

 public final class MultiFormatReader implements Reader {

   private Map<DecodeHintType,?> hints;
   private Reader[] readers;

   @Override
   public Result decode(BinaryBitmap image) throws NotFoundException {
     setHints(null);
     return decodeInternal(image);
   }

   @Override
   public Result decode(BinaryBitmap image, Map<DecodeHintType,?> hints) throws NotFoundException {
     setHints(hints);
     return decodeInternal(image);
   }

   public Result decodeWithState(BinaryBitmap image) throws NotFoundException {
     // Make sure to set up the default state so we don't crash
     if (readers == null) {
       setHints(null);
     }
     return decodeInternal(image);
   }

   public void setHints(Map<DecodeHintType,?> hints) {//根据设置的hint来设置reader
     this.hints = hints;

     boolean tryHarder = hints != null && hints.containsKey(DecodeHintType.TRY_HARDER);
     @SuppressWarnings("unchecked")
     Collection<BarcodeFormat> formats =
         hints == null ? null : (Collection<BarcodeFormat>) hints.get(DecodeHintType.POSSIBLE_FORMATS);
     Collection<Reader> readers = new ArrayList<Reader>();
     if (formats != null) {
       boolean addOneDReader =
           formats.contains(BarcodeFormat.UPC_A) ||
           formats.contains(BarcodeFormat.UPC_E) ||
           formats.contains(BarcodeFormat.EAN_13) ||
           formats.contains(BarcodeFormat.EAN_8) ||
           formats.contains(BarcodeFormat.CODABAR) ||
           formats.contains(BarcodeFormat.CODE_39) ||
           formats.contains(BarcodeFormat.CODE_93) ||
           formats.contains(BarcodeFormat.CODE_128) ||
           formats.contains(BarcodeFormat.ITF) ||
           formats.contains(BarcodeFormat.RSS_14) ||
           formats.contains(BarcodeFormat.RSS_EXPANDED);
       // Put 1D readers upfront in "normal" mode
       if (addOneDReader && !tryHarder) {
         readers.add(new MultiFormatOneDReader(hints));
       }
       if (formats.contains(BarcodeFormat.QR_CODE)) {
         readers.add(new QRCodeReader());
       }
       if (formats.contains(BarcodeFormat.DATA_MATRIX)) {
         readers.add(new DataMatrixReader());
       }
       if (formats.contains(BarcodeFormat.AZTEC)) {
         readers.add(new AztecReader());
       }
       if (formats.contains(BarcodeFormat.PDF_417)) {
          readers.add(new PDF417Reader());
       }
       if (formats.contains(BarcodeFormat.MAXICODE)) {
          readers.add(new MaxiCodeReader());
       }
       // At end in "try harder" mode
       if (addOneDReader && tryHarder) {
         readers.add(new MultiFormatOneDReader(hints));
       }
     }
     if (readers.isEmpty()) {
       if (!tryHarder) {
         readers.add(new MultiFormatOneDReader(hints));
       }

       readers.add(new QRCodeReader());
       readers.add(new DataMatrixReader());
       readers.add(new AztecReader());
       readers.add(new PDF417Reader());
       readers.add(new MaxiCodeReader());

       if (tryHarder) {
         readers.add(new MultiFormatOneDReader(hints));
       }
     }
     this.readers = readers.toArray(new Reader[readers.size()]);
   }

   @Override
   public void reset() {
     if (readers != null) {
       for (Reader reader : readers) {
         reader.reset();
       }
     }
   }

   private Result decodeInternal(BinaryBitmap image) throws NotFoundException {//最终都调用这个方法
     if (readers != null) {
       for (Reader reader : readers) {
         try {
           return reader.decode(image, hints);
         } catch (ReaderException re) {
           // continue
         }
       }
     }
     throw NotFoundException.getNotFoundInstance();
   }

 }

DecodeHintType的语法比较有意思，还在理解中

 public enum DecodeHintType {

   /**
    * Unspecified, application-specific hint. Maps to an unspecified {@link Object}.
    */
  OTHER(Object.class),

   /**
    * Image is a pure monochrome image of a barcode. Doesn't matter what it maps to;
    * use {@link Boolean#TRUE}.
    */
   PURE_BARCODE(Void.class),

   /**
    * Image is known to be of one of a few possible formats.
    * Maps to a {@link List} of {@link BarcodeFormat}s.
    */
   POSSIBLE_FORMATS(List.class),

   /**
    * Spend more time to try to find a barcode; optimize for accuracy, not speed.
    * Doesn't matter what it maps to; use {@link Boolean#TRUE}.
    */
   TRY_HARDER(Void.class),

   /**
    * Specifies what character encoding to use when decoding, where applicable (type String)
    */
   CHARACTER_SET(String.class),

   /**
    * Allowed lengths of encoded data -- reject anything else. Maps to an {@code int[]}.
    */
   ALLOWED_LENGTHS(int[].class),

   /**
    * Assume Code 39 codes employ a check digit. Doesn't matter what it maps to;
    * use {@link Boolean#TRUE}.
    */
   ASSUME_CODE_39_CHECK_DIGIT(Void.class),

   /**
    * Assume the barcode is being processed as a GS1 barcode, and modify behavior as needed.
    * For example this affects FNC1 handling for Code 128 (aka GS1-128). Doesn't matter what it maps to;
    * use {@link Boolean#TRUE}.
    */
   ASSUME_GS1(Void.class),

   /**
    * The caller needs to be notified via callback when a possible {@link ResultPoint}
    * is found. Maps to a {@link ResultPointCallback}.
    */
   NEED_RESULT_POINT_CALLBACK(ResultPointCallback.class),

   // End of enumeration values.
   ;

   /**
    * Data type the hint is expecting.
    * Among the possible values the {@link Void} stands out as being used for
    * hints that do not expect a value to be supplied (flag hints). Such hints
    * will possibly have their value ignored, or replaced by a
    * {@link Boolean#TRUE}. Hint suppliers should probably use
    * {@link Boolean#TRUE} as directed by the actual hint documentation.
    */
   private final Class<?> valueType;

   DecodeHintType(Class<?> valueType) {
     this.valueType = valueType;
   }

   public Class<?> getValueType() {
     return valueType;
   }

 }

然后我们看下android里面是如何调用的，入口是CaptureActivity,在com.google.zxing.client.android package中，以下描述一个通用的流程

CaptureAct中的onResume中的initCamera初始化CaptureActHandler,其构造函数中新起了一个DecodeThread去异步准备一个DecodeHandler，然后调用restartPreviewAndDecode方法，让DecodeHandler去处理R.id.decode的消息，当然这里需要处理一些线程同步问题，代码里用到了CountDownLatch来控制。DecodeHanlder处理R.id.decode消息后用传递R.id.decode_succeeded消息给CaptureActHanlder，最终再调用handleDecode传递给CaptureAct.