Android掃一掃：zxing的集成與優化

0.

最近項目裏須要實現二維碼的掃描功能，掃描兩個二維碼而後獲得數據進行綁定。目前比較常見的二維碼掃描庫就是zxing和zbar了，zxing是google官方的開源項目，有專門的維護，java編寫。zbar使用C語言寫的，並且github上多年沒有代碼提交了，因此我決定選用zxing。

1.

附上zxing的項目地址:
zxing
打開zxing的github地址，發現彷佛沒有如何接入的文檔。沒關心，沒有文檔，可是有demo，咱們要作的就是修改demo，移除無用的功能，只保留二維碼的掃描和識別。

2.

 

 

下載項目後，裏面不少東西咱們是不須要的，咱們須要的就是這個，如圖所示

screenshot.png

 

這個就是剛纔所說的android的demo，新建一個android項目，將這個module導入工程並命名爲zxinglib,在這個module裏的gradle文件裏添加依賴。

dependencies{
    api 'com.google.zxing:android-core:3.3.0'
    api 'com.google.zxing:core:3.3.2'
}

運行這個module，你會發現這就是一個已經集成好zxing二維碼掃描的app，同時還有一些不須要的功能，好比建立二維碼，歷史記錄等等，並且仍是相機預覽仍是橫屏。下面咱們就分析一下這個demo的二維碼識別流程

3.

首先打開AndroidManifest文件，找到含有

<intent-filter>
      <action android:name="android.intent.action.MAIN"/>
      <category     android:name="android.intent.category.LAUNCHER"/>
    </intent-filter>

的Activity，發現是CaptureActivity,發現他還有好多其餘intent-filter，CaptureActivity是能夠被其餘應用打開的，既然找到了入口，那就進去分析吧。
先看onCreate方法：

@Override
  public void onCreate(Bundle icicle) {
   super.onCreate(icicle);
Window window = getWindow();   window.addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);
setContentView(R.layout.capture);
hasSurface = false;
inactivityTimer = new InactivityTimer(this);
beepManager = new BeepManager(this);
ambientLightManager = new AmbientLightManager(this);

PreferenceManager.setDefaultValues(this, R.xml.preferences, false);}

window設置標誌位，保證屏幕常亮不會黑屏，inactivityTimer保證在電量較低的時候且一段時間沒有激活的時候，關閉CaptureActivity，
beepManager是用來掃碼時發出聲音和震動的，ambientLightManager是用來控制感光的，以此來控制閃光燈的開閉。而後咱們再來看佈局文件：

<SurfaceView android:id="@+id/preview_view"
           android:layout_width="fill_parent"
           android:layout_height="fill_parent"/>

  <com.google.zxing.client.android.ViewfinderView
  android:id="@+id/viewfinder_view"
  android:layout_width="fill_parent"
  android:layout_height="fill_parent"/>

佈局文件裏比較重要的就是這兩個，一個surfaceview和一個viewfinderView，一個是照相機用來預覽的界面，一個是取景框的界面，剩下的控價都是用來展現掃描結果的，和咱們的需求沒有太大關係，這裏就不說了。

4.

掃描二維碼天然不能少的就是相機的調用了，在AndroidManifest文件裏，咱們也看到了相關權限的聲明

<uses-permission android:name="android.permission.CAMERA"/>
  <uses-permission    android:name="android.permission.INTERNET"/>
  <uses-permission android:name="android.permission.VIBRATE"/>
  <uses-permission android:name="android.permission.FLASHLIGHT"/>

有相機，震動和閃光燈的權限。
接着看一下是在哪裏初始化相機並調用預覽的，在onReumse裏
有一段代碼

SurfaceView surfaceView = (SurfaceView) findViewById(R.id.preview_view);
SurfaceHolder surfaceHolder = surfaceView.getHolder();
if (hasSurface) {
  // The activity was paused but not stopped, so the surface still exists. Therefore
  // surfaceCreated() won't be called, so init the camera here.
  initCamera(surfaceHolder);
} else {
  // Install the callback and wait for surfaceCreated() to init the camera.
  surfaceHolder.addCallback(this);
}

這裏初始化了surfaceview，同時判斷surface是否爲true，true就調用initcamera，否在就爲surfaceholder添加回調。剛纔已經看到了，hasSurface在onCreate的時候賦值爲false，以後在onResume也沒有進行true的賦值，因此這裏在第一次打開的時候，hasSurface=false。
那咱們就要關注surfaceHolder的回調了

@Override
 public void surfaceCreated(SurfaceHolder holder) {
if (holder == null) {
  Log.e(TAG, "*** WARNING *** surfaceCreated() gave us a null surface!");
}
if (!hasSurface) {
  hasSurface = true;
  initCamera(holder);
}
  }

在這裏咱們發如今surface建立後仍是調用了initCamera，進入initCamera看看裏面有什麼

private void initCamera(SurfaceHolder surfaceHolder) {
if (surfaceHolder == null) {
  throw new IllegalStateException("No SurfaceHolder provided");
}
if (cameraManager.isOpen()) {
  Log.w(TAG, "initCamera() while already open -- late SurfaceView callback?");
  return;
}
try {
  cameraManager.openDriver(surfaceHolder);
  // Creating the handler starts the preview, which can also throw a RuntimeException.
  if (handler == null) {
    handler = new CaptureActivityHandler(this, decodeFormats, decodeHints, characterSet, cameraManager);
  }
  decodeOrStoreSavedBitmap(null, null);
} catch (IOException ioe) {
  Log.w(TAG, ioe);
  displayFrameworkBugMessageAndExit();
} catch (RuntimeException e) {
  // Barcode Scanner has seen crashes in the wild of this variety:
  // java.?lang.?RuntimeException: Fail to connect to camera service
  Log.w(TAG, "Unexpected error initializing camera", e);
  displayFrameworkBugMessageAndExit();
 }
  }

這裏咱們關心兩個點，cameraManager.openDriver(surfaceHolder)和CaptureActivityHandler，進入openDriver這個方法

public synchronized void openDriver(SurfaceHolder holder) throws IOException {
OpenCamera theCamera = camera;
if (theCamera == null) {
  theCamera = OpenCameraInterface.open(requestedCameraId);
  if (theCamera == null) {
    throw new IOException("Camera.open() failed to return object from driver");
  }
  camera = theCamera;
}

if (!initialized) {
  initialized = true;
  configManager.initFromCameraParameters(theCamera);
  if (requestedFramingRectWidth > 0 && requestedFramingRectHeight > 0) {
    setManualFramingRect(requestedFramingRectWidth, requestedFramingRectHeight);
    requestedFramingRectWidth = 0;
    requestedFramingRectHeight = 0;
  }
}

Camera cameraObject = theCamera.getCamera();
Camera.Parameters parameters = cameraObject.getParameters();
String parametersFlattened = parameters == null ? null : parameters.flatten(); // Save these, temporarily
try {
  configManager.setDesiredCameraParameters(theCamera, false);
} catch (RuntimeException re) {
  // Driver failed
  Log.w(TAG, "Camera rejected parameters. Setting only minimal safe-mode parameters");
  Log.i(TAG, "Resetting to saved camera params: " + parametersFlattened);
  // Reset:
  if (parametersFlattened != null) {
    parameters = cameraObject.getParameters();
    parameters.unflatten(parametersFlattened);
    try {
      cameraObject.setParameters(parameters);
      configManager.setDesiredCameraParameters(theCamera, true);
    } catch (RuntimeException re2) {
      // Well, darn. Give up
      Log.w(TAG, "Camera rejected even safe-mode parameters! No configuration");
    }
  }
}
cameraObject.setPreviewDisplay(holder);
  }

這裏能夠發現，主要是針對camera的一些參數的設定，另外還要說明的一點是sdk21之後，之前的camera類已經廢棄了，google又給出了camera2來替他他們，可是目前zxing這個庫裏尚未使用camera2，關於camera的相關問題，等之後有時間單獨來寫一篇文章，這裏咱們主要針對的是流程的分析。這個方法裏咱們發現調用了CameraConfigurationManager的initFromCameraParameters和setDesiredCameraParameters，這兩個方法裏找出了相機預覽的最佳大小和根據屏幕進行camera方向的旋轉，感興趣的話能夠看一下這兩個方法。
接下來咱們來看CaptureActivityHandler

CaptureActivityHandler(CaptureActivity activity,
                     Collection<BarcodeFormat> decodeFormats,
                     Map<DecodeHintType,?> baseHints,
                     String characterSet,
                     CameraManager cameraManager) {
this.activity = activity;
decodeThread = new DecodeThread(activity, decodeFormats, baseHints, characterSet,
    new ViewfinderResultPointCallback(activity.getViewfinderView()));
decodeThread.start();
state = State.SUCCESS;

// Start ourselves capturing previews and decoding.
this.cameraManager = cameraManager;
cameraManager.startPreview();
restartPreviewAndDecode();
}

在它的構造函數裏咱們發現它開啓了相機的預覽，同時啓動了DecodeThread,再看看DeocodeThread的run方法

@Override
  public void run() {
    Looper.prepare();
    handler = new DecodeHandler(activity, hints);
    handlerInitLatch.countDown();
    Looper.loop();
  }

這裏又實例化了一個DeoceHandler，好，那就進入DeoceHandler，看看這貨又是什麼

private void decode(byte[] data, int width, int height) {
long start = System.currentTimeMillis();
Result rawResult = null;
PlanarYUVLuminanceSource source = activity.getCameraManager().buildLuminanceSource(data, width, height);
if (source != null) {
  BinaryBitmap bitmap = new BinaryBitmap(new HybridBinarizer(source));
  try {
    rawResult = multiFormatReader.decodeWithState(bitmap);
  } catch (ReaderException re) {
    // continue
  } finally {
    multiFormatReader.reset();
  }
}

Handler handler = activity.getHandler();
if (rawResult != null) {
  // Don't log the barcode contents for security.
  long end = System.currentTimeMillis();
  Log.d(TAG, "Found barcode in " + (end - start) + " ms");
  if (handler != null) {
    Message message = Message.obtain(handler, R.id.decode_succeeded, rawResult);
    Bundle bundle = new Bundle();
    bundleThumbnail(source, bundle);        
    message.setData(bundle);
    message.sendToTarget();
  }
} else {
  if (handler != null) {
    Message message = Message.obtain(handler, R.id.decode_failed);
    message.sendToTarget();
  }
}
  }

這個decode方法，就是咱們心心念唸的用來解析二維碼的地方，multiFormatReader.decodeWithState(bitmap);獲得結果後，返回給CaptureActivityHandler，captureActivityHandler在接到R.id.decode_succeededde message後，會調用CaptureActivity的handleDecode方法，在這裏會調用

// Put up our own UI for how to handle the decoded contents.

private void handleDecodeInternally(Result rawResult, ResultHandler resultHandler, Bitmap barcode) {

maybeSetClipboard(resultHandler);

SharedPreferences prefs = PreferenceManager.getDefaultSharedPreferences(this);

if (resultHandler.getDefaultButtonID() != null && prefs.getBoolean(PreferencesActivity.KEY_AUTO_OPEN_WEB, false)) {
  resultHandler.handleButtonPress(resultHandler.getDefaultButtonID());
  return;
}

statusView.setVisibility(View.GONE);
viewfinderView.setVisibility(View.GONE);
resultView.setVisibility(View.VISIBLE);

ImageView barcodeImageView = (ImageView) findViewById(R.id.barcode_image_view);
if (barcode == null) {
  barcodeImageView.setImageBitmap(BitmapFactory.decodeResource(getResources(),
      R.drawable.launcher_icon));
} else {
  barcodeImageView.setImageBitmap(barcode);
}

TextView formatTextView = (TextView) findViewById(R.id.format_text_view);
formatTextView.setText(rawResult.getBarcodeFormat().toString());

TextView typeTextView = (TextView) findViewById(R.id.type_text_view);
typeTextView.setText(resultHandler.getType().toString());

DateFormat formatter = DateFormat.getDateTimeInstance(DateFormat.SHORT, DateFormat.SHORT);
TextView timeTextView = (TextView) findViewById(R.id.time_text_view);
timeTextView.setText(formatter.format(rawResult.getTimestamp()));


TextView metaTextView = (TextView) findViewById(R.id.meta_text_view);
View metaTextViewLabel = findViewById(R.id.meta_text_view_label);
metaTextView.setVisibility(View.GONE);
metaTextViewLabel.setVisibility(View.GONE);
Map<ResultMetadataType,Object> metadata = rawResult.getResultMetadata();
if (metadata != null) {
  StringBuilder metadataText = new StringBuilder(20);
  for (Map.Entry<ResultMetadataType,Object> entry : metadata.entrySet()) {
    if (DISPLAYABLE_METADATA_TYPES.contains(entry.getKey())) {
      metadataText.append(entry.getValue()).append('\n');
    }
  }
  if (metadataText.length() > 0) {
    metadataText.setLength(metadataText.length() - 1);
    metaTextView.setText(metadataText);
    metaTextView.setVisibility(View.VISIBLE);
    metaTextViewLabel.setVisibility(View.VISIBLE);
  }
}

CharSequence displayContents = resultHandler.getDisplayContents();
TextView contentsTextView = (TextView) findViewById(R.id.contents_text_view);
contentsTextView.setText(displayContents);
int scaledSize = Math.max(22, 32 - displayContents.length() / 4);
contentsTextView.setTextSize(TypedValue.COMPLEX_UNIT_SP, scaledSize);

TextView supplementTextView = (TextView) findViewById(R.id.contents_supplement_text_view);
supplementTextView.setText("");
supplementTextView.setOnClickListener(null);
if (PreferenceManager.getDefaultSharedPreferences(this).getBoolean(
    PreferencesActivity.KEY_SUPPLEMENTAL, true)) {
  SupplementalInfoRetriever.maybeInvokeRetrieval(supplementTextView,
                                                 resultHandler.getResult(),
                                                 historyManager,
                                                 this);
}

int buttonCount = resultHandler.getButtonCount();
ViewGroup buttonView = (ViewGroup) findViewById(R.id.result_button_view);
buttonView.requestFocus();
for (int x = 0; x < ResultHandler.MAX_BUTTON_COUNT; x++) {
  TextView button = (TextView) buttonView.getChildAt(x);
  if (x < buttonCount) {
    button.setVisibility(View.VISIBLE);
    button.setText(resultHandler.getButtonText(x));
    button.setOnClickListener(new ResultButtonListener(resultHandler, x));
  } else {
    button.setVisibility(View.GONE);
  }
}
}

用來展現最終的結果。
那麼DeoceHandler又是何時調用deocode方法的呢？

@Override
  public void handleMessage(Message message) {
   if (message == null || !running) {
  return;
}
switch (message.what) {
  case R.id.decode:
    decode((byte[]) message.obj, message.arg1, message.arg2);
    break;
  case R.id.quit:
    running = false;
    Looper.myLooper().quit();
    break;
}
  }

DeoceHandler在收到what==R.id.deocde的message時會調用decode方法，那麼是誰發送的這個message呢？還記得CaptureActivityHandler的構造函數裏，調用了restartPreviewAndDeocode方法

private void restartPreviewAndDecode() {
if (state == State.SUCCESS) {
  state = State.PREVIEW;
  cameraManager.requestPreviewFrame(decodeThread.getHandler(), R.id.decode);
  activity.drawViewfinder();
}

這個方法裏不只調用了drawViewFinder，繪製了取景框，還調用了
cameraManager.requestPreviewFrame(decodeThread.getHandler(), R.id.decode);這裏傳入了decodehandler，它又作了什麼？

public synchronized void requestPreviewFrame(Handler handler, int message) {
OpenCamera theCamera = camera;
if (theCamera != null && previewing) {
  previewCallback.setHandler(handler, message);
  theCamera.getCamera().setOneShotPreviewCallback(previewCallback);
}
  }

原來這個方法爲camera設置了previewcallback，同時previewcallback還持有decodehandler的引用，這個previewcallback的

public void onPreviewFrame(byte[] data, Camera camera) {
Point cameraResolution = configManager.getCameraResolution();
Handler thePreviewHandler = previewHandler;
if (cameraResolution != null && thePreviewHandler != null) {
  Message message = thePreviewHandler.obtainMessage(previewMessage, cameraResolution.x,
      cameraResolution.y, data);
  message.sendToTarget();
  previewHandler = null;
} else {
  Log.d(TAG, "Got preview callback, but no handler or resolution available");
}

}
在這裏Message message = thePreviewHandler.obtainMessage(previewMessage, cameraResolution.x,
cameraResolution.y, data);
message.sendToTarget();
發送了what=previewMessage，而這個previewMessage就是以前CaptureActivityHandler傳入的R.id.decode。
那麼camera的setOneShotPreviewCallback這個方法是用來幹什麼的？查看源碼看註釋
單個預覽幀將被返回給提供的處理程序。 數據將以byte []形式到達在message.obj字段中，寬度和高度編碼爲message.arg1message.arg2。
至此一個從預覽到識別解析的流程差很少就分析完了，圍繞這些，那些demo裏的不須要的東西就能夠刪除了。

最後附上git地址：
github

做者：滑板上的老砒霜 連接：https://www.jianshu.com/p/a4ba10da4231 來源：簡書 簡書著做權歸做者全部，任何形式的轉載都請聯繫做者得到受權並註明出處。