zxing開源庫工做流程源碼詳解

代碼獲取

做爲移動客戶端開發者來講，對二維碼識別或二維碼生成相關的開發需求確定並不陌生，Android開發二維碼相關的功能一般都會使用或參考大名鼎鼎的zxing庫。而本文則主要是經過源碼分析一下該開源庫掃描二維碼的工做流程，對這塊能有個更深的瞭解。

首先使用git將項目代碼clone到本地，新建項目，將zxing文件夾中的android以及core文件夾代碼覆蓋到對應的目錄下，稍做一些修改便可運行一個簡單的二維碼掃描的示例應用。

總體流程

Demo代碼運行起來後，會進入一個掃描的主功能界面，將掃描框對準一個二維碼便可彈出解析結果信息的浮框。經過AndroidManifest.xml文件中能夠得知這個頁面對應的類爲CaptureActivity.java，咱們便從這個類開始，分析整個二維碼掃描的流程。

要分析一個Activity，固然要從它的生命週期所對應的各個方法提及。首先咱們來看它的onCreate()方法：

@Override
public void onCreate(Bundle icicle) {
    super.onCreate(icicle);

    //保持屏幕常亮
    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);
}
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這個方法代碼很少，也很容易看懂，主要就是作一些初始化的工做。InactivityTimer主要是用來監聽當手機是使用電池而不是充電狀態時，若是5分鐘內沒有作任何操做，則主動finish掉activity。BeepManager負責掃描到結果後震動或鈴聲相關，AmbientLightManager則是負責控制閃光燈。

繼續往下走看onResume()方法：

@Override
protected void onResume() {
    super.onResume();

    ...

    // CameraManager must be initialized here, not in onCreate(). This is necessary because we don't
	// want to open the camera driver and measure the screen size if we're going to show the help on
	// first launch. That led to bugs where the scanning rectangle was the wrong size and partially
	// off screen.
	cameraManager = new CameraManager(getApplication());

    viewfinderView = (ViewfinderView) findViewById(R.id.viewfinder_view);
    viewfinderView.setCameraManager(cameraManager);

    ...

    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);
    }
}
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這個方法很重要，初始化了CameraManager，掃描二維碼毋庸置疑是須要用到相機，經過相機預覽的一幀一幀的圖片，去解析上面可能存在的二維碼信息。而在最後面還初始化了SurfaceView，經過hasSurface來決定是走initCamera(surfaceHolder)仍是surfaceHolder.addCallback(this)。在上面的onCreate()中咱們能夠看到hasSurface被初始化成false，因此這裏走的應該是else的代碼塊。CaptureActivity實現了SurfaceHolder.Callback接口，所以該方法綁定了surfaceHolder的回調。當SurfaceView添加到 activity 中時，會調用surfaceCreated()：

@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);
    }
}
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這裏咱們看到會改變hasSurface的狀態，而後走initCamera(holder)，和onResume()中 hasSurface爲true時作的操做是同樣的：

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);
}
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cameraManager 打開了驅動，而且把本身傳入一個CaptureActivityHandler對象中去，那這個CaptureActivityHandler看起來像是一個進行消息通知的 Handler，它的具體做用又是什麼呢？咱們來看看它的構造方法：

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();
}
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經過進入CaptureActivityHandler.java能夠看到該類確實繼承了Handler，而且在它的構造方法中開啓了一個DecodeThread的線程，而且調用了cameraManager的startPreview()方法：

Asks the camera hardware to begin drawing preview frames to the screen.

開啓相機預覽後，再看下面的restartPreviewAndDecode()：

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

public synchronized void requestPreviewFrame(Handler handler, int message) {
    OpenCamera theCamera = camera;
    if (theCamera != null && previewing) {
        previewCallback.setHandler(handler, message);
        theCamera.getCamera().setOneShotPreviewCallback(previewCallback);
    }
}
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能夠看到這個 handler 會一直傳遞到一個previewCallback對象中去，而PreviewCallback是setOneShotPreviewCallback()方法的一個回調，setOneShotPreviewCallback方法上的註釋說明：

Installs a callback to be invoked for the next preview frame in addition to displaying it on the screen. After one invocation, the callback is cleared. This method can be called any time, even when preview is live. Any other preview callbacks are overridden.

使用此方法註冊預覽回調接口時，會將下一幀數據回調給onPreviewFrame()方法，調用完成後這個回調接口將被銷燬，也就是隻會回調一次預覽幀數據。繼續順着這個方法走下去，看回調方法onPreviewFrame()：

@Override
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");
    }
}
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這裏將返回的 byte 數組數據和預覽幀的寬高信息經過 handler 進行通知，這個 handler 就是上文中傳過來的decodeThread.getHandler()，previewMessage爲R.id.decode，目的就是把圖片數據拿到該線程中進行解析。咱們跟進到DecodeHandler.java中查看handleMessage()方法：

@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;
    }
}

private void decode(byte[] data, int width, int height) {
    long start = System.currentTimeMillis();
  
    //省略具體解析代碼
    ...
    Handler handler = activity.getHandler();//CaptureActivityHandler
    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();
        }
    }
}
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上面代碼很清晰，DecodeHandler 接收到R.id.decode的消息後，會調用decode()方法去解析傳過來的圖片數據。通過一系列解析操做，獲得結果。若是結果爲不爲空，則經過CaptureActivityHandler將解析成功的消息傳到CaptureActivity中進行後續解析結果展現。而若是解析結果爲空呢，說明二維碼信息解析失敗了，傳了一個R.id.decode_failed到CaptureActivityHandler中：

@Override
public void handleMessage(Message message) {
    switch (message.what) {

    	...

        case R.id.decode_failed:
            // We're decoding as fast as possible, so when one decode fails, start another.
            state = State.PREVIEW;
            cameraManager.requestPreviewFrame(decodeThread.getHandler(), R.id.decode);
            break;
            
        ...
    }
}

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能夠看到，解析失敗時，從新調用requestPreviewFrame獲取下一幀預覽照片，再拿去解析，知道返回正確結果或者手動退出。

整個過程的時序圖以下：