Detailed explanation of Android Camera2 openCamera process (1)

This article uses Android 11 code to sort out.

The interface of Android Camera2 open Camera is the openCamera function of CameraManager, as follows:

frameworks/base/core/java/android/hardware/camera2/CameraManager.java
648      @RequiresPermission(android.Manifest.permission.CAMERA)
649      public void openCamera(@NonNull String cameraId,
650              @NonNull final CameraDevice.StateCallback callback, @Nullable Handler handler)
651              throws CameraAccessException {
    
    
652  
653          openCameraForUid(cameraId, callback, CameraDeviceImpl.checkAndWrapHandler(handler),
654                  USE_CALLING_UID);
655      }

1. Calling this function requires the android.Manifest.permission.CAMERA permission
2. The first parameter is the camera ID
3. The second parameter is the callback object for opening the camera operation. When the opening fails or succeeds, the corresponding function of the callback will be called back.
4. The third parameter is Handler. The callback will be executed in the thread corresponding to the handler. If the parameter is empty, the callback will be executed in the current thread.
5. Call the openCameraForUid function of CameraManager

Before calling openCameraForUid, some processing will be done on the parameter handler:

frameworks/base/core/java/android/hardware/camera2/impl/CameraDeviceImpl.java
2190      public static Executor checkAndWrapHandler(Handler handler) {
    
    
2191          return new CameraHandlerExecutor(checkHandler(handler));
2192      }

Use the checkHandler return value as a parameter to construct a CameraHandlerExecutor object and return it.

frameworks/base/core/java/android/hardware/camera2/impl/CameraDeviceImpl.java
2202      static Handler checkHandler(Handler handler) {
    
    
2203          if (handler == null) {
    
    
2204              Looper looper = Looper.myLooper();
2205              if (looper == null) {
    
    
2206                  throw new IllegalArgumentException(
2207                      "No handler given, and current thread has no looper!");
2208              }
2209              handler = new Handler(looper);
2210          }
2211          return handler;
2212      }

1. If handler is empty, construct a Handler object with the looper object of the current thread as a parameter and return
2. If handler is not empty, return handler directly.

frameworks/base/core/java/android/hardware/camera2/impl/CameraDeviceImpl.java
2144      private static class CameraHandlerExecutor implements Executor {
    
    
2145          private final Handler mHandler;
2146  
2147          public CameraHandlerExecutor(@NonNull Handler handler) {
    
    
2148              mHandler = Objects.requireNonNull(handler);
2149          }
2150  
2151          @Override
2152          public void execute(Runnable command) {
    
    
2153              // Return value of 'post()' will be ignored in order to keep the
2154              // same camera behavior. For further details see b/74605221 .
2155              mHandler.post(command);
2156          }
2157      }

3. Assign the parameter handler to the member variable mHandler. When the parameter of Objects.requireNonNull is null, a null pointer exception will be thrown.
4. When executing the execute function, the post function of mHandler is actually called.

Back to the analysis of the openCameraForUid function:

frameworks/base/core/java/android/hardware/camera2/CameraManager.java
第四个参数为USE_CALLING_UID值为-1
711      public void openCameraForUid(@NonNull String cameraId,
712              @NonNull final CameraDevice.StateCallback callback, @NonNull Executor executor,
713              int clientUid)
714              throws CameraAccessException {
    
    
715  
716          if (cameraId == null) {
    
    
717              throw new IllegalArgumentException("cameraId was null");
718          } else if (callback == null) {
    
    
719              throw new IllegalArgumentException("callback was null");
720          }
721          if (CameraManagerGlobal.sCameraServiceDisabled) {
    
    
722              throw new IllegalArgumentException("No cameras available on device");
723          }
724  
725          openCameraDeviceUserAsync(cameraId, callback, executor, clientUid);
726      }

1. The cameraId is empty and an IllegalArgumentException is thrown.
2. callback is empty and an IllegalArgumentException is thrown.
3. sCameraServiceDisabled is empty and an IllegalArgumentException is thrown.
4. Call the openCameraDeviceUserAsync function. This function has a lot of content and will be analyzed in sections.

(1) Get the CameraCharacteristics object based on cameraId.

 CameraCharacteristics characteristics = getCameraCharacteristics(cameraId);

Implementation of getCameraCharacteristics:

//获取CameraService代理对象
ICameraService cameraService = CameraManagerGlobal.get().getCameraService();
//获取屏幕显示尺寸参数
Size displaySize = getDisplaySize();
//调用CameraService的getCameraCharacteristics函数返回CameraMetadataNative对象
CameraMetadataNative info = cameraService.getCameraCharacteristics(cameraId);
//设置CameraMetadataNative对象的一些参数
try {
    
    
	info.setCameraId(Integer.parseInt(cameraId));
} catch (NumberFormatException e) {
    
    
	// For external camera, reaching here is expected.
	Log.v(TAG, "Failed to parse camera Id " + cameraId + " to integer");
}
boolean hasConcurrentStreams =
CameraManagerGlobal.get().cameraIdHasConcurrentStreamsLocked(cameraId);
info.setHasMandatoryConcurrentStreams(hasConcurrentStreams);
info.setDisplaySize(displaySize); 
//以CameraMetadataNative为参数构造CameraCharacteristics对象返回
characteristics = new CameraCharacteristics(info);

Continue to look at the getCameraCharacteristics function in CameraService

frameworks/av/services/camera/libcameraservice/CameraService.cpp

This chapter is over, let’s continue the next chapter.

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