[译] Android 的多摄像头支持

val cameraDevice = Camera.open(0)
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val cameraManager = activity.getSystemService(Context.CAMERA_SERVICE) as CameraManager
val cameraId = cameraManager.cameraIdList[0]
cameraManager.openCamera(cameraId, object : CameraDevice.StateCallback() {
    override fun onOpened(device: CameraDevice) {
        // Do something with `device`
    }
    override fun onDisconnected(device: CameraDevice) {
        device.close()
    }
    override fun onError(device: CameraDevice, error: Int) {
        onDisconnected(device)
    }
}, null)
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val cameraIdList = cameraManager.cameraIdList // may be empty
val characteristics = cameraManager.getCameraCharacteristics(cameraId)
val cameraLensFacing = characteristics.get(CameraCharacteristics.LENS_FACING)
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fun getFirstCameraIdFacing(cameraManager: CameraManager,
                           facing: Int = CameraMetadata.LENS_FACING_BACK): String? {
    val cameraIds = cameraManager.cameraIdList
    // Iterate over the list of cameras and return the first one matching desired
    // lens-facing configuration
    cameraIds.forEach {
        val characteristics = cameraManager.getCameraCharacteristics(it)
        if (characteristics.get(CameraCharacteristics.LENS_FACING) == facing) {
            return it
        }
    }
    // If no camera matched desired orientation, return the first one from the list
    return cameraIds.firstOrNull()
}
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fun filterCameraIdsFacing(cameraIds: Array<String>, cameraManager: CameraManager,
                          facing: Int): List<String> {
    return cameraIds.filter {
        val characteristics = cameraManager.getCameraCharacteristics(it)
        characteristics.get(CameraCharacteristics.LENS_FACING) == facing
    }
}

fun getNextCameraId(cameraManager: CameraManager, currCameraId: String? = null): String? {
    // Get all front, back and external cameras in 3 separate lists
    val cameraIds = cameraManager.cameraIdList
    val backCameras = filterCameraIdsFacing(
            cameraIds, cameraManager, CameraMetadata.LENS_FACING_BACK)
    val frontCameras = filterCameraIdsFacing(
            cameraIds, cameraManager, CameraMetadata.LENS_FACING_FRONT)
    val externalCameras = filterCameraIdsFacing(
            cameraIds, cameraManager, CameraMetadata.LENS_FACING_EXTERNAL)

    // The recommended order of iteration is: all external, first back, first front
    val allCameras = (externalCameras + listOf(
            backCameras.firstOrNull(), frontCameras.firstOrNull())).filterNotNull()

    // Get the index of the currently selected camera in the list
    val cameraIndex = allCameras.indexOf(currCameraId)

    // The selected camera may not be on the list, for example it could be an
    // external camera that has been removed by the user
    return if (cameraIndex == -1) {
        // Return the first camera from the list
        allCameras.getOrNull(0)
    } else {
        // Return the next camera from the list, wrap around if necessary
        allCameras.getOrNull((cameraIndex + 1) % allCameras.size)
    }
}
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