1. CameraX 最小预览

最小预览的组成与生命周期

CameraX 最小预览由四个对象协作完成：PreviewView 负责显示画面，Preview 用例负责从相机取流，ProcessCameraProvider 负责把用例绑定到相机，CameraSelector 决定使用哪颗镜头。绑定发生在 onCreate 之后、界面可见之前；解绑由 LifecycleOwner 自动完成，无需手动调用 unbind。

权限申请与绑定实现

CameraX 需要 CAMERA 权限。使用 registerForActivityResult 注册权限请求，在回调中判断授权结果，授权成功后再执行绑定。绑定前必须确保 PreviewView 已完成布局，否则 SurfaceProvider 可能为空。

class MainActivity : AppCompatActivity() {
    private lateinit var previewView: PreviewView
    private lateinit var cameraProvider: ProcessCameraProvider

    private val requestPermission =
        registerForActivityResult(ActivityResultContracts.RequestPermission()) { granted ->
            if (granted) startCamera()
        }

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        previewView = PreviewView(this)
        setContentView(previewView)

        if (ContextCompat.checkSelfPermission(this, Manifest.permission.CAMERA)
            == PackageManager.PERMISSION_GRANTED) {
            startCamera()
        } else {
            requestPermission.launch(Manifest.permission.CAMERA)
        }
    }

    private fun startCamera() {
        val future = ProcessCameraProvider.getInstance(this)
        future.addListener({
            cameraProvider = future.get()
            val preview = Preview.Builder().build().also {
                it.setSurfaceProvider(previewView.surfaceProvider)
            }
            cameraProvider.unbindAll()
            cameraProvider.bindToLifecycle(
                this,
                CameraSelector.DEFAULT_BACK_CAMERA,
                preview
            )
        }, ContextCompat.getMainExecutor(this))
    }
}

验证与常见问题