如何使用CameraX和ViewModel

CameraX是Android Jetpack的一部分，它提供了一个易用、可测试且可扩展的相机应用接口，使得开发者能够更容易地使用Android的相机功能。而ViewModel是Jetpack的另一个组件，它主要用于管理UI相关数据，并且在配置更改（如屏幕旋转）时能够保持数据不丢失。以下是使用CameraX和ViewModel的一个基本示例：

1. 首先，在项目的`build.gradle`文件中添加CameraX依赖项：

dependencies {
    // CameraX核心库
    implementation "androidx.camera:camera-camera2:1.0.0"
    // CameraX生命周期库
    implementation "androidx.camera:camera-lifecycle:1.0.0"
    // CameraX视图库
    implementation "androidx.camera:camera-view:1.0.0-alpha15"
}

2. 在ViewModel中管理CameraX的状态和数据。例如，创建一个ViewModel来存储相机相关的参数和状态：

import androidx.lifecycle.LiveData
import androidx.lifecycle.MutableLiveData
import androidx.lifecycle.ViewModel
import androidx.lifecycle.viewModelScope
import androidx.camera.view.PreviewView
import androidx.camera.core.Camera
import androidx.camera.core.ImageAnalysis
import androidx.camera.core.Preview
import androidx.camera.lifecycle.ProcessCameraProvider
import kotlinx.coroutines.launch

class CameraViewModel : ViewModel() {
    private val _camera: MutableLiveData<Camera> = MutableLiveData()
    val camera: LiveData<Camera> = _camera

    fun startCamera(previewView: PreviewView) {
        val cameraProviderFuture = ProcessCameraProvider.getInstance(previewView.context)
        cameraProviderFuture.addListener(Runnable {
            val cameraProvider = cameraProviderFuture.get()
            bindPreview(cameraProvider, previewView)
        }, ContextCompat.getMainExecutor(previewView.context))
    }

    private fun bindPreview(cameraProvider: ProcessCameraProvider, previewView: PreviewView) {
        val preview = Preview.Builder().build().also {
            it.setSurfaceProvider(previewView.createSurfaceProvider())
        }

        val cameraSelector = CameraSelector.Builder()
            .requireLensFacing(CameraSelector.LENS_FACING_BACK)
            .build()

        val imageAnalysis = ImageAnalysis.Builder()
            .build()
            .also {
                // 可以在这里添加分析器，例如用于图像处理或机器学习的代码
            }

        try {
            cameraProvider.unbindAll()
            cameraProvider.bindToLifecycle(this, cameraSelector, preview, imageAnalysis, _camera)
        } catch (e: Exception) {
            // 处理错误，例如相机正在使用或错误绑定
        }
    }
}

3. 在Activity或Fragment中使用CameraViewModel来启动相机预览。首先创建ViewModel实例，然后在视图准备好之后（例如在Activity的`onCreate()`方法或者Fragment的`onViewCreated()`方法中），调用ViewModel的`startCamera()`方法。

class CameraActivity : AppCompatActivity() {
    private lateinit var cameraPreview: PreviewView
    private lateinit var cameraViewModel: CameraViewModel

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_camera)

        cameraPreview = findViewById(R.id.camera_preview)
        cameraViewModel = ViewModelProvider(this).get(CameraViewModel::class.java)
        cameraViewModel.startCamera(cameraPreview)
    }
}