android previewview怎样实现

在Android中，PreviewView 是一个用于预览相机捕获的图像和视频的视图。要在应用中使用 PreviewView，你需要遵循以下步骤：

添加依赖项

在你的项目的 build.gradle 文件中，添加以下依赖项：

dependencies {
    implementation 'androidx.camera:camera-core:1.0.0'
    implementation 'androidx.camera:camera-预览:1.0.0'
    implementation 'androidx.camera:camera-view:1.0.0'
}

在布局文件中添加 PreviewView

在你的应用的布局文件（例如 activity_main.xml）中，添加一个 PreviewView 控件：

<androidx.camera.core.PreviewView
    android:id="@+id/previewView"
    android:layout_width="match_parent"
    android:layout_height="match_parent"/>

配置相机源和预览

在你的 Activity 或 Fragment 中，配置相机源和预览。首先，获取 PreviewView 的引用，然后创建一个 CameraProvider 实例。接下来，为 PreviewView 设置一个预览回调，以便在捕获图像时显示预览。

import android.Manifest
import android.content.Context
import android.content.pm.PackageManager
import androidx.camera.core.*
import androidx.camera.core.ImageAnalysis
import androidx.camera.core.ImageProxy
import androidx.camera.lifecycle.ProcessCameraProvider
import androidx.camera.view.PreviewView
import androidx.core.app.ActivityCompat
import androidx.core.content.ContextCompat
import java.util.concurrent.ExecutorService
import java.util.concurrent.Executors

class MainActivity : AppCompatActivity() {

    private lateinit var previewView: PreviewView
    private lateinit var cameraProviderFuture: ProcessCameraProvider
    private lateinit var cameraProvider: ProcessCameraProvider
    private lateinit var imageAnalyzer: ImageAnalyzer

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

        previewView = findViewById(R.id.previewView)

        // 检查相机权限
        if (ContextCompat.checkSelfPermission(this, Manifest.permission.CAMERA) != PackageManager.PERMISSION_GRANTED) {
            ActivityCompat.requestPermissions(this, arrayOf(Manifest.permission.CAMERA), REQUEST_CAMERA_PERMISSION)
            return
        }

        // 创建一个单线程的 ExecutorService 用于运行相机操作
        val cameraExecutor = Executors.newSingleThreadExecutor()

        // 获取 CameraProvider
        cameraProviderFuture = ProcessCameraProvider.getInstance(this, cameraExecutor)

        // 创建一个 ImageAnalyzer
        imageAnalyzer = ImageAnalyzer()

        // 设置预览回调
        previewView.setSurfaceProvider(previewView.surfaceProvider)
        cameraProviderFuture.addListener({
            cameraProvider = cameraProviderFuture.get()
            bindPreviewView()
        }, ContextCompat.getMainExecutor(this))
    }

    private fun bindPreviewView() {
        cameraProvider.unbindAll()
        val preview = Preview.Builder().build().also {
            it.setSurfaceProvider(previewView.surfaceProvider)
        }

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

        try {
            cameraProvider.bindToLifecycle(this, cameraSelector, preview, imageAnalyzer)
        } catch (exc: Exception) {
            // 处理异常
        }
    }

    override fun onRequestPermissionsResult(requestCode: Int, permissions: Array<out String>, grantResults: IntArray) {
        super.onRequestPermissionsResult(requestCode, permissions, grantResults)
        if (requestCode == REQUEST_CAMERA_PERMISSION && grantResults.isNotEmpty() && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
            // 相机权限被授予，重新尝试绑定预览视图
            bindPreviewView()
        }
    }

    companion object {
        private const val REQUEST_CAMERA_PERMISSION = 1
    }
}

创建 ImageAnalyzer

创建一个继承自 ImageAnalysis 的类，用于分析捕获的图像。在这个类中，你可以使用 ImageProxy 对象来处理图像数据。

import android.renderscript.Allocation
import android.renderscript.Element
import android.renderscript.RenderScript
import androidx.camera.core.ImageProxy
import java.nio.ByteBuffer

class ImageAnalyzer : ImageAnalysis {

    private lateinit var rs: RenderScript
    private lateinit var inAllocation: Allocation
    private lateinit var outAllocation: Allocation

    override fun createImageAnalyzer(context: Context): ImageAnalyzer {
        rs = RenderScript.create(context)
        inAllocation = Allocation.createFromBitmap(rs, BitmapFactory.decodeResource(context.resources, R.drawable.ic_launcher_background))
        outAllocation = Allocation.createTyped(rs, Element.U8_4(rs))
        return this
    }

    override fun analyze(imageProxy: ImageProxy) {
        val buffer = imageProxy.image?.planes?.firstOrNull()?.buffer
        val byteArray = ByteArray(buffer?.capacity() ?: 0)
        buffer?.get(byteArray)

        // 处理图像数据，例如识别物体、人脸等

        imageProxy.close()
    }

    override fun close() {
        inAllocation.destroy()
        outAllocation.destroy()
        rs.destroy()
    }
}

现在，当你运行应用时，你应该能够看到一个预览窗口，实时显示相机捕获的图像和视频。你可以根据需要对 ImageAnalyzer 类进行扩展，以实现更高级的图像处理功能。

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