CameraX :如何显示16:9的比例预览？

dependencies {
    def camerax_version = "1.1.0-alpha07"
    implementation "androidx.camera:camera-core:$camerax_version"
    implementation "androidx.camera:camera-camera2:$camerax_version"
    implementation "androidx.camera:camera-lifecycle:$camerax_version"
    implementation "androidx.camera:camera-view:1.0.0-alpha23"
}

<androidx.camera.view.CameraView
    android:id="@+id/cameraView"
    android:layout_width="match_parent"
    android:layout_height="match_parent" />

val cameraView: CameraView = findViewById(R.id.cameraView)
cameraView.post {
    val ratio = Rational(16, 9)
    val previewConfig = PreviewConfig.Builder().setTargetAspectRatio(ratio).build()
    val preview = Preview(previewConfig)
    preview.setOnPreviewOutputUpdateListener { previewOutput ->
        val parent = cameraView.parent as ViewGroup
        parent.removeView(cameraView)
        parent.addView(cameraView, 0)
        cameraView.surfaceTexture = previewOutput.surfaceTexture
    }

    val cameraProviderFuture = ProcessCameraProvider.getInstance(this)
    cameraProviderFuture.addListener(Runnable {
        val cameraProvider = cameraProviderFuture.get()
        val cameraSelector = CameraSelector.DEFAULT_BACK_CAMERA
        cameraProvider.unbindAll()
        cameraProvider.bindToLifecycle(this, cameraSelector, preview)
    }, ContextCompat.getMainExecutor(this))
}