调用摄像头和相册

调用摄像头拍照
虽然Android赋予我们了在应用程序中打开摄像头拍照的能力，但是

从相册中选择图片
虽然调用摄像头拍照既方便又快捷，但我们并不是每次都需要当场拍一张照片的。因为每个人的手机相册里应该都会存有许多张图片，直接从相册里选取一张现有的图片会比打开相机拍一张照片更加常用。示例代码如下：
class MainActivity : AppCompatActivity() {

    override fun onCreate(savedInstanceState: Bundle?) {
        val intent = Intent(Intent.ACTION_OPEN_DOCUMENT)
        intent.addCategory(Intent.CATEGORY_OPENABLE) 
        intent.type = "image/*"
        startActivityForResult(intent, fromAlbum)
    }

    override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)
        when (requestCode) {
            fromAlbum -> {
                if (resultCode == Activity.RESULT_OK && data != null) {
                    data.data?.let { uri ->
                        val bitmap = getBitmapFromUri(uri)
                        imageView.setImageBitmap(bitmap)
                    }
                }
            }
        }
    }

    private fun getBitmapFromUri(uri: Uri) = contentResolver.openFileDescriptor(uri, "r")?.use {
        BitmapFactory.decodeFileDescriptor(it.fileDescriptor)
    }
    …
}

通常情况下并没有必要这么做，因为我们可以直接打开系统的相机程序进行拍照，然后获取返回的图片即可。示例代码如下：
class MainActivity : AppCompatActivity() {
    lateinit var imageUri: Uri
    lateinit var outputImage: File
    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        outputImage = File(externalCacheDir, "output_image.jpg")
        if (outputImage.exists()) {
            outputImage.delete()
        }
        outputImage.createNewFile()
        imageUri = if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) {
            FileProvider.getUriForFile(this, "com.example.cameraalbumtest.fileprovider", outputImage)
        } else {
            Uri.fromFile(outputImage)
        }
        val intent = Intent("android.media.action.IMAGE_CAPTURE")
        intent.putExtra(MediaStore.EXTRA_OUTPUT, imageUri)
        startActivityForResult(intent, takePhoto)
    }

    override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)
        when (requestCode) {
            takePhoto -> {
                if (resultCode == Activity.RESULT_OK) {val bitmap = BitmapFactory.decodeStream(contentResolver.openInputStream(imageUri))
                    imageView.setImageBitmap( bitmap)
                }
            }
        }
    }
    ...
}