Android 10下Jetpack Compose读取外部存储图片失败求助：权限已获但文件不可读

问题背景 #

正在将一款大型天文应用迁移至Jetpack Compose，已完成Room数据库、OpenGL视图与渲染器、多个复杂页面等迁移工作，但卡在读取外部存储图片的基础功能上。

使用环境 #

• Android 10（API 29）
• AndroidManifest已添加<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>权限
• 依赖库：

  implementation "com.google.accompanist:accompanist-permissions:0.29.1-alpha"
  implementation "io.coil-kt:coil-compose:2.2.2"
  

问题现象 #

测试Composable函数中：

• 外部存储目录可访问，目标文件存在且大小正确
• 权限检查结果为已授予
• 但file.canRead()返回false，图片无法加载显示

测试代码 #

@OptIn(ExperimentalMaterialApi::class, ExperimentalPermissionsApi::class)
@Composable
fun SheetContent() {

    val context = LocalContext.current

    val readExternalMemoryState = rememberPermissionState(
        Manifest.permission.READ_EXTERNAL_STORAGE
    )
    if (readExternalMemoryState.status.isGranted) {
        Text("Read External memory permission Granted")
    } else {
        AskPermission(permissionState = readExternalMemoryState)
    }

    val path = Environment.getExternalStorageDirectory().toString() + "/Download/"
    val file = File(path + "solar-system.png")
    val filesize = file.length()
    
    if (file.exists()) {
        Text("File exist with size: $filesize")
        when (ContextCompat.checkSelfPermission(context, Manifest.permission.READ_EXTERNAL_STORAGE
        )) {
            PackageManager.PERMISSION_GRANTED -> {
                //File read logic here
                Text("checkSelfPermission gives status: Granted")
                if (file.canRead()) {
                    Text("File can be read")
                    Image(
                        rememberAsyncImagePainter(file),
                        contentDescription = ""
                    )
                } else {
                    Text("File not readable")
                }
            }
        }
    } else {
        Text("Not available")
    }
}

输出结果 #

Read External memory permission Granted
File exist with size: 20534
checkSelfPermission gives status: Granted
File not readable

解决方案 #

方案1：启用旧版存储访问兼容（快速解决） #

Android 10引入了分区存储机制，默认限制了直接通过File路径访问外部存储文件的权限。在AndroidManifest.xml的<application>标签中添加以下属性，即可暂时禁用分区存储，恢复旧的文件访问逻辑：

<application
    ...
    android:requestLegacyExternalStorage="true">
    ...
</application>

添加后重新编译运行，file.canRead()应该会返回true，Coil也能正常加载图片。

方案2：适配分区存储（推荐长期方案） #

如果希望遵循Android 10及以上的分区存储规范，建议通过MediaStore查询文件Uri，再用Coil加载Uri而非直接操作File：

1. 编写工具函数获取Download目录下指定文件名的Uri：

fun getImageUriFromDownloads(context: Context, fileName: String): Uri? {
    val projection = arrayOf(MediaStore.Images.Media._ID)
    val selection = "${MediaStore.Images.Media.DISPLAY_NAME} = ?"
    val selectionArgs = arrayOf(fileName)
    val sortOrder = "${MediaStore.Images.Media.DATE_ADDED} DESC"

    context.contentResolver.query(
        MediaStore.Images.Media.EXTERNAL_CONTENT_URI,
        projection,
        selection,
        selectionArgs,
        sortOrder
    )?.use { cursor ->
        if (cursor.moveToFirst()) {
            val idColumn = cursor.getColumnIndexOrThrow(MediaStore.Images.Media._ID)
            val id = cursor.getLong(idColumn)
            return ContentUris.withAppendedId(MediaStore.Images.Media.EXTERNAL_CONTENT_URI, id)
        }
    }
    return null
}

2. 在Compose中修改加载逻辑：

@OptIn(ExperimentalMaterialApi::class, ExperimentalPermissionsApi::class)
@Composable
fun SheetContent() {
    val context = LocalContext.current
    val readExternalMemoryState = rememberPermissionState(
        Manifest.permission.READ_EXTERNAL_STORAGE
    )

    if (readExternalMemoryState.status.isGranted) {
        Text("Read External memory permission Granted")
        val imageUri = remember { getImageUriFromDownloads(context, "solar-system.png") }
        imageUri?.let { uri ->
            Text("File found via MediaStore")
            Image(
                painter = rememberAsyncImagePainter(uri),
                contentDescription = "Solar system image"
            )
        } ?: run {
            Text("File not found in MediaStore")
        }
    } else {
        AskPermission(permissionState = readExternalMemoryState)
    }
}

补充说明 #

• 分区存储是Android 10及以上的默认行为，requestLegacyExternalStorage在Android 11（API 30）及以上会被忽略，所以长期来看推荐采用方案2适配。
• Coil支持直接加载Uri对象，无需额外转换，适配分区存储时使用Uri加载是更合规的方式。

内容的提问来源于stack exchange，提问作者Jelle