KMM项目中如何在iOS端用Ktor上传30MB以上的视频？

针对30MB+的大视频上传，核心思路是流式上传避免全量加载文件到内存，通过KMM的expect/actual机制封装平台特定的文件流处理，统一跨平台上传逻辑。

1. 跨平台文件封装（Common层） #

先定义平台无关的文件抽象，通过expect声明类，两端分别实现：

// commonMain/kotlin/com/yourpackage/PlatformFile.kt
expect class PlatformFile(path: String) {
    fun asUploadPart(): PartData.FileItem
}

// 共用上传接口
suspend fun uploadVideo(
    file: PlatformFile,
    uploadUrl: String,
    contentType: ContentType = ContentType.Video.MP4
): Result<Unit> {
    return runCatching {
        val client = HttpClient(getPlatformClient())
        client.post(uploadUrl) {
            setBody(MultiPartFormDataContent(formData {
                append(
                    "video_file",
                    file.asUploadPart(),
                    headersOf(HttpHeaders.ContentType, contentType.toString())
                )
            }))
            // 添加其他必要请求头，比如Authorization
            headers {
                append(HttpHeaders.Authorization, "Bearer YOUR_TOKEN")
            }
        }
    }
}

expect fun getPlatformClient(): HttpClientEngine

2. Android端实现（AndroidMain） #

Android直接用File和输入流转换为ByteReadChannel：

// androidMain/kotlin/com/yourpackage/PlatformFile.kt
actual class PlatformFile actual constructor(path: String) {
    private val file = File(path)

    actual fun asUploadPart(): PartData.FileItem {
        return PartData.FileItem(
            provider = { file.inputStream().toByteReadChannel() },
            headers = Headers.build {
                append(HttpHeaders.ContentDisposition, "filename=\"${file.name}\"")
            }
        )
    }
}

actual fun getPlatformClient(): HttpClientEngine = OkHttp.create()

3. iOS端实现（iOSMain） #

iOS通过NSURL获取文件输入流，再转换为Kotlin协程的ByteReadChannel：

// iosMain/kotlin/com/yourpackage/PlatformFile.kt
import kotlinx.coroutines.io.reader
import kotlinx.coroutines.io.writeFully
import kotlinx.coroutines.io.ByteReadChannel
import platform.Foundation.NSFileHandle
import platform.Foundation.NSURL
import platform.Foundation.NSInputStream

actual class PlatformFile actual constructor(path: String) {
    private val fileUrl = NSURL.fileURLWithPath(path)

    actual fun asUploadPart(): PartData.FileItem {
        val inputStream = NSFileHandle.fileHandleForReadingFromURL(fileUrl, null)?.inputStream()
            ?: error("Failed to open file for reading: $path")

        return PartData.FileItem(
            provider = { inputStream.toByteReadChannel() },
            headers = Headers.build {
                append(HttpHeaders.ContentDisposition, "filename=\"${fileUrl.lastPathComponent}\"")
            }
        )
    }
}

// 扩展方法：将NSInputStream转换为ByteReadChannel
private fun NSInputStream.toByteReadChannel(): ByteReadChannel = reader {
    val buffer = ByteArray(8192) // 8KB缓冲区，可根据需求调整
    open()
    try {
        while (true) {
            val bytesRead = read(buffer, 0, buffer.size.toULong()).toInt()
            if (bytesRead <= 0) break
            channel.writeFully(buffer, 0, bytesRead)
        }
    } finally {
        close()
    }
}

actual fun getPlatformClient(): HttpClientEngine = Ios.create()

关键说明 #

• 流式处理：两端均使用固定大小缓冲区读取文件，避免加载全量文件到内存，解决大文件OOM问题。
• MultiPart上传：采用标准的表单上传格式，适配大多数后端接口，同时支持断点续传扩展（需后端配合）。
• 权限处理：iOS端需确保文件路径在沙箱可访问范围内（如Documents目录），并在Info.plist中声明必要的文件访问权限。

内容的提问来源于stack exchange，提问作者Akshat