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Android Host Service

Host Service lets the host application register named native functions that journey JavaScript can call with await HostService.invokeFunction(name, args, callbacks). It also provides host-side utilities such as the image picker.

Functions​

registerFunction​

fun registerFunction(
    name: String,
    handler: HostFunctionHandler
): Unit

Registers a named host function callable from journey JavaScript as await HostService.invokeFunction(name, args, callbacks). Re-registering the same name replaces the previous handler and logs a warning.

Parameters:

  • name: String — Unique function name as seen from JavaScript.
  • handler: HostFunctionHandler — Invoked when a script calls this function, see HostFunctionHandler.

unregisterFunction​

fun unregisterFunction(name: String): Unit

Removes a registered host function. No-op if not present.

Parameters:

  • name: String — Name of the function to remove.

cancelAllFunctions​

fun cancelAllFunctions(reason: String = "journey_cancelled"): Unit

Rejects every in-flight HostService.invokeFunction call with the given reason. Called when the journey hosting the JavaScript runtime is dismissed or cancelled.

Parameters:

  • reason: String — Rejection reason delivered to the scripts. Defaults to "journey_cancelled".

pickImage​

fun pickImage(
    activity: Activity,
    source: ImagePickerSource = ImagePickerSource.CHOOSE,
    maxCount: Int = 1,
    maxWidth: Int = 512,
    maxHeight: Int = 512,
    quality: Double = 0.7,
    onSuccess: (List<String>) -> Unit,
    onError: (Throwable) -> Unit
): Unit

Presents the system image picker (gallery, camera, or a chooser), resizes the selected images, and returns their local file paths.

Parameters:

  • activity: Activity — Activity used to present the picker UI.
  • source: ImagePickerSource — Image source, see ImagePickerSource. Defaults to CHOOSE.
  • maxCount: Int — Maximum number of images the user can select. Defaults to 1.
  • maxWidth: Int — Maximum width of the resized images. Defaults to 512.
  • maxHeight: Int — Maximum height of the resized images. Defaults to 512.
  • quality: Double — JPEG compression quality between 0 and 1. Defaults to 0.7.
  • onSuccess: (List<String>) -> Unit — Called with local file paths of the processed images.
  • onError: (Throwable) -> Unit — Called when picking fails or the user cancels.

Classes​

HostFunctionHandler​

typealias HostFunctionHandler = (
    args: HostFunctionArgs,
    callbacks: HostCallbacks,
    resolve: (Any?) -> Unit,
    reject: (String) -> Unit,
) -> Unit

Handler signature registered with Host Service. Exactly one of resolve / reject must be called: resolve fulfills the script's promise with the given value, reject rejects it with the given message.

HostFunctionArgs​

class HostFunctionArgs(
    val raw: Map<String, Any?>,
) {
    operator fun get(key: String): Any?
    inline fun <reified T> value(key: String): T?
    inline fun <reified T> require(key: String): T
    inline fun <reified T> decode(key: String): T
}

Wraps the JSON args map delivered to a host function handler. value returns the value cast to T or null; require throws HostFunctionError if absent or the wrong type; decode deserializes a JSON-compatible field into a typed object.

HostCallbacks​

class HostCallbacks {
    fun call(name: String, value: Any? = null)
    fun has(name: String): Boolean
}

Proxy passed to host handlers for the callback functions the script supplied. call fires a named callback with an optional value (no-op if the script did not provide it); has returns whether the script provided a callback with this name.

HostFunctionError​

sealed class HostFunctionError(message: String) : Exception(message) {
    class InvalidArgs(detail: String) : HostFunctionError("invalid_args: $detail")
}

Errors a host function handler can throw before invoking resolve/reject.

ImagePickerSource​

enum class ImagePickerSource {
    GALLERY,
    CAMERA,
    CHOOSE,
}