The flutter command-line tool supports a daemon server mode for use by IDEs and other tools.
flutter daemon
It runs a persistent, JSON-RPC based server to communicate with devices. IDEs and other tools can start the flutter tool in this mode and get device addition and removal notifications, as well as being able to programmatically start and stop apps on those devices.
A set of flutter daemon commands/events are also exposed via flutter run --machine and flutter attach --machine which allow IDEs and tools to launch and attach to flutter applications and interact to send commands like Hot Reload. The command and events that are available in these modes are documented at the bottom of this document.
The daemon speaks JSON-RPC to clients. It uses stdin and stdout as the protocol transport. To send a command to the server, create your command as a JSON-RPC message, encode it to json, surround the encoded text with square brackets, and write it as one line of text to the stdin of the process:
[{"method":"daemon.version","id":0}]
The response will come back as a single line from stdout:
[{"id":0,"result":"0.1.0"}]
All requests and responses should be wrapped in square brackets. This ensures that the communications are resilient to stray output in the stdout/stdin stream.
id is an opaque type to the server, but ids should be unique for the life of the server. A response to a particular command will contain the id that was passed in for that command.
Each command should have a method field. This is in the form ‘domain.command’.
Any params for that command should be passed in through a params field. Here's a example request/response for the device.getDevices method:
[{"method":"device.getDevices","id":2}]
[{"id":2,"result":[{"id":"702ABC1F-5EA5-4F83-84AB-6380CA91D39A","name":"iPhone 6","platform":"ios_x64","available":true}]}]
Events that come from the server will have an event field containing the type of event, along with a params field.
[{"event":"device.added","params":{"id":"1DD6786B-37D4-4355-AA15-B818A87A18B4","name":"iPhone XS Max","platform":"ios","emulator":true}}]
The version() command responds with a String with the protocol version.
The shutdown() command will terminate the flutter daemon. It is not necessary to call this before shutting down the daemon; it is perfectly acceptable to just kill the daemon process.
The getSupportedPlatforms() command will enumerate all platforms supported by the project located at the provided projectRoot. It returns a Map with the key ‘platforms’ containing a List of strings which describe the set of all possibly supported platforms. Possible values include:
The daemon.connected event is sent when the daemon starts. The params field will be a map with the following fields:
version: The protocol version. This is the same version returned by the version() command.pid: The pid of the daemon process.This is sent when user-facing output is received. The params field will be a map with the field log. The log field is a string with the output text. If the output indicates an error, an error boolean field will be present, and set to true.
The daemon.showMessage event is sent by the daemon when some if would be useful to show a message to the user. This could be an error notification or a notification that some development tools are not configured or not installed. The JSON message will contain an event field with the value daemon.showMessage, and an params field containing a map with level, title, and message fields. The valid options for level are info, warning, and error.
It is up to the client to decide how best to display the message; for some clients, it may map well to a toast style notification. There is an implicit contract that the daemon will not send too many messages over some reasonable period of time.
The daemon.logMessage event is sent whenever a log message is created - either a status level message or an error. The JSON message will contain an event field with the value daemon.logMessage, and an params field containing a map with level, message, and (optionally) stackTrace fields.
Generally, clients won‘t display content from daemon.logMessage events unless they’re set to a more verbose output mode.
The restart() restarts the given application. It returns a Map of { int code, String message } to indicate success or failure in restarting the app. A code of 0 indicates success, and non-zero indicates a failure.
appId: the id of a previously started app; this is required.fullRestart: optional; whether to do a full (rather than an incremental) restart of the applicationreason: optional; the reason for the full restart (eg. save, manual) for reporting purposespause: optional; when doing a hot restart the isolate should enter a paused modeThe callServiceExtension() allows clients to make arbitrary calls to service protocol extensions. It returns a Map - the result returned by the service protocol method.
appId: the id of a previously started app; this is required.methodName: the name of the service protocol extension to invoke; this is required.params: an optional Map of parameters to pass to the service protocol extension.The detach() command takes one parameter, appId. It returns a bool to indicate success or failure in detaching from an app without stopping it.
appId: the id of a previously started app; this is required.The stop() command takes one parameter, appId. It returns a bool to indicate success or failure in stopping an app.
appId: the id of a previously started app; this is required.This is sent when an app is starting. The params field will be a map with the fields appId, directory, deviceId, and launchMode.
This is sent when an observatory port is available for a started app. The params field will be a map with the fields appId, port, and wsUri. Clients should prefer using the wsUri field in preference to synthesizing a uri using the port field. An optional field, baseUri, is populated if a path prefix is required for setting breakpoints on the target device.
This is sent once the application launch process is complete and the app is either paused before main() (if startPaused is true) or main() has begun running. When attaching, this even will be fired once attached. The params field will be a map containing the field appId.
This is sent when output is logged for a running application. The params field will be a map with the fields appId and log. The log field is a string with the output text. If the output indicates an error, an error boolean field will be present, and set to true.
This is sent when an operation starts and again when it stops. When an operation starts, the event contains the fields id, an opaque identifier, and message containing text describing the operation. When that same operation ends, the event contains the same id field value as when the operation started, along with a finished bool field with the value true, but no message field.
This is sent when an app is stopped or detached from. The params field will be a map with the field appId.
Return a list of all connected devices. The params field will be a List; each item is a map with the fields id, name, platform, and emulator (a boolean).
Turn on device polling. This will poll for newly connected devices, and fire device.added and device.removed events.
Turn off device polling.
Forward a host port to a device port. This call takes two required arguments, deviceId and devicePort, and one optional argument, hostPort. If hostPort is not specified, the host port will be any available port.
This method returns a map with a hostPort field set.
Removed a forwarded port. It takes deviceId, devicePort, and hostPort as required arguments.
This is sent when a device is connected (and polling has been enabled via enable()). The params field will be a map with the fields id, name, platform, and emulator.
This is sent when a device is disconnected (and polling has been enabled via enable()). The params field will be a map with the fields id, name, platform, and emulator.
Return a list of all available emulators. The params field will be a List; each item is a map with the fields id and name.
The launch() command allows launching an emulator/simulator by its id.
emulatorId: the id of an emulator as returned by getEmulators.The create() command creates a new Android emulator with an optional name.
name: an optional name for this emulatorThe returned params will contain:
success - whether the emulator was successfully createdemulatorName - the name of the emulator created; this will have been auto-generated if you did not supply oneerror - when success=false, a message explaining why the creation of the emulator failedWhen running flutter run --machine or flutter attach --machine the following subset of the daemon is available:
The following subset of the daemon domain is available in flutter run --machine. Refer to the documentation above for details.
The following subset of the app domain is available in flutter run --machine. Refer to the documentation above for details.
See the source for the daemon protocol and implementation.
app.detach commandflutter attach --machineemulator.create commanddaemon.connected event at startup