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](http://json-rpc.org/) 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:
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:
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.
The `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.
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`.
#### app.progress
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.
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`, `category`, `platformType`, `ephemeral`, and `emulator` (a boolean).
`category` is string description of the kind of workflow the device supports. The current categories are "mobile", "web" and "desktop", or null if none.
`platformType` is a string description of the platform sub-folder the device
supports. The current catgetories are "android", "ios", "linux", "macos",
"fuchsia", "windows", and "web". These are kept in sync with the response from `daemon.getSupportedPlatforms`.
`ephemeral` is a boolean which indicates where the device needs to be manually connected to a development machine. For example, a physical Android device is ephemeral, but the "web" device (that is always present) is not.
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.
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`, `category`, `platformType`, `ephemeral`, and `emulator`. For more information on `platform`, `category`, `platformType`, and `ephemeral` see `device.getDevices`.
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`, `category`, `platformType`, `ephemeral`, and `emulator`. For more information on `platform`, `category`, `platformType`, and `ephemeral` see `device.getDevices`.
See the [source](https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/commands/daemon.dart) for the daemon protocol and implementation.
