Engine specific Service Protocol extensions

The Flutter engine adds several extensions to the Dart VM Service Protocol. The Flutter Engine specific extensions are documented here. Applications may also choose to register their own extensions.

List views: _flutter.listViews

Tooling requests this very early in the application lifecycle to ask for the details of the root isolate.

No arguments.

Response:

{
  "type": "FlutterViewList",
  "views": [
    {
      "type": "FlutterView",
      "id": "_flutterView/0x1066096d8",
      "isolate": {
        "type": "@Isolate",
        "fixedId": true,
        "id": "isolates/453229818",
        "name": "main.dart$main-453229818",
        "number": 453229818
      }
    }
  ]
}

Isolate Restart or Cold Reload: _flutter.runInView

The IDE has requested (or the user pressed ‘R’ from the ‘flutter run’ interactive console) a cold reload. For example, this happens when the user presses the green play button.

Used to "cold reload" a running application where the shell (along with the platform view and its rasterizer bindings) remains the same but the root isolate is torn down and restarted with the new configuration. Only used in the development workflow.

The previous root isolate is killed and its identifier invalidated after this call. Callers can query the new isolate identifier using the _flutter.listViews method.

Four arguments:

viewId = _flutterView/0x14ba08c68
mainScript = /path/to/application/lib/main.dart
packagesFile = /path/to/application/.packages
assetDirectory = /path/to/application/build/flutter_assets

Response:

{
  "type": "Success",
  "view": {
    "type": "FlutterView",
    "id": "_flutterView/0x104e0ab58",
    "isolate": {
      "type": "@Isolate",
      "fixedId": true,
      "id": "isolates/1056589762",
      "name": "main.dart$main-1056589762",
      "number": 1056589762
    }
  }
}

The object in the view key is constructed in the same way as the views in the List Views method.

Flush UI thread tasks: _flutter.flushUIThreadTasks

Does nothing but waits for all pending tasks on the UI thread to be completed before returning success to the service protocol caller.

No arguments.

Response:

{"type": "Success"}

Get screenshot of view as PNG: _flutter.screenshot

Get the screenshot as PNG of a random Flutter view on the device. The screenshot data will be base64 encoded in the response body.

No arguments.

Response:

{
  "type": "Screenshot",
  "screenshot": "<base64_data>"
}

Get screenshot of view as Skia picture: _flutter.screenshotSkp

Get the Skia SKP of a random Flutter view on the device. The SKP data will be base64 encoded in the response body.

No arguments.

Response:

{
  "type": "ScreenshotSkp",
  "skp": "<base64_data>"
}

Update asset bundle path: _flutter.setAssetBundlePath

In case of a hot-reload, the service protocol handles source code updates. However, there may be changes to assets. The DevFS updates assets in an separate directory that needs to be used by the engine.

Two arguments:

viewId = _flutterView/0x15bf057f8
assetDirectory = /path/to/flutter_assets

Response:

{
  "type": "Success",
  "view": {
    "type": "FlutterView",
    "id": "_flutterView/0x104e0ab58",
    "isolate": {
      "type": "@Isolate",
      "fixedId": true,
      "id": "isolates/1056589762",
      "name": "main.dart$main-1056589762",
      "number": 1056589762
    }
  }
}

The object in the view key is constructed in the same way as the views in the List Views method.

Get the display refresh rate: _flutter.getDisplayRefreshRate

Get the display refresh rate of the actual device that runs the Flutter view. For example, most devices would return an fps of 60, while iPad Pro would return an fps of 120.

One argument:

viewId = _flutterView/0x15bf057f8

Response:

{
  "type": "DisplayRefreshRate",
  "fps": 60.0
}

Get Skia SkSL shader artifacts: _flutter.getSkSLs

Get Skia SkSL shader artifacts from an actual device that runs the Flutter view. Such artifacts can be used to warm up shader compilations and avoid jank. One has to first tell Flutter to prepare SkSL shaders by flutter run --cache-sksl or flutter drive --cache-sksl, and trigger some shader compilations by going through some animations/transitions. Otherwise, this service protocol extension may return an empty set of SkSLs.

The key of the returned SkSLs map will be Base32 encoded. It should be used directly as the filename of the shader artifact. The value in that map is the Base64 encoded SkSL shader. Once decoded, it should be the content of the shader artifact file.

One argument:

viewId = _flutterView/0x15bf057f8

Response:

{
  "type": "GetSkSLs",
  "SkSLs": {
    "CAZAAAACAAAAAAAAAAABGAABAAOAAFAADQAAGAAQABSQAAAAAAAAAAAAAABAAAAAEAAGGAA": "eQ=="
  }
}

Estimate raster cache memory usage: _flutter.estimateRasterCacheMemory

Estimate the memory usage of both picture and layer raster cache. For each picture or layer cached, there is a rasterized SkImage of that picture or layer to speed up future draws. Only SkImage's memory usage is counted as other objects in the cache system are often much smaller compared to SkImage. Function SkImageInfo::computeMinByteSize is used to estimate the SkImage memory usage.

One argument

viewId = _flutterView/0x15bf057f8

Response:

{
    "type": "EstimateRasterCacheMemory",
    "layerBytes": 40000,
    "pictureBytes": 400
}