Debugging the engine

This page has some hints about debugging the engine.

See also Crashes for advice on handling engine crashes (specifically around obtaining stack traces, and reporting crashes in AOT Dart code).

Running a Flutter app with a local engine

Once the appropriate version of the engine is built (see Compiling the engine), run your Flutter app with:

$ flutter run --local-engine=XXXX`

to run an app with the local engine where XXXX should be replaced with the version you wish to use. For example, use --local-engine=android_debug_unopt to run a debug android engine or --local-engine=ios_debug_sim_unopt to run a debug iOS simulator engine.

It is important to always have a host_XXXX version of the engine built when using a local engine since Flutter uses the host build's version of Dart.

Bisecting a roll failure

If the engine roll is failing (see Autorollers), you can use git bisect on the engine repo to track down the offending commit, using the --local-engine command as described above to run the failing framework test with each version of the engine.

Tracing OpenGL calls in Skia

All OpenGL calls in Skia are guarded by either the GR_GL_CALL_NOERRCHECK or GR_GL_CALL_RET_NOERRCHECK macros. Trace events may be added in these macros to trace all GL calls made by Skia, for example in a patch like this.

Due to the number of events traced to the timeline, the trace buffer may be filled up very quickly. Unless you want to see only the traces for the past few frames, use an endless trace buffer (flutter run --endless-trace-buffer turns on an endless trace buffer).

Also, make sure to run your application with the --trace-skia flag.

Debugging iOS builds with Xcode

Building with flutter --local-engine will set a LOCAL_ENGINE Xcode build setting in your Flutter application Generated.xcconfig file. This will be set until you run flutter run again with either a different --local-engine option, or with none at all (which will unset it).

You can speed up your workflow by adding the --config-only flag to set up the Xcode build settings and plugins, but not compile the app. For example:

$ flutter build ios --local-engine ios_debug_unopt --config-only

To start debugging, open your Flutter app ios/Runner.xcworkspace file in Xcode. Ensure Product > Scheme > Edit Scheme > Run > Build Configuration matches your engine runtime mode (defaults to Debug).

Add an engine symbol breakpoint via Debug > Breakpoints > Create Symbolic Breakpoint.... The Symbol field should be the engine symbol you're interested in, like -[FlutterEngine runWithEntrypoint:] (note the -[ prefix has no space).

You can also set a breakpoint directly with lldb by expanding Flutter > Runner > Supporting Files > main.m in the Runner Project Navigator. Put a breakpoint in main() and start the application by clicking the Run button (CMD + R). Then, set your desired breakpoint in the engine in lldb via breakpoint set -....

Debugging Android builds with gdb

See https://github.com/flutter/engine/blob/master/sky/tools/flutter_gdb#L13

Debugging Android builds with Android Studio

First, import the Android embedding into Android studio.

1. Import the engine/src/flutter/shell/platform/android subdirectory as a new project. It's important to pick this specific directory. IntelliJ needs this as the root in order to make sense of the package structure.
2. Mark the project as depending on the engine's SDK and Java versions (currently 29 and 8). The option should be visible under File > Project Structure > Project Settings > Project.
3. (Optional) Manually tell the IDE to look for any JARs needed by the embedding code in engine/src/third_party/android_embedding_dependencies/lib to fix "Missing import" errors. The option should be visible under File > Project Structure > Modules, then by selecting the android module and clicking on the Dependencies tab.

Next, build and run a flutter app using flutter run --local-engine. It may be helpful to configure the Android app to wait for the local debugger on start.

Then hit the "Attach debugger" button in Android Studio, click "Show all processes" in the pop up, and select your app from the list and hit OK.

Logging in the engine

Flutter tool will by default parse out any non-error output from the engine. Error logs will be displayed. Logging is handled though the FML library's logging.h