[jnigen] Add doc about threading (#1751)

* Close #611

pkgs/jnigen/doc/threading.md

@@ -0,0 +1,67 @@
+## Threading considerations
+
+Unlike method channels, JNIgen uses FFI calls. This means that the calls happen
+on the calling thread.
+
+Dart callbacks are executed on the isolate where they were created. This means
+the Dart isolate remains active until the associated Java objects are garbage
+collected by the JVM.
+
+Java calls Dart callbacks directly when both are on the same thread. If they are
+on different threads, Java sends a message to the Dart isolate and waits for a
+response. However, you can
+[configure void-returning callbacks to avoid waiting](<(../interface_implementation.md#implement-as-a-listener)>),
+as they don't produce a return value.
+
+### Deadlocks
+
+When implementing Java/Kotlin interfaces in Dart, it is possible to create
+deadlocks. Suppose we have created an object that implements `Runnable` in Dart
+in the main isolate. This means that the code will always be run on the platform
+thread.
+
+```dart
+// Dart
+final runnableFromDart = Runnable.implement(
+  $Runnable(run: () => print('hello'))
+);
+```
+
+If Java creates a second thread from the platform thread and calls
+`runnableFromDart.run()` from that thread and then attempts to join or
+synchronize with the main thread, it will cause a deadlock.
+
+This is because the body of the `runnableFromDart` needs to be run on the
+platform thread, so JNIgen waits until platform thread is available. However in
+this setting the platform thread will not be available until
+`runnableFromDart.run` is executed.
+
+If the callback does not need to be blocking,
+[making it a listener](../interface_implementation.md#implement-as-a-listener)
+solves this issue:
+
+```dart
+// Dart
+final runnableFromDart = Runnable.implement($Runnable(
+  run: () => print('hello'),
+  run$async: true,
+));
+```
+
+Of course, only void-returning methods can be non-blocking.
+
+### Calling thread restricted APIs
+
+When developing for Flutter Android, certain APIs can be thread-restricted to
+the platform thread which is currently different than the Flutter's UI thread.
+
+These two threads will be merged in
+[the near future](https://github.com/flutter/flutter/issues/150525).
+
+Until then you can wrap your calls with `runOnPlatformThread` which is available
+in `dart:ui`.
+
+### Dart-standalone
+
+On Dart-standalone, call `Jni.setDylibsDir` in each new isolate, since each
+isolate loads the dynamic libararies separately.