Background Platform Channels for desktop

[gaaclarke]

Background Platform Channels (a way for Platform Channels to execute their host platform handlers on a background thread instead of forcing them to use the platform thread) are implemented for iOS and Android. This issue represents the work to implement them on the desktop platforms (and embedder.h).

Related Issue: #91635

[niuhuan]

I think it is necessary to execute method channel in other threads, because a long time will block the UI.
Call database or other network operation can take a lot of time, I usually create a thread pool, this is almost inevitable, and every platform has done this.

[niuhuan]

Result unique pointer can't move to another thread , I think it not about windows (platform) . I tried any way, can't move it, may flutter to give result time life delete at method call over? sqflite not support windows / Linux too.

[stuartmorgan]

Result unique pointer can't move to another thread

You can pass the results pointer between threads as much as you want, you just need to return to original thread before calling it.

[niuhuan]

How can i post a task or result callback to original thread? ( like kotlin " Handler(Looper.getMainLooper()).post {} " , or like ios/mac Thread { result() }).

I only want async result, this does not block the UI.

[stuartmorgan]

How can i post a task or result callback to original thread?

See comments in #79213 for some pointers. If you need more help beyond that, try a resource like StackOverflow.

[gaaclarke]

I drew up a sequence diagram that shows how this might work here.

One interesting thing about implementing this is that the platform message handler is something that is modular. You can specify it when you setup the embedder. Changing the thread that runs on would be a breaking change.

[stuartmorgan]

Generally we handle things like this by adding another field to FlutterProjectArgs. In this case either a bool to enable background channels, or an alternate callback param that can be set instead of the original one that is documented to be called on any channel (and then we would internally use whichever is set).

[alexmercerind]

How can i post a task or result callback to original thread? ( like kotlin " Handler(Looper.getMainLooper()).post {} " , or like ios/mac Thread { result() }).

I only want async result, this does not block the UI.

@niuhuan, not the best approach but still better alternative than hacking around in native code (for now):

• You should launch a separate thread in detached mode when making a call from Dart to C/C++ for heavy computation.
• Instead of sending the final result/reply through the method channel (success/error), invoke new method on the same channel from C/C++ to Dart & send the final data as argument.
• Receive it on Dart side using Completer.
• UI won't freeze.

---

Downside:

Note that your code written like this may differ from existing platform channel implementations (for other platforms) & may require refactor in future when this feature lands on engine.

[stuartmorgan]

• Instead of sending the final result/reply through the method channel (success/error), invoke new method on the same channel from C/C++ to Dart & send the final data as argument.

These use the same mechanism and have the same threading requirements, and replies can already be asynchronous, so this won't make a difference.

• Receive it on Dart side using Completer.

This has no relation to the threading on the native side.

[alexmercerind]

These use the same mechanism and have the same threading requirements, and replies can already be asynchronous, so this won't make a difference.

Apologies if I'm missing something, but sending a reply from another thread on Linux seems to raise some failed assertions.

[ +149 ms] ** (libmpv_tagger_example:206236): CRITICAL **: 00:49:36.602: FlBinaryMessengerResponseHandle was not responded to
[   +3 ms] ** (libmpv_tagger_example:206236): CRITICAL **: 00:49:36.606: gboolean fl_method_call_respond(FlMethodCall *, FlMethodResponse *, GError **): assertion 'FL_IS_METHOD_CALL(self)'

This has no relation to the threading on the native side.

I just wanted him to know how to wait for the response (since await channel.invokeMethod would no longer work), if I didn't know 😅.

Thanks!

[stuartmorgan]

Apologies if I'm missing something, but sending a reply from another thread on Linux seems to raise some failed assertions.

Sending replies and calling new methods are both main-thread-only on all desktop platforms at the moment.

If you are currently calling any channel methods on another thread, your code has undefined behavior.

[flutter-triage-bot]

The triaged-desktop label is irrelevant if there is no team-desktop label or fyi-desktop label.