C++ library drawing on a Flutter Canvas?

[Odev]

Hi,

I have a library written in C++ for drawing on a canvas. It leaves the implementation of the canvas up to the user, depending on which platform the library is meant to be used on; it just expects various functions like canvas.setColor() and canvas.drawLine() to be available. I was wondering if it'd be at all possible to use this library in conjunction with a Flutter Canvas (which supports all those functions), via FFI?

Of course I'd be happy to write some kind of wrappers in Flutter/Dart and C/C++, but my understanding is that even with this, this might be tricky. Is there some reasonable way to work around this kind of issue? Is the easiest way to implement a dummy canvas in C++ that caches all the instructions in some data type, and then pull this from Flutter using FFI, and draw the instructions? This could get rather tedious; is there some best practice for encoding this kind of thing?

Kind regards,

Odev

[Daco Harkes 🔵]

Hi Odev,

Nice to meet you!

Assuming you're talking about the Dart API Canvas in Flutter: You can use callbacks from C back in to Dart to invoke the relevant Dart functions on Canvas.

If you want to do the callbacks from an arbitrary thread (instead of the main Dart thread), you need to use native ports. How that works is documented in these 2 samples.

If you have more than one Canvas object, currently the workaround is to have a Map<int,Canvas> in Dart and pass the integer through C. (I'm working on something that allows you to pass objects through the FFI, but this is not ready yet.)

Kind regards,

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Daco Harkes

[Odev]

Many thanks for your reply, Daco! I am indeed talking about the Dart API Canvas in Flutter.

Are there any examples that demonstrate using simple callbacks from C back into Dart, with both the C code and the Dart code? In the (I presume simpler) case where everything's happening in the same thread, that is. (I'm afraid I don't understand yet how the threading will work when calling an external library, even if it is all being built from source.)

I found sample_ffi_functions_callbacks.dart which I assume is one half of this, just missing the C code. Is that a good place to start?

Great work with FFI!

Thanks!
Odev

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Daco Harkes

[Daco Harkes 🔵]

Hi Odev,

Yes, those samples are synchronous. The C code is here: ffi_test_functions.cc.

Have a nice day!

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Daco Harkes