[flutter_tools] Make test asset directory visible on web platform testing

[bleroux]

Description

This PR makes the test assets directory content available to web tests.
The goal of this PR is to unblock #144314 which activates ink sparkle animation on CanvasKit. The ink sparkle animation relies on a shader.

#144314 works when launching a flutter app but fails when launching a web test with a 404 error for the ink_sparkle.frag file.

Related Issue

Needed for #144314.

Tests

Adds 1 test.

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[jonahwilliams]

ink_sparkle should be loaded via asset bundles. if the asset bundle is broken, then a fix should be made there, not by adjusting the test harness loading behavior.

[bleroux]

@jonahwilliams
Ink sparkle is correctly loaded when launching an app using 'flutter run -d chrome' (once activated on canvas kit, see the PR linked on the description). That's why I assumed, it was a test only issue.
Can you point me to someone who can work on this? Or give my guidance on where to fix this?

[jonahwilliams]

The test assets are built here:

https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/commands/test.dart#L656-L680

maybe those arguments need to be adjusted for web tests?

[bleroux]

The test assets are built here:

https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/commands/test.dart#L656-L680

maybe those arguments need to be adjusted for web tests?

Thanks for pointing out this code.
I figured out that test assets are correctly generated for web tests (this code is called before launching the internal web server). But the asset directory was not sent to FlutterWebPlatform.
I have updated the PR to make the test assets directory visible on the internal web server (and I added a test).
Let me know it this is the right way or if there was a reason for not exposing this directory?

[bleroux]

@christopherfujino
Can you point me to someone who can review this PR?
Based on Jonah feedback I updated this PR some weeks ago.
Thanks!

[christopherfujino]

I'm gonna defer to @andrewkolos to make the final review on this one, he's currently out of office but will be back ~1 week.

[andrewkolos]

Hello, I'm back and trying to catch up on this. I'm starting out from the same place @jonahwilliams did:

ink_sparkle should be loaded via [the] asset bundle. if the asset bundle is broken, then a fix should be made there, not by adjusting the test harness loading behavior.

I need to figure out where this shader is being loaded, how it's being loaded, and why it's not working for web tests.

[bleroux]

@andrewkolos
Hey! For context, before Jonah review, my first implementation was to add a specifc rule to the Shelf server in order to serve ink_sparkle file.
That's why Jonah commented:

ink_sparkle should be loaded via [the] asset bundle. if the asset bundle is broken, then a fix should be made there, not by adjusting the test harness loading behavior.

Since then I updated the PR to serve the test asset directory which was already correctly built for web tests but whose path was not known by the FlutterWebPlatform.

[bleroux]

@andrewkolos Did you get a chance to look at this PR?

[andrewkolos]

Hello, I got preoccupied with other things, but I have no excuse for not communicating this. My apologies.

Since then I updated the PR to serve the test asset directory which was already correctly built for web tests but whose path was not known by the FlutterWebPlatform.

If the path was not known by FlutterWebPlatform, wouldn't any test that depends on an asset fail when running on a browser? To my knowledge, this is not the case, and I've verified this locally¹ (assuming I did so correctly). I'm missing something here.

Footnotes

1. I created a simple Flutter app that displays text loaded from an asset file. I wrote an integration test that verifies that the text is displayed and I ran this test on chrome. The test passed. ↩

[andrewkolos]

To extend the previous comment: I believe the code that actually loads ink_sparkle.frag is unique compared to the more general asset-loading code, but I have to imagine that the file would still be visible on the web server hosted by flutter test since it's bundled in the same directory as any other asset file.

[bleroux]

@andrewkolos Thanks for your insight. I’m far from having a full understanding of asset loading, shaders and flutter tools inner workings. That makes it hard to draw conclusions on this area.
I will close this PR for the moment and I will try to come up with a narrowed investigation.

For context, here is what led me to this PR:

• In order to fix Activate InkSparkle on Canvaskit (Android only) #138487, I filled Reland Activate InkSparkle on CanvasKit #144314 which activates the ink sparkle shader animation on CanvasKit. With that PR, the shader works correctly when launching an app using flutter run but several tests are failing.
• When I filed it the main cause for test failure was a 404 response while retrieving the ink_sparkle.frag shader file, see Activate InkSparkle on CanvasKit #138545 (comment). That’s why I worked on this flutter tools PR.
• As of today, this work is not enough to get Reland Activate InkSparkle on CanvasKit #144314 passing all tests locally (I think it was when I first filed it). Especially, the problem is not specific to integration test, some widget tests fail because the shader is not loaded. For reference, the loading logic on framework side is:

flutter/packages/flutter/lib/src/material/ink_sparkle.dart

Lines 443 to 452 in 13a05de

	static void initializeShader() {
	if (!_initCalled) {
	ui.FragmentProgram.fromAsset('shaders/ink_sparkle.frag').then(
	(ui.FragmentProgram program) {
	_program = program;
	},
	);
	_initCalled = true;
	}
	}

I will try to get a better understanding of how the shader is supposed to be loaded on web tests and I will try to narrow it down to make progress.

[bleroux]

Just found #31207 which is a great starting point to understand how assets are loaded in non-web tests (it mocks flutter/assets channel to bypass the engine).

[andrewkolos]

Thanks for the recap!

I’m far from having a full understanding of asset loading, shaders and flutter tools inner workings. That makes it hard to draw conclusions on this area.

I work on the tool full-time, and I could still say the same 😆. I would have loved to have a deep dive here and figure all of this out, but I unfortunately do not have the time. If you do figure this out or this turns out to be a higher priority than I thought, I'd be glad to look at this again.

[dvaruas]

Hello @bleroux I am also facing the same issue where shaders asset is not found in unit tests for flutter app.
Were you able to find anything more during your investigation?

@andrewkolos this really blocks users from using updated Flutter version (since 3.16, and the latest SDK is 3.22). Could the flutter team also take a look into this with priority?

[bleroux]

Hello @dvaruas!
I have not work on this issue since my last comment. It would help if you can file an issue with a minimal repro. My work was based on #138487 which is a specific case (a shader provided by the framework). Having a proper issue with a simple repro based on a Flutter app that loads a shader would be interesting because it extends the scope of this issue.

this really blocks users from using updated Flutter version (since 3.16, and the latest SDK is 3.22).

Do you mean that it worked before 3.16? (I'm not sure when shaders support was added to Flutter Web)
For Flutter 3.16 and 3.19, there was another issue related to shaders (#141296), it was fixed in flutter/engine#49754 which is included in Flutter 3.22.

[dvaruas]

Hey @bleroux , thank you for your response. I'll try to create a issue with a minimal example once I isolate it.

Do you mean that it worked before 3.16? (I'm not sure when shaders support was added to Flutter Web)

yes, I was using 3.13.9 previously and everything works fine.
Encountered this issue when I tried to update to 3.16

For now I am removing the --no-test-assets flag to bypass this issue.