first time use of mojo

[Laura Lucas Alday (xWF)]

Hello, I'm trying to follow the documentation here https://chromium.googlesource.com/chromium/src/+/refs/heads/main/docs/mojo_and_services.md

After pasting 

 mojo::Remote<math::mojom::MathService> math_service =
   content::ServiceProcessHost::Launch<math::mojom::MathService>(
     content::ServiceProcessHost::Options().WithDisplayName("Math!").Pass());

in my code and trying to build, I get:

Error:  fatal error: 'chrome/services/math/public/mojom/math_service.mojom.h' file not found

How do I generate this file? thanks

[Alex Gough]

Hello,

You'll need to ensure the .mojom file is in a build.gn mojom("")[0] or mojom_component() target that is a dep of the target that contains your .cc files.

[0] https://source.chromium.org/search?q=lang:gn%20%22mojom(%22%20-file:out%20-file:third_party%2F%5CS%2B%2Fthird_party&ss=chromium%2Fchromium%2Fsrc

[Laura Lucas Alday (xWF)]

I'm working on credential_provider, I don't quite understand what I need to do, where do I need to add it? and what do I need to add? in this case for the math example to work. I'm getting fatal error: 'chrome/services/math/public/mojom/math_service.mojom.h' file not found. Could you point at a BUILD file in credential_provider that is the right one? 

[Alex Gough]

This is probably easier to figure out if you can upload a CL with what you're trying already - or you can chat on the chromium slack's #mojo channel.

[Joe Mason]

If you're following the example exactly , you created a math_service.mojom file in the .../chrome/services/math/public/mojom directory, not in .../chrome/credential_provider. If you actually put the file somewhere in the .../chrome/credential_provider, you need to change all the paths in the example to where you actually put math_services.mojom.

Assuming you put the example code in chrome/services/math, and you want to USE the services in .../chrome/credential_provider (using that code you pasted), there are two BUILD.gn files involved:

The one in chrome/services/math that's described in https://chromium.googlesource.com/chromium/src/+/main/docs/mojo_and_services.md#defining-the-service:

# src/chrome/services/math/public/mojom/BUILD.gn
import("//mojo/public/tools/bindings/mojom.gni")

mojom("mojom") {
  sources = [
    "math_service.mojom",
  ]
}

That defines a target called "//chrome/services/math/public/mojom:mojom" that generates math_service.mojom.h and some other files from math_service.mojom. (It can also just be "//chrome/services/math/public/mojom" for short, since the target name ":mojom" matches the directory name.)

And whatever .cc file you're putting that "math_service = ..." line in should be in a BUILD.gn file in credential_provider. For instance if you're adding it to chrome/credential_provider/test/gcp_setup_unittest.cc, I see that file in the "gcp_unittests" target of chrome/credential_provider/test/BUILD.gn. So you need to tell the build system that target now depends on "//chrome/services/math/public/mojom:mojom" to make sure it builds it first, by adding it to the "deps" list:

  deps = [
    "../extension:app_inventory",
    "../extension:common",

    ...
    "//chrome/common:version_header",

    "//chrome/services/math/public/mojom:mojom", <-- Add this.
    "//content/public/browser",

I see there's an existing gaia_credential_provider_win_hid.mojom in credential_provider which might be a better example. You can see the "mojom" target for it is defined in the BUILD.gn file in the same directory, and it's used in credential_provider_broker_win.h. The "gaiacp_lib" target in chrome/credential_provider/gaiacp/BUILD.gn contains credential_provider_broker_win.h and has //chrome/credential_provider/gaiacp/mojom as a depedendency.

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[Laura Lucas Alday (xWF)]

Very helpful, thanks! So If I want to use these methods declared in credential_provider from chrome, how should I do it? For example, I want to call OpenDevice implemented in credential_provider from a chrome process, do I need to add or change anything from chrome side?

[Joe Mason]

For anyone else who wants a more complete intro to Mojo than the docs, there's this Chrome University presentation:  https://youtu.be/ZdB5P88-w8s?si=iAqW0wEnSSwsCGJo

And this What's Up With Mojo? video is a deeper dive: https://www.youtube.com/watch?v=at_35qCGJPQ