WebIDL describes interfaces web browsers are supposed to implement.
The interaction between WebIDL and the build system is somewhat complex. This document will attempt to explain how it all works.
.webidl files throughout the tree define interfaces the browser
implements. Since Gecko/Firefox is implemented in C++, there is a
mechanism to convert these interfaces and associated metadata to
C++ code. That’s where the build system comes into play.
All the code for interacting with .webidl files lives under
dom/bindings. There is code in the build system to deal with
WebIDLs explicitly.
Not all .webidl files are created equal! There are several flavors,
each represented by a separate symbol from mozbuild Sandbox Symbols.
Refers to regular/static .webidl files. Most WebIDL interfaces
are defined this way.
In addition to generating a binding, these .webidl files also
generate a source file implementing the event object in C++
The .webidl files are generated by preprocessing an input file.
They otherwise behave like WEBIDL_FILES.
Like WEBIDL_FILES but the interfaces are for testing only and aren’t shipped with the browser.
Like TEST_WEBIDL_FILES except the .webidl is obtained via
preprocessing, much like PREPROCESSED_WEBIDL_FILES.
The .webidl for these is obtained through an external
mechanism. Typically there are custom build rules for producing these
files.
The most complicated part about WebIDLs is the process by which
.webidl files are converted into C++.
This process is handled by code in the mozwebidlcodegen
package. mozwebidlcodegen.WebIDLCodegenManager is
specifically where you want to look for how code generation is
performed. This includes complex dependency management.
This section aims to document the build and developer workflow requirements for WebIDL.
There are parser tests provided by dom/bindings/parser/runtests.py
that should run as part of make check. There must be a mechanism
to run the tests in human mode so they output friendly error
messages.
The current mechanism for this is mach webidl-parser-test.
There are various mochitests under dom/bindings/test. They should
be runnable through the standard mechanisms.
TestExampleGenBinding.cpp calls into methods from the
TestExampleInterface, TestExampleProxyInterface,
TestExampleThrowingConstructorInterface,
and TestExampleWorkerInterface interfaces.
These interfaces need to be generated as part of the build. These
interfaces should not be exported or packaged.
There is a compiletests make target in dom/bindings that
isn’t part of the build that facilitates turnkey code generation
and test file compilation.
Reprocessing every output for every change is expensive. So we don’t
inconvenience people changing .webidl files, the build system
should only perform a minimal rebuild when sources change.
This logic is mostly all handled in
mozwebidlcodegen.WebIDLCodegenManager. The unit tests for
that Python code should adequately test typical rebuild scenarios.
Bug 940469 tracks making the existing implementation better.
There needs to be an explicit method for invoking code generation. It needs to cover regular and test files.
This is implemented via make export in dom/bindings.
So developers touching .webidl files are not inconvenienced,
no-op binding generation should be fast. Watch out for the build system
processing large dependency files it doesn’t need in order to perform
code generation.
Any interface can have example .h/.cpp files generated.
There must be a mechanism to facilitate this.
This is currently facilitated through mach webidl-example. e.g.
mach webidl-example HTMLStyleElement.