Friday, July 18, 2018

Written on Jul 20, 2018

Things I could explain

• https://wiki.mozilla.org/XPConnect_object_wrapping

Things relevant to this new bug

• https://searchfox.org/mozilla-central/source/GENERATED/xpcom/build/Services.cpp#390
• https://searchfox.org/mozilla-central/source/js/xpconnect/src/XPCComponents.cpp#416 are these the resolve hooks?
• https://searchfox.org/mozilla-central/source/dom/base/ChromeUtils.cpp#253
  • Will I have to create a class similar to this?

Guide to understanding XPCOM

• https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XUL/Tutorial/XPCOM_Interfaces
• https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Language_bindings/XPConnect
  • https://developer.mozilla.org/en-US/docs/Mozilla/Gecko/Script_security
• Some of the functionality the browser needs to provide is not possible to implement in pure JavaScript. So some of it is implemented in C++ and is exposed to JavaScript and Rust.
• http://books.mozdev.org/html/
  • A book someone made a long time ago. It has a lot of information, and even though some of it is outdated, there are parts that are well documented and are still relevant.
• https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Guide/Creating_components
• https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Language_bindings/XPConnect/XPConnect_wrappers

Trying to figure out what to do

I need to figure out how the newly created Service object will be used by the js consumers. I found some classes where they also create a new object in js and now I need to look for classes where the new component is created using JS_NewPlainObject and returned.

• example ChromeUtils
  • https://searchfox.org/mozilla-central/source/dom/base/ChromeUtils.cpp#253
    • Is this how I would prepare Services object to be imported?
  • https://searchfox.org/mozilla-central/source/devtools/shared/builtin-modules.js#32
    • And is this how I will be exposing it?
  • Actually, I noticed that ChromeUtils is not exactly the same as Services.jsm because one can do ChromeUtils.import("resource://gre/modules/Services.jsm")
• example nsFrameMessageManager
  • Does not apply because it is not created via JS_NewPlainObject Perhaps I should look for how other .jsm files are treated. Maybe there are some that are implemented in C++.

Update

Actually, I realized it’s okay that ChromeUtils is not exactly like Services.jsm because I just need an example of how C++ interfaces get exposed to JS code. Perhaps, it is through webidl.

Actually

It won’t work because ChromeUtils as an object is already exposed through webidl, and it uses low level JS APIs to create a copy of it. But the method that created a copy of itself is defined on an alerady existing object. So it won’t work 100% for me.

3:28 pm

Perhaps, I will look instead at functions that have the following function declaration myFunc(JSContext* aCx, JS::MutableHandleValue aResult) and see they are exposed. I found this https://searchfox.org/mozilla-central/source/widget/GfxInfoBase.cpp#1205.

• It seems like I will have to write an .idl file.

4:01 pm

I don’t think I should be creating new webidl file because that means I will be creating Services object in C++. Furthermore, if I can’t really generate a webidl file.

4:54 pm

I discovered a Components object. Per https://developer.mozilla.org/en-US/docs/Mozilla/Tech/XPCOM/Language_Bindings/Components_object

The Components object is the object through which XPConnect functionality is reflected into JavaScript.

5:49 pm

Success!!! I have managed to create an API in C++ in the Components class and expose it to js. TODO This is how I did it.

Things to do

• Create wrapper
• Add additional interfaces
• Overwrite this getter
• Use resolve hooks

To draw