paperlesspost/cef2go

This is a fork of https://github.com/CzarekTomczak/cef2go/.

@CzarekTomczak has done a ton of amazing work in wrapping Chromium Embedded Framework in Go and making it cross platform. If you're interested in cross platform CEF please visit the main repo.

This fork includes a lot of changes very specific to our needs and is not attempting to be anything other than a tool for our use case. Specifically we:

Are only running in Linux
Want/need to use cef2go as a pkg from another project
Use offscreen rendering and render handlers
Want callbacks from JS into Go

Known issues/constraints

This fork is only tested on linux and currently only tested on Ubuntu (13 & 14).
This fork is specifically geared towards building headless applications that need offscreen rendering and JS integration.
JS callbacks into the application have some interesting properties when running in multi-process mode as they callback to the inidividual browser process, not the main process (where your go code is actually running). We're working on better ways around this, but currently we run in single-process mode to get around this constraint.
If you're running on a machine that does not have a Graphics processor, it is best to run with the --disable-gpu flag to avoid warnings and potential errors.
There is an issue where in certain circumstances an application based on cef2go does not boot properly and then can not create browsers. There is a bunch of discussion about this here cztomczak#16 and here https://code.google.com/p/chromiumembedded/issues/detail?id=1362

JS Callbacks

We've added a simple mechanism for making callbacks into go from running JS. Though cef allows to create C/C++ backed "native" js functions, allowing for the definition of those functions at runtime involves some pretty gnarly APIs. In order to simplify the creation and use of callbacks, we define a single "native" JS function cef.callback() that allows you to register any arbitrary functions with any number of arguments. It is up to the user to convert the callback arguments to their native go types (though helpers are provided).

In your go program you register a callback that takes a V8Callback func that will execute when the callback is invoked in the browser:

cef.RegisterV8Callback("mycallback", cef.V8Callback(func(args []*cef.V8Value) {
    // executed when called from js.
}))

You would call this from the browser by executing:

cef.callback('mycallback', "arg1", 2);

Through the magic of JS the number of arguments is not set explicitly and is returned to the go side as a slice of V8Value pointers.

You have to convert the values to their native go types through explicit helper methods:

cef.RegisterV8Callback("mycallback", cef.V8Callback(func(args []*cef.V8Value) {
    arg0 := args[0].ToString()
    arg1 := args[0].ToInt32()
}))

This is obviously very powerful but there are a number of caveats:

Callbacks need to be registered before browsers are created.
Currently only basic type conversions are supported (Int, Float, Bool, String). Objects and Arrays are possible, but not done. [You can flatten an array into arguments on the JS side, though].
If you are running in multi-process mode (the default - as opposed to single-process) the callback will be executed in the browser process and not in the main process, so if you want to callback to your main process you need to use IPC (possible, but I havent tested) or run in single-process mode.

CEF Version and Compatibility

This fork is compiled against CEF 3 branch 3163 revision 1670 (Chrome 61) from http://cefbuilds.com

Building and running an application

Building an application based on cef2go on linux requires some special setup and dependencies.

After downloading the specific cef build outlined above and extracting it to a directory you need to move the libraries and resources to their proper locations.

cd cef_binary_3.3163.1670.g934eaba_linux64
# Move libcef.so to the shared lib directory (/usr/lib) 
# so it can be found by the linker (ld) (will probably require sudo)
mv Release/*.so /usr/lib
# Make a shared directory to hold the resources files and copy the resources there
mkdir /var/lib/cef
cp Resources/* /var/lib/cef/

See the extract_cef script for a working example of extraction.

Because of hardcoded paths in Chromium itself, every application needs the resource files in the same directory as the compiled application binary. The easiest thing to do is move all the resources to a shared directory (as in the script above) and then symlink each file into your binary working directory.

See the link_cef script for a full example of linking.

Messages like:

[0218/214957:ERROR:icu_util.cc(152)] Couldn't mmap /opt/src/paperlesspost/go/src/github.com/paperlesspost/cef2go/icudtl.dat
[0218/214957:FATAL:content_main_runner.cc(749)] Check failed: base::i18n::InitializeICU().

mean that the resources are not correctly linked or are in the wrong location.

Updating the CEF Version

The process for updating CEF to a newer version is relatively simple.