Category: gtk+

As I mentioned in my overview of the upcoming Builder release, a lot of this development cycle focused on improving machinery the user does not see. In the past four years of development, a number of patterns emerged. When I started this project, I had some ideas of how to keep things organized, but Builder quickly grew beyond my ability to keep the whole design in my head at once.

Thankfully, I’ve worked on many large software teams during my tenure at companies like VMware and MongoDB. These company’s products have something in common, in that they’re a number of sub-systems performing specialized tasks that coordinate with each other. Not surprising, I know. But sometimes, knowing where to draw the lines between sub-systems is what differentiates products. (Interestingly, if you want to build a database engine, you better have a good reason to deviate from page 144 of Architecture of a Database System).

Now days, Builder segments the code into a series of static libraries. We have a core library that provides the basic functionality used by all sub-systems. We have a process and threading library. One library deals with representing and manipulating source code. Yet another focuses on building, executing, testing, and packaging software. To enable Language Servers, we have one too. Even the gui and editor are somewhat compartmentalized.

In the end, we link all of these static libraries into the final executable, which is then processed to generate GObject Introspection data; the *.gir and *.typelib. That means that our plugins, written in C, C++, Python, or Vala, do not need to rely on linking against any shared libraries. All symbols come from the executable with -export-dynamic. (One side-effect of this, is that we no longer allow linking our Vala based plugin into the executable, since it relies on *.gir generation).

To keep startup fast, all of our bundled C-based plugins are compiled into the final executable. That reduces the number of link loader work, file-system directory scanning and iops, and allows for more aggressive compiler optimizations. We also avoid loading some plugins in the early initialization phase, instead deferring that work until later.

The biggest change in the refactoring is IdeObject. This object has become somewhat like a multi-threaded version of what GtkObject used to be. It allows us to build a top-down object tree. The root of the tree is a specialized form, IdeContext. Each sub-system mounts itself on this tree. Plugins may have or work with objects that are descendants of the sub-systems. In the end, it makes things much easier for me to debug at runtime because I can get a clearer picture of how things interact and what plugins are active. One interesting feature of IdeObject is that we guarantee they are only ever finalized in the main thread.

The sub-system that I think resulted in the most cleanup was libide-foundry.a. This contains the build manager, pipelines, and extension points for plugins to provide the the abstraction glue. It also contains runtime abstractions, execution management, testing infrastructure, device management (phone, tablet, simulator), and toolchains. Currently, the debugger engine is a separate library, but I may fold it into this library for completeness.

When cleaning up libide-code.a, I simplified the buffer management. This code was a bit temperamental because I put a few too many features into the base class. I think that happened simply because I couldn’t yet foresee many of the ways it would be extended. Seeing how the code evolved over time allowed me to create better abstractions and extract those features.

What kicked off this whole process was an hour hack to add multi-monitor support. That somehow turned into a 2 month project that involved me ripping everything apart and putting it back together because of our initial design. I’m happy to say that I think things turned out quite well and we are poised to keep moving fast for the next few development cycles to come.

We’re getting many more requests to support external plugins in Builder. I’ve always taken the Linux kernel approach here in that I very much want to encourage upstream contributions over external plugins. However, I am starting to loosen up just a bit and we’ve added the X-Builder-ABI=3.31 key to plugin definitions. This allows us to continue to evolve our APIs but promise to not break them in micro releases. My expectation, however, is that this major refactoring will set us down the path towards some amount of ABI stability.

In my next post I’d like to cover how the UI components are abstracted and give you an idea of which plugin interfaces to use and when. In the mean time, I have plenty of documentation to write.

One of the things we’ve wanted in Builder for a while is a designer. We’ve had various prototypes in the past to see how things would have worked out, and mostly just punted on the idea because it seemed like Glade served users better than we would be able to directly.

Last week, Juan Pablo, Matthias Clasen and I met up in San Francisco to see what we could do in the short term. We discussed a couple of options that we have going forward.

• Integrate glade 3 into Builder using libgladeui.
• Integrate glade 3 using the external Glade application and use D-Bus to inter-operate.

Like all projects, we have some constraints.

• Gtk 4 is in progress, and our hope is that most new application development moves towards that because the benefits are outstanding. That means the value of a Gtk 3 designer is depreciating.
• Gtk 4 changes many fundamental designs behind the scenes. While much effort has been done to reduce the friction in porting applications, porting an UI designer is no trivial task as they necessarily reach into library internals. It is likely Gtk 4 will require creating a new designer from the ground up. Doing this as part of Gtk itself is probably worthwhile.
• We want the designer to know about all of your .ui files so that it is easier to see widgets created using composition.
• Allow generating signal callbacks into your existing code-base in a variety of languages.

With that in mind, I want to get the maximal benefit with the least amount of time to ship. I made a new plugin for Builder last week that gets us moving in that direction. It still needs more work to integrate with signal editing, templates, and other more advanced Glade features.

Hopefully that happens soon because I know we’ve all been waiting for it. Get it now with the Builder Nightly flatpak.

flatpak install --from https://git.gnome.org/browse/gnome-apps-nightly/plain/gnome-builder.flatpakref

One dead give-away of a GNOME/Gtk programmer is how they format their headers. For the better part of two decades, many of us have been trying to keep things aligned. Whether this is cargo-culted or of real benefit depends on the reader. Generally, I find them easier to filter through.

Unfortunately, none of indent, clang-format, nor uncrustify have managed to exactly represent our style which makes automated code formatting tools rather problematic. Especially in an automated fashion.

For example, notice how the types and trailing asterisks, stay aligned, in multiple directions.

FOO_EXPORT
void   foo_do_something_async  (Foo                  *self,
                                const gchar * const  *params,
                                GCancellable         *cancellable,
                                GAsyncReadyCallback   callback,
                                gpointer              user_data);
FOO_EXPORT
Bar   *foo_do_something_finish (Foo                  *self,
                                GAsyncResult         *result,
                                GError              **error);

Keeping that sort of code aligned is quite a pain. Even for vim users who can fairly easily repeat commands. Worse, it can explode patches into unreadable messiness.

Anyway, I added a new command in Builder last night that will format these in this style so long as you don’t do anything to trip it up. Just select a block of function declarations, and run format-decls from the command bar.

It doesn’t yet handle vtable entries, but that shouldn’t be too painful. Also, it doesn’t handle miscellaneous other C code in-between declarations (except G_GNUC_* macros, __attribute_() etc.

Since my initial announcement of Builder at GUADEC in 2014, I’ve had a vision in the back of my mind about how I’d like completion to work in Builder. However, there have been more important issues to solve and I’m just one person. So it was largely put on the back burner because after a few upstream patches, the GtkSourceView design was good enough.

However, as we start to integrate more external tooling into Builder, the demands and design of what those completion layers expect of the application have changed. And some of that is in conflict with the API/ABI we have in the long-term stable versions of GtkSourceView.

So over the past couple of weeks, I’ve built a new completion engine for Builder that takes these new realities into account.

It has a number of properties I wanted for Builder such as:

Reduced Memory and CPU Usage

Some tooling wants to give you a large set of proposals for completion and then expects the IDE to filter in the UI process. Notably, this is how Clang works. That means that a typical Gtk application written in C could easily have 25,000 potential completion proposals.

In the past we mitigated this through a number of performance tricks, but it still required creating thousands of GObjects, linked lists, queues, and such. That is an expensive thing to do on a key-press, especially when communicating with a sub-process used for crash-isolation.

So the new completion provider API takes advantage of GListModel which is an interface that focuses on how to have a collection of GObjects which don’t need to be “inflated” until they’ve been requested. In doing so, we can get our GVariant IPC message from the gnome-builder-clang sub-process as a single allocation. Then, as results are requested by the completion display, a GObject is inflated on demand to reference an element of that larger GVariant.

In doing so, we provide a rough upper bound on how many objects need to be created at any time to display the results to the user. We can also still sort and filter the result set without having to create a GObject to represent the proposal. That’s a huge win on memory allocator churn.

Consistent and Convenient Refiltering

Now that we have external tooling that expects UI-side refiltering of proposals, we need to make that easier for tooling to do without having to re-query. So the fuzzy search and highlighting tools have been moved into IdeCompletion for easy access by completion providers.

As additional text is provided for completion, the providers are notified to perform filters on their result set. Since the results are GListModel-based, everything updates in the UI out-of-band nicely with a minimal number of gsignal emissions. Compare this to GtkTreeModel which has to emit signals for every row insertion, change, or deletion!

Alternative Styling

When working with completions for programming languages, we’re often dealing with 3 potential groups of content. The return value, the name and possible parameters, and miscellaneous data. To get the styling we want for all of this, I chose to forgo the use of GtkTreeView and use widgets directly. That means that we can use CSS like we do everywhere else. But also, it means that some additional engineering is required.

We only want to create widgets for the visible rows, because otherwise we’re wasting memory and time crunching CSS for things that won’t be seen. We also want to avoid creating new widgets every time the visible range of proposals is changed.

The result is IdeCompletionListBox which is a GtkBox containing GtkListBoxRow and some GtkSizeGroups to give things a columnar effect. Because the number of created widgets is small things stay fast and snappy while giving us the desired effect. Notably, it implements GtkScrollable so if you place it in a GtkScrolledWindow you still get the expected behavior.

Further more, we can adjust the window sizing and placement to be more natural for code-related proposals.

Dynamic Priority Control

We want the ability to change the priority of some completion providers based on the context of the completion. The new design allows for providers to increase their priority when they know they have something of high-importance given some piece of contextual knowledge.

Long term, we also want to provide an API for providers to register a small number of suggested completions that will be raised to the top-level, regardless of what provider gave them. This is necessary instead of having global “scoring” since that would require both O(n) scans of the data set as well as coming up with a strategy to score disparate systems (and search engines prove that rarely works well).

More to do

There are still a couple things that I think we should address that may influence the API design more. For example:

• How should we handle string interpolation? A simplified API for completions when working inside of strings might be useful. Think strftime(), printf(), etc as potential examples here.
• The upcoming Gtk+ 3.24 release will give us access to the move_to_rect() API. Combined with some Wayland xdg_popup improvements, this could allow us to make our display widget more flexible.
• Parameter completion is still a bit of an annoying process. We could probably come up with a strategy to make the display look a lot better here.
• Give some tweaks and knobs for how much and what to complete (just function name vs parameters and types).

Conclusions

Rarely do I write any code that doesn’t have bugs. Now that this is landing in Builder Nightly soon, I could use some more testing and bug filing from the community at large.

I’m very happy with the improvements over the past couple of months. Between getting Clang out of process and this to allow us to make clang completion fast, I think we’re in a much better place.

We can’t get this design into older GtkSourceView releases, but we can probably look at some form of integration into what will eventually integrate with Gtk4. I would be very happy if it influenced new API releases of the library so that we don’t need to carry the implementation downstream.