How the the A/B Street UI & drawing work

When I started A/B Street in June 2018, the Rust UI ecosystem had nothing that clearly fit my needs, so I wound up rolling something custom. This doc explains conceptually how it works and how to use it. Eventually some of this should become proper docs for widgetry.

Best advice on how to use stuff in practice is to work from examples. grep is your friend.

widgetry overview

First, the crate-level view. widgetry is the generic drawing and UI library, independent of A/B Street. map_gui builds on top of it, providing ways to render the map model used by all of the projects. Finally, game, fifteen_min, santa, and others are the runnable applications making use of this.

This section will explain how widgetry works from bottom-up. Conceptually we'll walk through how it was built from scratch, skipping all of the false turns made along the way.


Let's start just by drawing stuff and handling keyboard/mouse events. The basic loop of any winit program is to handle input events and, when winit says to, redraw everything. The earliest A/B Street prototype expressed the primitive map model imported from OpenStreetMap as a bunch of polygons, and hooked up basic mouse controls to pan and zoom over the canvas.

And that hasn't really changed -- we still only draw 2D colored polygons. Widgetry's use of OpenGL shaders is dirt simple. With the exception of some barely used texture code, all of the icons and images in A/B Street are SVGs, which can be transformed into colored polygons through the magic of the usvg and lyon crates. This includes all text -- even the text is just colored vector polygons! (Text rendering usually works by uploading a table of raster glyphs to the GPU and drawing textured quads.)

The brief story of how we got here: by November 2019, there was some basic support for uploading raster texture and drawing them. At the second Democracy Lab hackathon, a developer on Mac hit a 16 texture object limit that was different than Linux. This is also when Yuwen joined and started designing using Figma, which... conveniently had SVG export. I was also frustrated by rendering text separately from everything else; finding the bounding boxes was buggy and there were z-order issues. All of this prompted me to poke around and discover an example using lyon to tesellate the output from usvg. I thought, there's no way vectorizing EVERYTHING could be performant. But happily I was wrong.

Finally getting to the practical consequence here. It's expensive to upload stuff to the GPU, but it's cheap to draw something already uploaded. So you use a GeomBatch to build up that list of colored polygons, then upload it by doing ctx.upload(batch). Later on, you can g.redraw(&drawable) as many times as you want and it's fast. You don't keep the GeomBatch around; it's just the builder for a Drawable.

How should you batch stuff? Issuing redraw calls is fast, but not when there's lots of them. So for example, one Drawable per every building on the map would be a nightmare. Since buildings don't change, there's a single batch and Drawable for all of them. There's a balance here; sometimes you have to experiment to find it. But generally, if recalculating a batch only needs to happen every so often, just lump everything together in one for simplicity.

Stack of states

So at this point, there's logically one method to handle input events, and one method to draw. No built-in organization; all application state is just lumped somewhere. The basic insight is that an app has a stack of smaller states layered on top of each other. For example, you start with a title screen, then enter the main simulation interface, then open up a menu to show extra layers. You still want to draw the simulation underneath the menu, but not allow it to handle events. When you exit the simulation, you want to go back to the title screen and preserve any local state there.

So a widgetry app mainly consists of a stack of States. Each state implements a method to handle events and draw. Some states want to draw what's underneath them, while some want to clear the screen and fully handle everything -- so draw_baselayer specifies this.

When a state handles an event, it returns a Transition. This manipulates the stack. Transition::Keep doesn't do anything; the current state remains. Transition::Pop deletes the current state from the stack, and the previous one takes over. Transition::Push introduces a new state, preserving the current underneath. And so on.

There's actually two types of "state" (as in, data managed by the app): local and global. Local "state" is owned by the struct implementing the State trait. This is usually stuff like any UI panels (which we'll get to soon) and stuff like which road we're editing, or what building we're examining. But usually an app has a bunch of "state" that lasts for the entire lifetime of the program -- the map, the simulation, global settings. This stuff can change through the program, like loading a new map, but every single State probably wants to use it. This global stuff is stored in the App struct, which gets plumbed around. So, each State has fn event(&mut self, ctx: &mut EventCtx, app: &mut App) -> Transition -- self is the local State on the stack, ctx is a handle into the generic widgetry system, and app is that global "state". And there's also fn draw(&self, g: &mut GfxCtx, app: &App), with GfxCtx being the hook to draw stuff. Note draw uses immutable borrows; generally you shouldn't modify anything while drawing. (When you need to, like for caching, usually RefCell is the answer.)

This system of states and transitions mostly works, but there's one super awkward problem. Sometimes, state1 needs to push on state2 in order to prompt the user for input (free-form text, a menu, or even something more complicated) and use the result of state2 in order to do something else. In normal programming, this would just be calling a function and using the return value. How do we make that work with the event/draw interface? The answer is for state2 to return Transition::Pop and Transition::ModifyState, downcast the previous State into a particular struct, and shove the return value somewhere. This is incredibly gross, but I'm not sure what else to do.


What's been described so far is kind of only useful for drawing and interacting with stuff in "map-space", aka, the scrollable canvas. What about normal GUI elements that live in "screen-space" on top of everything else -- buttons, dropdowns, checkboxes, frobnozzlers? There needs to be a way to create these, arrange them in some kind of layout, and use them for interaction. There's a bunch of ways that GUI frameworks manage the problem of synchronizing application "state" with the UI widgets, and it's more complex than usual in Rust, because you hit crazy lifetime and borrowing issues if you try to do anything with callbacks.

So sticking to the widgetry philosophy of seeing how far the low-level abstractions stretch, widgets are just temporary "state" managed by a State. They're always managed as part of a Panel, even if you have just a single button. Constructing Panels is hopefully straightforward from examples; you assemble a tree of rows and columns, with some occasional styling and layouting hints thrown in. Underneath, widgetry uses stretch for CSS Flexbox-style layouting. There are some quirks, most of which we've worked out and hopefully papered over (like padding and margin don't work on most widgets directly; you have to wrap them in a Widget::row or Widget::col).

So how do you know if a button has been clicked, a toggle toggled, a slider slidden, a frobnozzler frobnuzzled? In some languages, you might expect callbacks, but here in widgetry, you have to explicitly ask. Most States will match self.panel.event(ctx) somewhere near the top. This takes the current event (a low-level keypress or mouse movement) and lets all of the widgets inside the Panel possibly use it. If the event caused the widgets to do something interesting, the entire Panel will return Some(Outcome::Clicked("button name")) or Some(Outcome::Changed("spinner name")). The State code can then interpret that UI-level event appropriately.

Currently, the widgets in a Panel are identified by lovely type-unsafe hardcoded strings. This isn't the best, but in practice, it's rare to get out of sync between the two places in one file that talk about the same widget.

Some widgets have more information than just "the button was clicked". Whenever you need to, you can just query their state -- self.panel.slider("time").get_percent(), self.panel.dropdown_value("mode"), self.panel.spinner("duration"). These last two are generic, and the compiler usually infers the type of the value contained. (Internally, we just downcast to that type, so you'd get a runtime panic if you mess up.)

Updating panels

Sometimes you need to change a Panel, often in response to something done on that panel -- like say you toggle between showing raw data points versus aggregating in a heatmap, and want to expose extra heatmap settings. There are two choices for how to do this: build a new panel entirely, or replace one widget.

Often it's simplest to just split the method that builds a Panel into its own method, and call it again with some different parameters. Very very occasionally when you take this approach, you'll need to do new_panel.restore(ctx, &self.panel); self.panel = new_panel; to retain internal widgetry state, such as "this scrollbar is in the process of being dragged by the mouse."

Or you can just replace one widget (which may be an entire row or column of stuff; it's just based on the string ID you specify). self.panel.replace(ctx, "edit", new_button) does the trick.


The free-formed nature of State::event is sometimes overwhelming; how do you order all of the things that need to happen? You could also implement SimpleState when you only have a single Panel. This gives a slightly more opinionated interface, telling you when a button was clicked, slider was changed, when the mouse was moved, etc. If you're confused, see how it implements fn event -- it's just organizing some typical different things that happen to handle an event.

Higher layers

Someday we want to release widgetry for general use to the Rust community. But there's also lot of code shared between game, fifteen_min, and other apps that handles UI concerns specific to map_model, which isn't something most people will care about. This stuff goes in map_gui.

Lots of the map_gui code implements widgetry States, but those are parameterized by a particular App struct. Since each of the top-level crates uses a different struct, there's an AppLike trait to handle this level of indirection. If it walks like a duck...

Rendering maps

There are generally two strategies for drawing the map. In the unzoomed view, we tend to have a single Drawable for all buildings, another for all roads, etc. In the zoomed view, only a few map elements (bus stops, lanes, buildings) are visible at a time, so we can afford to show more detail, and store a Drawable per object. In fact, there's no need to even calculate all of this zoomed-in detail upfront; many maps are huge, and a player won't zoom into every section in a particular session. So internally, most of the Renderables use RefCell and lazily calculate what to draw.

DrawMap internally manages a quadtree to figure out what to draw and to help figure out what object is under the mouse cursor.

async madness

If you want to see some scary Rust, check out There's a fatal flaw with the core winit event loop -- it was written before Rust async landed. It's generally bad to spend more than a few milliseconds in either event or draw; the app will appear sluggish, and at some point, the window manager warns that the app is frozen. This happens when we synchronously/blockingly load a big file from disk, or worse, from the network.

We're starting to figure out some of the workarounds. When there's "proper" async Rust code, like for downloading a file on native, the trick is to spawn a separate thread to execute the async block. The main thread (where event and draw and most things run) stashes a Future inside of the State. In event, it non-blockingly polls the future to see if its done. If not, immediately return control to the window manager and just ask it to wake things up again in a few milliseconds. Proper loading screens can be drawn this way.

All of this gets more complicated on the web, because you can't really spawn a thread without somme web worker magic. It's still possible to make async HTTP fetches work, but it's not all wired together yet.