Why Widgets?

Pavel Panchekha

This is a different kind of blog post—perhaps you can call it a menifesto. When Chris and I published Chapter 5, we included two interactive widgets that let you explore how the element tree and layout tree interact. We plan to do more. This post is about why we are building these widgets and what we hope to accomplish. I’ll follow up with more posts later on the technical details, which I think are quite subtle and interesting!

An example widget

Consider the following widget, which steps you through how line height is computed when different font sizes are mixed:

Click the buttons to start the simulations and go through the steps one by one. You can see how baseline computation:

In the widget, pink and blue represent ascents and descents, with dark red and dark blue showing the maximum ascent / descent (once that’s computed). The variable names (in typewriter font on the right) correspond to the variables in Chapter 3’s flush method.

Of course, we didn’t invent this idea of interactive widgets. Perhaps it all goes back to Logo and the work of Seymore Pappert. Bret Victor is of course the modern intellectual source. Bartosz Ciechanowski’s visual explanations are a more direct inspiration. Pierre-Marie Dartus’s essay on shadow DOM event propagation gave us a browser-related example to study. In any case, I think widgets like this will help readers better understand our book.

Goals for widgets

So let me dissect this widget a bit—how is it better or worse than the bulleted list above?

First off, of course, it is visual, and might be easier to pick up that way. If you click through the widget you could probably figure out the steps without reading anything. Some people prefer that, and also if you already read the text the visual can reinforce things.

Second, the visual representation actually reinforces some key concepts. For example, layout has two phases, you first lay it out horizontally, and only later lay it out vertically. Putting two things on the screen, labeled “Phase 1” and “Phase 2”, reinforces that.

Third, the widget can actually reveal things that were hidden in the description. For example, the bullet points in the previous section didn’t mention leading, but it clearly shows up in the widget. In fact, in a proper browser, the amount of leading above and below the text should be the same—the widget shows that (for simplicity) this book’s toy browser doesn’t do that.

Finally, the widget can use color or shape to highlight relationships and states. My widget above shows the maximum ascent and descent in a different color, and uses those colors to hint that the line’s height is computed based on a maximum ascent and descent, but each word is aligned relative to its own ascent and descent. That hints at the correct algorithm without having to explain it.

To summarize: Just clicking through the widget should explain the algorithm. The widget should make key concepts visual. It should be real enough that it can surprise you. And it should use color, alignment, and size to suggest real relationships in the code.

Technical requirements

Ok—these are nice goals. But achieving them means surmounting some real challenges:

For the widget to explain the algorithm, the user needs to execute the code step by step. But you normally don’t want the browser to pause randomly, and adding code to support it would clutter up the browser code and make it much harder to explain. A debugger could work, but most debuggers, like the built-in pdb, can’t step backwards, and that’s quite useful when learning.

To visually present key concepts each widget would need to be custom-written. Each chapter means new concepts means new visuals! That means any other overhead—coding up the widget, styling things, adding step controls—needs to be minimized. If each widget is a pain, Chris and I aren’t going to write very many!

For the widget to be real, it should run the same code described in the book. But the book is written in Python, not JavaScript, so it can’t be run in the reader’s browser. And neither I nor Chris want to maintain the book in two languages. Anyway, bugs in the translation would mean the confusing or deceptive widgets.

And for the widgets to use visual relationships to signify logical ones, each widget would need custom styling and custom visuals. Yet the widgets should also fit naturally into the book and have similar styles and controls, so readers would know what to do when they saw one.

These are pretty stringent requirements, so the solution Chris and I have found is crazy and bizarre—I’ll be explaining it over the next few blog posts. Stay tuned!

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