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Chapter 5 - Low button refactored


The intent of this section is to organize the code better.


Up to now, we’ve built the program by bolting on functionality, bit by bit. This has served us well, allowing us to start with an empty canvas, iterate by changing the minimum amount of lines, while still making meaningful progress.

Going forward however, it’s starting to look a little unwieldy. Having all the code inside one big main() is can make it harder to understand, and harder to continue building. Hence we’ll refactor the program a bit, simply breaking it up into smaller pieces.

Refactoring is transforming code in a safe and rapid way is vital to keeping it cheap and easy to modify for future needs. Martin Fowler

In other words, no new functionality will be added, but we’ll clear the way for better things to come.


No 1 - main() is too long

Main is too long and does too much. It’s better if main() starts and controls the program, but apart from that delegates to others. Here’s the new one:

func main() {
  go func() {
    // create new window
    w := app.NewWindow(
      app.Title("Egg timer"),
      app.Size(unit.Dp(400), unit.Dp(600)),
    if err := draw(w); err != nil {

Now, inside main() we create a window w as before, and immediately hand it over to a dedicated function draw().

By storing the result of draw() in err, we can examine if the execution went well, and we can handle any errors in an orderly fashion.

For that we use os.Exit() and it’s close cousin log.Fatal(err). Both come from the standard library and are included as imports.

As mentioned before, the convention is that a zero exit code indicates success, which is what we send from os.Exit(0)if err is nil. If not, we call log.Fatal(err) which prints the error message en exits with os.Exit(1).

No 2 - Constraints and Dimensions - A handy shortcut

We talked at length about Constraints and Dimensions earlier. Since we’re using them quite a lot, it’s handy to define two shortcuts, C and D. Constraints are part of the Context.

type C = layout.Context
type D = layout.Dimensions

No 3 - The draw( ) function

A simplified version of draw( ) shows the structure.

func draw(w *app.Window) error {
    // ...

  // listen for events in the window.
  for e := range w.Events() {

    // detect what type of event
    switch e := e.(type) {

    // this is sent when the application should re-render.
    case system.FrameEvent:
        // ...

        // this is sent when the application is closed.
    case system.DestroyEvent:
      return e.Err
  return nil

As before we range through w.Events(), detecting their type.

  • system.FrameEvent is handled as before,
  • we add a new case for system.DestroyEvent, which returns nil for normal window closures, but Err if something else is the cause.


Refactoring is a matter of taste, and this is my take on it. If you have different needs, do what’s right for your app. The main point is to keep your applications flexible enough to support continued improvements and future needs. Good luck.

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