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HomeProgrammingNon-Robot Programming2026-2027 TrainingModule 2: Git & FlutterLesson 8

Lesson 8

🎯 Goal

Learn a system for fixing bugs, use it on a page full of them, then turn your own checklist into a real checklist in your app.

Everybody’s code breaks. Good programmers just have a plan.

The Debug Checklist ✅

Go in order.

  1. Expected vs. happened. What did you want? What actually happened?
  2. Read the error. Start at the top. Click the blue file link.
  3. Red squiggles? Hover over them and fix those first.
  4. What did I change last? Open the Commit window, right-click your file, and choose Show Diff. The bug is usually in what’s green or red.
  5. Check the usual suspects: ; · matching () and {} · quotes · spelling and CAPITALS · did I forget setState? · did I forget the $?
  6. Print it. print('cycles is $cycles'); shows up in the Run panel.
  7. Make it smaller. Comment out code (Ctrl + /, Mac Cmd + /) until the bug disappears.
  8. Hot restart. Still weird? Stop the app and run it again.
  9. 15-minute rule. Stuck for 15 minutes? Ask. Say what you expected, what happened, and what you tried.

Start here

Do The Pull First Rule from Lesson 3.

Task 1: Break it on purpose

The best way to learn errors is to cause them. On your Training Page, do each of these one at a time:

#Break it by…Then
1Deleting one ; from a lineFind the red squiggle
2Spelling setState as setstateRead the error message
3Deleting a )Count how many red errors appear
4Writing onPressed: myFunction() instead of onPressed: myFunction (use any button with a named function, or make a tiny one)Read the error message

For each one: read the error out loud in plain English (to a neighbor or to yourself), then undo it with Ctrl + Z (Mac Cmd + Z). Don’t commit these.

✅ Checkpoint: your page runs again with no red errors.

Task 2: Bug Hunt

On your Training Page, tap Open Bug Hunt (Lesson 8) at the bottom. This page runs, but it has 5 bugs.

The symptoms (what you’ll see):

  1. Tapping + on Cycles seems to do nothing, until you tap something else
  2. The text says Climbed: climbed instead of Climbed: true or false
  3. The counters go below zero
  4. Adding fouls makes the score go up
  5. Goal reached! shows up at 6 cycles instead of 5

Open lib/Training/BugHuntPage.dart. Use the checklist to find the cause of each symptom and fix it.

Need a nudge? (don’t open until you’ve tried)

  1. Look at the Cycles callback and compare it to the Fouls one right below it. What’s different?
  2. Look at that line carefully. What’s missing in the text?
  3. Look at how you stopped negatives in Lesson 7.
  4. Look at totalScore(). What should happen to fouls?
  5. Look at the comparison in goalMessage(). Is it > or >=?

✅ Checkpoint: all 5 symptoms are gone. Test it by tapping things in a different order. A bug you can’t see is one you didn’t test.

Task 3: Make a checklist in your app

Now turn the checklist into real code. On your Training Page, under the heading LESSON 8: CHECKLIST, make a checklist of 5 debug steps (use your own words!).

  1. Give each step its own bool variable, starting false.
  2. Show each one with buildCheckBox, like this:
buildCheckBox('Read the error', readError, (bool value) {
  setState(() {
    readError = value;
  });
}),
  1. Write a function that counts the steps not done yet:
int stepsLeft() {
  int left = 0;
  if (!readError) {
    left++;
  }
  // one more "if" for each of your other steps
  return left;
}

(! means NOT. !readError means “readError is false”.)

  1. Under the checklist, show a Text:
    • All steps done! in green when stepsLeft() is 0
    • N steps left in orange otherwise

✅ Checkpoint: you can’t see All steps done! until all 5 boxes are ticked.

Ship it

Click Git → Push… and push your branch.

To complete this lesson

Paste your branch name or link (training-yourname) into this lesson’s tab in your training doc.

Lesson 7Lesson 9