Wait for a press

Make the micro:bit wait for a button press or a shake, then react.

A doorbell, a game controller, a TV remote: each one waits for a press, then does one job.

So far the micro:bit ran its blocks and carried on by itself. Most devices wait instead: nothing happens until you press something. A block that waits like this is called an event block. You find them in Input.

In the simulator, click the A and B buttons with the mouse. To shake, click SHAKE above the micro:bit.

Predict: Read the code and say what it will do before you run anything. A wrong guess is fine: it shows you what to look for.
Run: Run it and compare the result with your prediction.
Investigate: Trace how each line of code connects to what you see.
Modify: Change the code on purpose and check the result.
Make: Build something new using what you have learned. Expect a few tries; that is normal.

Explain it to me

Some blocks wait for an event · Sound on for the voiceover

Some blocks wait. on button A pressed runs its blocks every time A is pressed, and never before. That is an event.

So a micro:bit program has three kinds of block holder:

on start: runs once, at the start
forever: runs again and again, by itself
on button pressed, on shake: runs each time that thing happens

Show as code

basic.showIcon(IconNames.Asleep)
input.onButtonPressed(Button.A, function () {
    basic.showIcon(IconNames.Happy)
})
input.onButtonPressed(Button.B, function () {
    basic.showIcon(IconNames.Sad)
})
input.onGesture(Gesture.Shake, function () {
    basic.showIcon(IconNames.Surprised)
})
basic.forever(function () {
    basic.showIcon(IconNames.Asleep)
    basic.pause(5000)
})

The example, as blocks

What it makes

PredictRead the code and say what it will do before you run anything. A wrong guess is fine: it shows you what to look for.

Predict

Show as code

lesson.ts

basic.showIcon(IconNames.Asleep)
input.onButtonPressed(Button.A, function () {
    basic.showIcon(IconNames.Happy)
})
input.onButtonPressed(Button.B, function () {
    basic.showIcon(IconNames.Sad)
})

There are three groups of blocks here. Which ones run straight away, and which ones wait?

What does the micro:bit show when it starts, and then after you press B?Asleep, then sad when B is pressedNothing until a button is pressedHappy, then sad, straight away when it startsAsleep, then happy and sad together

RunRun it and compare the result with your prediction.

Run

Press A, then B, then A again. Compare with your prediction.

Show as code

lesson.ts

basic.showIcon(IconNames.Asleep)
input.onButtonPressed(Button.A, function () {
    basic.showIcon(IconNames.Happy)
})
input.onButtonPressed(Button.B, function () {
    basic.showIcon(IconNames.Sad)
})

Try it yourselfYour edits are saved in this browser.

It takes a few seconds to load. Your work in this step is kept.

I pressed A and B in the simulator and compared what happened with my prediction.

InvestigateTrace how each line of code connects to what you see.

Investigate

This program has four groups of blocks. Try it in your head first.

Show as code

lesson.ts

basic.showIcon(IconNames.Asleep)
input.onButtonPressed(Button.A, function () {
    basic.showIcon(IconNames.Happy)
})
input.onButtonPressed(Button.B, function () {
    basic.showIcon(IconNames.Sad)
})
input.onGesture(Gesture.Shake, function () {
    basic.showIcon(IconNames.Surprised)
})
basic.forever(function () {
    basic.showIcon(IconNames.Asleep)
    basic.pause(5000)
})

Match each block with when its blocks run.

Work out how often an event block runs.

Ben's program used an if block from Logic. Read this for now: it asks a yes-or-no question, is A pressed right now?, and runs the blocks inside only if the answer is yes. You will use if properly in a later lesson.

Show as code

if (input.buttonIsPressed(Button.A)) {
    basic.showIcon(IconNames.Heart)
}

Match each block with when its blocks run.

on start

forever

on button A pressed

on shake

You press A four times. How many times does the happy face appear?

Ben wants A to show a heart. He put this in on start. He presses A and nothing happens. Why?He needs a pause firston start checks the button once, when the micro:bit switches on, before he presses itHearts only work in foreverA is broken in the simulator

ModifyChange the code on purpose and check the result.

Modify

Choose different pictures for A and B.

Need a hint?

Click the picture in a show icon block to choose a different one.

Add a third event block for A+B (both buttons at once). Make it show a picture of your choice.

Need a hint?

'on button A pressed' is in Input. Drag in another one.

Another hint

Click the A in the new block and choose A+B.

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I changed both faces and pressed each button to check.

Add an event for pressing A and B together, and show a picture.

This ticks by itself when your blocks do what is asked.

InvestigateTrace how each line of code connects to what you see.

Mix it up

Read the animation from the top, the way forever goes round it.

Show as code

lesson.ts

basic.forever(function () {
    basic.showIcon(IconNames.Heart)
    basic.pause(200)
    basic.showIcon(IconNames.SmallHeart)
    basic.pause(200)
    basic.showIcon(IconNames.Yes)
    basic.pause(1000)
})

This animation runs in forever. Which picture stays on the longest each time round?The heartThey all stay the same timeThe small heartThe tick

MakeBuild something new using what you have learned. Expect a few tries; that is normal.

Make

Build a signal board for a bike rider:

A shows an arrow pointing left.
B shows an arrow pointing right.
A+B shows a cross for "stop".
Shake shows a surprised face.

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Need a hint?

You need four event blocks: A, B, A+B and shake. Each one has one picture inside.

Another hint

show arrow is in Basic (click more...). Choose the direction with the dropdown.

A signal board: A, B, A+B and shake each show their own picture.

This ticks by itself when your blocks do what is asked.

ModifyChange the code on purpose and check the result.

Change a bike indicator

Real bike indicators flash. This one flashes each arrow twice. Make it flash three times, then make A+B flash a warning (both arrows, or a picture you draw).

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For teachers

Students make programs that wait for a button press or a shake, and see that an event block runs its blocks every time its event happens, and only then. About 45 minutes.

Open the teacher guide: answers, solutions, slides and a worksheet.

Students will be able to

I can make the micro:bit react when a button is pressed or it is shaken.
I can say when on start, forever and an event block each run.
I can add a new event block to react to something new.

Before class

An event is something that happens: a button press, a shake. An event block (such as on button A pressed) holds blocks that run every time that event happens, and only then. Compare with the last two lessons: on start runs once, forever runs again and again by itself. In the simulator, students click the A and B buttons, and click SHAKE for a shake. Try the Make yourself: four event blocks, one picture in each.

Running the lesson

Predict: ask which blocks run straight away and which wait. Many students expect the faces to show at the start.
Run: press A, B, A. Say it plainly: nothing happens until a button is pressed, and each press runs its own blocks.
Investigate: the match task names when each kind of block runs. Ben's button is the classic mistake: a check in on start happens once, before anyone presses.
Modify: changing a face changes what an event does. Adding A+B is a new event block, not a change to an old one.
Mix it up: a quick look back at forever and pause.
Make: four event blocks. show arrow is under Basic › more.

The hard part (on purpose)

An event block runs only when its event happens. When a press does nothing, ask which block the picture is inside.

When students get stuck

Pressing a button does nothing. Ask: Which block is your picture inside? Fix: Put it inside 'on button A pressed' (from Input), not in on start.
A+B never fires: A or B fires instead. Ask: Did you press both at once? Fix: In the simulator, click the A+B button below the micro:bit. On a real micro:bit, press both together.
Two event blocks for the same button, and only one seems to work. Ask: How many 'on button A pressed' blocks are there? Fix: Keep one block per event, and put everything for that press inside it.

Stretch and support

Stretch: Make the signal board flash each arrow three times, and have shake show a short animation of three frames.

Support: Give students the four event blocks already placed and empty, and ask them only to fill in the pictures.

Where it leads

Next unit adds variables, so a button can count presses. Class ideas: a quiz buzzer where A and B are two teams. With real micro:bits: wear one on a bag strap as a bike signal.

What each step is for

Investigate: An event block runs its blocks every time its event happens, and only then. on start and forever are not events you trigger: they run by themselves.
Modify: To change what an event does, change the blocks inside it. To react to something new, add a new event block.
Mix it up: forever goes round its blocks in order, and each pause sets how long that picture stays.

Now you can

Make the micro:bit react when a button is pressed or it is shaken.
Say when on start, forever and an event block each run.
Add a new event block to react to something new.

Next lesson4. Count with a variableUse a variable to remember a number, and change it each time a button is pressed.

Source: content/code/microbit/microbit-buttons/lesson.md