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Robot PU Actions

🤖 Lesson: Robot PU Actions (Asynchronous Motion)​

Most Robot PU actions are designed to be asynchronous to prevent the micro:bit from locking up.

Many motion blocks return a status code:

  • 0: the action has reached a completion boundary (a "step" / state finished)
  • 1: the action is still running (keep calling it)

This lets you build "synchronous" behavior yourself by repeatedly calling the action until you observe enough 0 return events.

1. Action APIs (What you can call)​

Motion actions (return status number)​

  • robotPu.walk(speed, turn)
  • robotPu.sideStep(direction)
  • robotPu.explore()
  • robotPu.dance()
  • robotPu.kick()
  • robotPu.jump()
  • robotPu.rest()
  • robotPu.stand()

Motion actions (statement versions, return void)​

These are the same actions but do not return a code, so they are harder to sequence precisely:

  • robotPu.walkDo(speed, turn)
  • robotPu.sideStepDo(direction)
  • robotPu.exploreDo()
  • robotPu.danceDo()
  • robotPu.kickDo()
  • robotPu.jumpDo()
  • robotPu.restDo()
  • robotPu.standDo()

Non-motion actions (usually synchronous)​

  • robotPu.greet()
  • robotPu.talk(text)
  • robotPu.sing(text)
  • robotPu.setMode(mode) (switch the internal behavior state machine)

2. Why actions are asynchronous​

A motion like walking or sidestepping is not "one motor command". It is a sequence of body poses that must be updated repeatedly over time.

If a block tried to do the whole motion in one call (blocking), it could:

  • freeze button/radio events
  • starve background tasks
  • make the whole system feel "locked up"

So Robot PU action APIs are designed to be called repeatedly in a loop.

3. Comparing "synchronous" vs "asynchronous" patterns​

A. Synchronous (blocking) idea (what we avoid)​

This is the style that can cause lock-ups:

// (Concept only) A blocking API would look like this.
// robotPu.walkBlocking(3)
// robotPu.sideStepBlocking(-1)

You call the action many times. Each call advances the motion.

let rc = robotPu.walk(2, 0)
if (rc == 0) {
    // A completion boundary happened (a gait state finished)
}

3.5 Example: call walk() 100 times (no return checking)​

Sometimes you just want to "drive" the motion for a fixed amount of time and you do not care about counting steps.

This pattern ignores the return value and simply calls walk(...) repeatedly:

for (let i = 0; i < 100; i++) {
    robotPu.walk(2, 0) // ignore rc
}

3.6 JavaScript "function pointers" (callbacks)​

In JavaScript / TypeScript, you can store a function in a variable and pass it to another function. This is often called a callback (similar idea to a "function pointer" in C).

In this lesson we pass a function like () => number into helpers such as doCompletions(...).

Example:

function myAction(): number {
    return robotPu.walk(2, 0)
}

function do400Times(run: () => number): void {
    let done = 0
    while (done < 400) {
        const rc = run()
        done += 1
    }
}

// Pass a function *reference* (do not call it here)
do400Times(myAction)

// Or pass an inline anonymous function (arrow function)
do400Times(() => robotPu.walk(2, 0))

Key idea:

  • robotPu.walk(2, 0) calls the function immediately and produces a number.
  • () => robotPu.walk(2, 0) produces a function that we can call later, many times.

4. How to "wait for completion" (build synchronous behavior safely)​

The safest pattern is:

  1. Call the action
  2. If it returns 1, it is still running (keep calling)
  3. When it returns 0, treat that as a completion boundary for counting

A. Wait until one completion event​

function waitOneCompletion(run: () => number): void {
    while (true) {
        const rc = run()
        if (rc == 0) return
    }
}

B. Count completions inside loops​

function doCompletions(run: () => number, completions: number): void {
    let done = 0
    while (done < completions) {
        const rc = run()
        if (rc == 0) done += 1
    }
}

Notes:

  • The meaning of a "completion" depends on the action (it is typically a gait/state boundary).
  • For some gaits, one physical "step" is made of multiple internal states. If your gait uses 2 states per step, then you will see rc == 0 twice per step.

5. Final program: walk forward, sidestep left, jump, stand​

Requirements:

  • walk forward for 3 steps (if it is 2 states per step, count return == 0 for 6 times)
  • side step left for 3 steps
  • jump 1 time
  • stand
function doCompletions(run: () => number, completions: number): void {
    let done = 0
    while (done < completions) {
        const rc = run()
        if (rc == 0) done += 1
    }
}

// 1) Walk forward: 3 steps
// If your gait uses 2 states per step, count 0 six times
doCompletions(() => robotPu.walk(2, 0), 6)

// 2) Side step left: 3 steps
// direction: negative = left, positive = right
doCompletions(() => robotPu.sideStep(-0.2), 6)

// 3) Jump one time
doCompletions(() => robotPu.jump(), 4)

// 4) Stand (return to neutral)
doCompletions(() => robotPu.stand(), 1)