Compose playback controls, device recovery contracts, and event cleanup — Boundary detective

PART I

Warm-up

Reconnect with a useful idea before starting compose playback controls, device recovery contracts, and event cleanup. In one or two sentences, describe the input, output or host boundary from the previous exercise that this lesson builds on. If you are arriving directly, skim the worked example first; there is no locked prerequisite.

A rough note is enough. You can revise it after the example.

PART II

Instruction & guided example

Handle completion racing with close

Event cleanup is an ownership obligation across registration, delivery, close and late completion. Unregistering a callback and cancelling active native work are different guarantees. WS cancellation signatures remain unresolved, so these lessons specify required host behavior without inventing runtime calls.

Completion for V is queued at t4. P close begins at t5; R release acknowledged at t6. Host contract says already-queued delivery may arrive after release, but callback must check owner-open before visible updates.

Worked reasoning: Delivery at t7 finds P closed and must not show Finished or reopen P. R release prevents future enqueueing under this contract, but does not erase the already-queued event.

Failed alternative: Assuming release retroactively deletes queued callbacks permits a late visible write without an owner check.

Worked reasoning and supplied artifact

t4:completion queued
t5:P closed
t6:R release acknowledged,future enqueue disabled
t7:queued callback arrives ->owner closed ->no visible apply
Queue and registration validity are distinct

Illustrative example from the proposed WillowScript design. The language and WUIX/WMATH/WDB/WAUDIO/WTMP services are not implemented in the supplied source. This site has no interpreter, VM or native host and does not execute or certify the snippet.

Try the guided change

Reconstruct the supplied worked case first: list its inputs, intermediate decisions and final artifact. Compare each row with the explanation and identify exactly where the rejected alternative fails. Then use the changed inputs in Part IV for independent work.

• Write the supplied inputs and initial state before following the example.
• Reconstruct each intermediate calculation or host decision, then compare with the worked artifact.
• Explain why the contrasting alternative fails before attempting the changed inputs in Part IV.

PART III

Take a breather

Step back after the guided example. Look away from the screen, stretch, or reread your prediction. This is an optional learning pause, not a timer or an assessment gate.

When you return, try to name the lesson’s key idea without copying its wording. If that feels difficult, return to Part II and trace one line rather than restarting the entire lesson.

PART IV

Independent practice

Project context: Media operations console. Build a small, understandable change around compose playback controls, device recovery contracts, and event cleanup.

Trace queued progress at 2, close 3, release 4, delivery 5. Specify the owner guard and remaining native cleanup evidence.

What to produce

A race timeline with enqueue, release, delivery and visible-effect decisions.

• State the intended behavior in a sentence before editing.
• Use only source-backed syntax and the host services whose contract you have been given.
• Record a normal case and a boundary or unavailable-resource case.
• Label hand predictions separately from anything actually measured in a supplied toolchain.

Syntax colors are a reading aid, not a compiler. The line you are editing stays uncolored until you move to a different line; capitalization is never changed. Drafts stay on this browser and code is not executed.

import wdb;
domain Inventory {
    const lowStock = wdb.query({
        from: products,
        select: [id, name, stock],
        where: [[ stock < 10 ]],
        order: { by: stock, direction: ascending }
    });
}

PART V

Self-check

Use this check to notice what to revisit. It does not award a private course grade, block another page or claim to verify compiled code. Every answer has a learning explanation.

You may keep exploring even if you are unsure. Use the hint, compare the trace, and revise your explanation.

PART VI

Recap & small task

Bring the idea back to the mod feature you are building. A useful recap connects the mechanism to a decision you can make in your own code.

• Handle completion racing with close
• Trace queued progress at 2, close 3, release 4, delivery 5. Specify the owner guard and remaining native cleanup evidence.
• Separate source-backed behavior, host requirements and untested predictions.

One small recap task

Read whether release blocks future enqueueing or also drains already-queued work.

This marks your personal practice record only. It is not a certification or an official workspace assessment.