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OffscreenCanvas · 19 Cleanup Protocol
Series contents · Technology map
Stage: Rendering Lifecycle and Recovery · Editable example · Storage ID: offscreen-canvas-step-19
Goals and learning outcomes
- Learning outcome: explain the role of Dispose. Input cases: dispose during rendering.
- Deliverable: Counts of remaining frames, messages, and observers. Explain the rationale for handling Acknowledgment and termination, linking it to the code.
Prerequisites
Prepare the experiment results for Visibility and Synchronization from Unit 18, “Page Visibility Policy”.
Key concepts and explanations
Dispose
Frames, events, observers, and the Worker must each be stopped or cleaned up. Removing the DOM alone does not clean up every task.
Acknowledgment and termination
Wait for the cleanup acknowledgment, but avoid waiting indefinitely. Terminate from the main thread with terminate if needed.
Practice setup
First check the execution scope guidance on the example screen. The following describes the environment needed to extend the assignment in a separate project. The existing offscreen-canvas-01·02·03 are starting examples for supported browsers. Implement the new units in main-thread and Worker files in a separate local HTTP project. Check API and context support, and provide a main-thread fallback on a new Canvas if transfer fails. Preserve CSP and isolation, and record performance measurements yourself.
The linked example is a starting point. It does not implement every task below. Complete work beyond its supported scope in a separate project.
Practice tasks
- In src/day-19.js in a separate practice project, implement dispose messages·ack, observer.disconnect, and a terminate path. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: dispose during rendering. Baseline: Remove only the DOM. Run under the baseline conditions, then record observations and errors in notes/day-19.md.
- Variable: Shutdown; changed value: Full dispose. Run again through the same input path. Deliverable to save: Counts of remaining frames, messages, and observers. Hypothesis to check: No recurring tasks remain after removal. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Shutdown | Remove only the DOM | Full dispose | No recurring tasks remain after removal. |
Predictions in this table are not measured results. Change one condition at a time and record why the actual result differs.
Completion criteria
- Required deliverable: Counts of remaining frames, messages, and observers. Results are available for both the baseline and changed conditions, allowing comparison of 2 conditions; input values and the execution environment are recorded together.
- The explanation of differences between the two conditions connects both concepts: Dispose and Acknowledgment and termination. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Dispose affect in this exercise? Input cases: dispose during rendering.
- Variable: Shutdown. Hypothesis to check: No recurring tasks remain after removal. Did the actual results agree with the hypothesis? Explain the evidence in terms of Acknowledgment and termination.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive Cleanup Protocol experiment with cleanup of the resources it uses.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.
Next learning
Previous: Page Visibility Policy · Next: Restarting with a New Canvas