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OffscreenCanvas · 20 Restarting with a New Canvas
Series contents · Technology map
Stage: Rendering Lifecycle and Recovery · Editable example · Storage ID: offscreen-canvas-step-20
Goals and learning outcomes
- Learning outcome: explain the role of Retransfer limits. Input cases: 5 start·stop cycles.
- Deliverable: Canvas·Worker counts and first frame for each cycle. Explain the rationale for handling Restart state, linking it to the code.
Prerequisites
Prepare the experiment results for Dispose and Acknowledgment and termination from Unit 19, “Cleanup Protocol”.
Key concepts and explanations
Retransfer limits
Trying to transfer an already transferred HTML Canvas again may fail. Begin the lifecycle with a new DOM element.
Restart state
A new renderer must receive scene data again. Attach pointer and size listeners to the new element as well.
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-20.js in a separate practice project, create a new canvas after stop and resend the init state. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: 5 start·stop cycles. Baseline: Reuse the existing Canvas. Run under the baseline conditions, then record observations and errors in notes/day-20.md.
- Variable: Restart; changed value: New Canvas. Run again through the same input path. Deliverable to save: Canvas·Worker counts and first frame for each cycle. Hypothesis to check: Rendering restarts without duplicate-transfer errors. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Restart | Reuse the existing Canvas | New Canvas | Rendering restarts without duplicate-transfer errors. |
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: Canvas·Worker counts and first frame for each cycle. 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: Retransfer limits and Restart state. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Retransfer limits affect in this exercise? Input cases: 5 start·stop cycles.
- Variable: Restart. Hypothesis to check: Rendering restarts without duplicate-transfer errors. Did the actual results agree with the hypothesis? Explain the evidence in terms of Restart state.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive Restarting with a New Canvas 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: Cleanup Protocol · Next: Recovering from Worker Failure