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OffscreenCanvas · 15 Updating Scene State
Series contents · Technology map
Stage: Rendering Lifecycle and Recovery · Editable example · Storage ID: offscreen-canvas-step-15
Goals and learning outcomes
- Learning outcome: explain the role of Patches. Input cases: Valid and stale patches.
- Deliverable: Final scene values for each version. Explain the rationale for handling Versions, linking it to the code.
Prerequisites
Prepare the experiment results for Unit contract and Preserving order from Unit 14, “Resize and Input Protocol”.
Key concepts and explanations
Patches
Sending the entire state each time increases data volume. You can define a patch contract that sends only changed fields.
Versions
A patch needs a baseline state to apply to. Provide a full-state resynchronization path when versions disagree.
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-15.js in a separate practice project, implement color·speed patches and stateVersion checks. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: Valid and stale patches. Baseline: Current. Run under the baseline conditions, then record observations and errors in notes/day-15.md.
- Variable: Patch version; changed value: Stale version. Run again through the same input path. Deliverable to save: Final scene values for each version. Hypothesis to check: Stale patches are rejected and the current state is retained. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Patch version | Current | Stale version | Stale patches are rejected and the current state is retained. |
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: Final scene values for each version. 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: Patches and Versions. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Patches affect in this exercise? Input cases: Valid and stale patches.
- Variable: Patch version. Hypothesis to check: Stale patches are rejected and the current state is retained. Did the actual results agree with the hypothesis? Explain the evidence in terms of Versions.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive Updating Scene State 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: Resize and Input Protocol · Next: Batching Particle Data