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OffscreenCanvas · 09 Resize Sequence Numbers
Series contents · Technology map
Stage: Synchronizing Size and Input · Editable example · Storage ID: offscreen-canvas-step-09
Goals and learning outcomes
- Learning outcome: explain the role of Latest size. Input cases: Input in seq 1·3·2 order.
- Deliverable: The applied seq and final size. Explain the rationale for handling Acknowledgment, linking it to the code.
Prerequisites
Prepare the experiment results for Measurement and Cost of changes from Unit 8, “Resize Messages”.
Key concepts and explanations
Latest size
Rapid resizing generates multiple messages. Attach a seq so that only the latest size remains valid.
Acknowledgment
Returning the applied seq lets you match it to the main-thread request. Do not interpret an old acknowledgment as completion of the latest request.
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-9.js in a separate practice project, implement resize seq and lastApplied checks. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: Input in seq 1·3·2 order. Baseline: 1·2·3. Run under the baseline conditions, then record observations and errors in notes/day-9.md.
- Variable: Delivery order; changed value: 1·3·2. Run again through the same input path. Deliverable to save: The applied seq and final size. Hypothesis to check: seq 2 does not overwrite the latest size. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Delivery order | 1·2·3 | 1·3·2 | seq 2 does not overwrite the latest size. |
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: The applied seq and final size. 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: Latest size and Acknowledgment. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Latest size affect in this exercise? Input cases: Input in seq 1·3·2 order.
- Variable: Delivery order. Hypothesis to check: seq 2 does not overwrite the latest size. Did the actual results agree with the hypothesis? Explain the evidence in terms of Acknowledgment.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive Resize Sequence Numbers experiment with cleanup of the resources it uses.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.