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OffscreenCanvas · 14 Resize and Input Protocol
Series contents · Technology map
Stage: Synchronizing Size and Input · Editable example · Storage ID: offscreen-canvas-02
Goals and learning outcomes
- Learning outcome: explain the role of Unit contract. Input cases: Selecting the center during rapid resize.
- Deliverable: Records of size seq, selected ID, and sharpness. Explain the rationale for handling Preserving order, linking it to the code.
Prerequisites
Prepare the experiment results for Capture and Cancel from Unit 13, “Pointer Capture and Cancellation”.
Key concepts and explanations
Unit contract
Size, DPR, and pointer input must use the same coordinate basis. The Worker does not infer DOM layout.
Preserving order
Record the latest resize seq together with input timing. An old size must not overwrite the latest selection.
Practice setup
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-14.js in a separate practice project, extend the existing offscreen-canvas-02 with integrated ResizeObserver·seq·local input handling. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: Selecting the center during rapid resize. Baseline: 1. Run under the baseline conditions, then record observations and errors in notes/day-14.md.
- Variable: DPR; changed value: 2. Run again through the same input path. Deliverable to save: Records of size seq, selected ID, and sharpness. Hypothesis to check: Sharpness and the selection position are preserved. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| DPR | 1 | 2 | Sharpness and the selection position are preserved. |
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: Records of size seq, selected ID, and sharpness. 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: Unit contract and Preserving order. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Unit contract affect in this exercise? Input cases: Selecting the center during rapid resize.
- Variable: DPR. Hypothesis to check: Sharpness and the selection position are preserved. Did the actual results agree with the hypothesis? Explain the evidence in terms of Preserving order.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.
Next learning
Previous: Pointer Capture and Cancellation · Next: Updating Scene State