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OffscreenCanvas · 12 Coalescing Input
Series contents · Technology map
Stage: Synchronizing Size and Input · Editable example · Storage ID: offscreen-canvas-step-12
Goals and learning outcomes
- Learning outcome: explain the role of Latest value. Input cases: Rapid movement and clicks.
- Deliverable: Message counts and a record of down/up pairs. Explain the rationale for handling Event semantics, linking it to the code.
Prerequisites
Prepare the experiment results for Local coordinates and Matching scales from Unit 11, “Transforming Pointer Coordinates”.
Key concepts and explanations
Latest value
Some scenes do not need every pointer event. You can send only the latest position once per frame.
Event semantics
Do not discard boundary events such as down·up. Distinguish continuous move events from discrete events.
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-12.js in a separate practice project, coalesce move in rAF and forward down/up immediately. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: Rapid movement and clicks. Baseline: Every move. Run under the baseline conditions, then record observations and errors in notes/day-12.md.
- Variable: Transmission; changed value: Last move per frame. Run again through the same input path. Deliverable to save: Message counts and a record of down/up pairs. Hypothesis to check: Click boundaries are preserved even as the number of move messages decreases. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Transmission | Every move | Last move per frame | Click boundaries are preserved even as the number of move messages decreases. |
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: Message counts and a record of down/up pairs. 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 value and Event semantics. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Latest value affect in this exercise? Input cases: Rapid movement and clicks.
- Variable: Transmission. Hypothesis to check: Click boundaries are preserved even as the number of move messages decreases. Did the actual results agree with the hypothesis? Explain the evidence in terms of Event semantics.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive Coalescing Input 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: Transforming Pointer Coordinates · Next: Pointer Capture and Cancellation