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OffscreenCanvas · 13 Pointer Capture and Cancellation
Series contents · Technology map
Stage: Synchronizing Size and Input · Editable example · Storage ID: offscreen-canvas-step-13
Goals and learning outcomes
- Learning outcome: explain the role of Capture. Input cases: Dragging inside, moving outside, and cancellation.
- Deliverable: The dragging value after input ends. Explain the rationale for handling Cancel, linking it to the code.
Prerequisites
Prepare the experiment results for Latest value and Event semantics from Unit 12, “Coalescing Input”.
Key concepts and explanations
Capture
A pointer can leave the Canvas during a drag. Use capture to maintain the input lifecycle.
Cancel
pointercancel differs from a normal up. The Worker's dragging state must also end.
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-13.js in a separate practice project, connect main-thread setPointerCapture with pointercancel messages. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: Dragging inside, moving outside, and cancellation. Baseline: pointerup. Run under the baseline conditions, then record observations and errors in notes/day-13.md.
- Variable: End event; changed value: pointercancel. Run again through the same input path. Deliverable to save: The dragging value after input ends. Hypothesis to check: Both paths clear the drag state. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| End event | pointerup | pointercancel | Both paths clear the drag state. |
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 dragging value after input ends. 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: Capture and Cancel. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Capture affect in this exercise? Input cases: Dragging inside, moving outside, and cancellation.
- Variable: End event. Hypothesis to check: Both paths clear the drag state. Did the actual results agree with the hypothesis? Explain the evidence in terms of Cancel.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive Pointer Capture and Cancellation 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: Coalescing Input · Next: Resize and Input Protocol