Open code, practice, and notes →
Download source with English comments · for reading
OffscreenCanvas · 18 Page Visibility Policy
Series contents · Technology map
Stage: Rendering Lifecycle and Recovery · Editable example · Storage ID: offscreen-canvas-step-18
Goals and learning outcomes
- Learning outcome: explain the role of Visibility. Input cases: Hide the tab and return.
- Deliverable: Computation count while hidden and state on return. Explain the rationale for handling Synchronization, linking it to the code.
Prerequisites
Prepare the experiment results for Pause and Time baseline from Unit 17, “Pausing and Resuming”.
Key concepts and explanations
Visibility
A Worker can receive the document visibility policy from the main thread. Continuing to compute an invisible scene still has a cost.
Synchronization
Choose whether to preserve state or catch up with elapsed time on return. Document the policy according to the simulation's purpose.
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-18.js in a separate practice project, translate visibilitychange into pause/resume. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: Hide the tab and return. Baseline: Keep computing. Run under the baseline conditions, then record observations and errors in notes/day-18.md.
- Variable: Policy; changed value: pause while hidden. Run again through the same input path. Deliverable to save: Computation count while hidden and state on return. Hypothesis to check: Computation stops while hidden and continues according to the return policy. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Policy | Keep computing | pause while hidden | Computation stops while hidden and continues according to the return policy. |
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: Computation count while hidden and state on return. 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: Visibility and Synchronization. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Visibility affect in this exercise? Input cases: Hide the tab and return.
- Variable: Policy. Hypothesis to check: Computation stops while hidden and continues according to the return policy. Did the actual results agree with the hypothesis? Explain the evidence in terms of Synchronization.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
Visibility changes send pause messages and reset the time baseline. Hidden state suppresses drawing; schedule cancellation and computation-count measurement are not implemented.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.