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OffscreenCanvas · 04 Context Creation Failure
Series contents · Technology map
Stage: Control and the First Render · Editable example · Storage ID: offscreen-canvas-step-04
Goals and learning outcomes
- Learning outcome: explain the role of Context. Input cases: Forced context failure.
- Deliverable: Failure guidance and rendering results from the replacement Canvas. Explain the rationale for handling Replacement Canvas, linking it to the code.
Prerequisites
Prepare the experiment results for Ready and Pending input from Unit 3, “Initialization Handshake”.
Key concepts and explanations
Context
getContext may fail to create the requested context type. Do not treat null as a working renderer.
Replacement Canvas
Do not transfer an already transferred HTML Canvas again as it is. Replace it with a new DOM Canvas for the fallback.
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-4.js in a separate practice project, check the Worker's getContext result and send failures to the main thread. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: Forced context failure. Baseline: Normal. Run under the baseline conditions, then record observations and errors in notes/day-4.md.
- Variable: Initialization; changed value: Inject failure. Run again through the same input path. Deliverable to save: Failure guidance and rendering results from the replacement Canvas. Hypothesis to check: Main-thread rendering recovers on a new Canvas. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Initialization | Normal | Inject failure | Main-thread rendering recovers on a new Canvas. |
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: Failure guidance and rendering results from the replacement Canvas. 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: Context and Replacement Canvas. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Context affect in this exercise? Input cases: Forced context failure.
- Variable: Initialization. Hypothesis to check: Main-thread rendering recovers on a new Canvas. Did the actual results agree with the hypothesis? Explain the evidence in terms of Replacement Canvas.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
The button calls fallback directly rather than injecting a getContext failure. The shared initialization path checks for a null Context and replaces the Canvas.
Source code and records are not overwritten automatically. Resetting an existing example also preserves your notes.