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OffscreenCanvas · 02 Renderer Contract
Series contents · Technology map
Stage: Control and the First Render · Editable example · Storage ID: offscreen-canvas-step-02
Goals and learning outcomes
- Learning outcome: explain the role of Pure state. Input cases: The same three circles.
- Deliverable: A table of coordinates and colors for both adapters. Explain the rationale for handling Ownership boundaries, linking it to the code.
Prerequisites
Prepare the experiment results for Ownership and Fallback path from Unit 1, “Transferring Canvas Control”.
Key concepts and explanations
Pure state
The renderer draws from the state it receives rather than from the DOM. The same draw function can run in different execution contexts.
Ownership boundaries
The main thread manages the DOM and input. The Worker manages the Canvas and scene state.
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-2.js in a separate practice project, separate draw(ctx,state) from the main·worker adapters. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: The same three circles. Baseline: Main thread. Run under the baseline conditions, then record observations and errors in notes/day-2.md.
- Variable: Execution context; changed value: Worker. Run again through the same input path. Deliverable to save: A table of coordinates and colors for both adapters. Hypothesis to check: The same state produces the same shapes. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Execution context | Main thread | Worker | The same state produces the same shapes. |
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: A table of coordinates and colors for both adapters. 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: Pure state and Ownership boundaries. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Pure state affect in this exercise? Input cases: The same three circles.
- Variable: Execution context. Hypothesis to check: The same state produces the same shapes. Did the actual results agree with the hypothesis? Explain the evidence in terms of Ownership boundaries.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive Renderer Contract 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: Transferring Canvas Control · Next: Initialization Handshake