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OffscreenCanvas · 08 Resize Messages
Series contents · Technology map
Stage: Synchronizing Size and Input · Editable example · Storage ID: offscreen-canvas-step-08
Goals and learning outcomes
- Learning outcome: explain the role of Measurement. Input cases: Changes to width and height.
- Deliverable: A table of backing-store sizes and shape styles. Explain the rationale for handling Cost of changes, linking it to the code.
Prerequisites
Prepare the experiment results for Equivalence and Validation scope from Unit 7, “Basic Rendering Validation”.
Key concepts and explanations
Measurement
A Worker cannot read DOM dimensions. A ResizeObserver on the main thread sends the observed values.
Cost of changes
Changing Canvas dimensions may reset its state. Reapply the transform and styles after setting the dimensions.
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-8.js in a separate practice project, send cssWidth·cssHeight·dpr from ResizeObserver and reset the Worker renderer accordingly. Refer to the existing Unit 1·14·28 examples only as starting points.
- Input conditions: Changes to width and height. Baseline: 400×240. Run under the baseline conditions, then record observations and errors in notes/day-8.md.
- Variable: Size; changed value: 240×400. Run again through the same input path. Deliverable to save: A table of backing-store sizes and shape styles. Hypothesis to check: Styles and coordinate rules are preserved after the aspect ratio changes. Compare it with the actual results, and record whether they agree and why they differ.
Variables to observe
| Variable | Baseline | Change | Expected change / hypothesis |
|---|---|---|---|
| Size | 400×240 | 240×400 | Styles and coordinate rules are preserved after the aspect ratio changes. |
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 backing-store sizes and shape styles. 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: Measurement and Cost of changes. For unexpected items, at least 1 cause or reason they remain unresolved is recorded.
Self-check
- Which processing stage does Measurement affect in this exercise? Input cases: Changes to width and height.
- Variable: Size. Hypothesis to check: Styles and coordinate rules are preserved after the aspect ratio changes. Did the actual results agree with the hypothesis? Explain the evidence in terms of Cost of changes.
Official references
Linked example and execution scope
Example source · Runtime environments and limits
An interactive Resize Messages 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: Basic Rendering Validation · Next: Resize Sequence Numbers