Guarded Call Runtime Toggle

Run the Guarded Call Runtime Toggle MicroSim Fullscreen
Edit in the p5.js Editor

About This MicroSim

A teacher may paste an instrumented p5.js MicroSim into the p5.js web editor, where none of the xAPI runtime files exist. A guarded call is an instrumentation call wrapped in a test that the runtime is present, so the MicroSim runs unchanged, and silently, without it. The idiom from Chapter 17 is: the instance variable lrs starts as null, is created only inside if (window.LRSSim), and every use is written if (lrs).

This MicroSim shows a small bouncing-ball sketch beside its instrumentation code. Two switches set up the experiment:

• Runtime loaded on means the runtime files are present; off simulates the p5.js editor.
• Call style chooses the unguarded code (lrs = LRSSim.create(...) in setup() and speedEvidence.input(speed) in the slider handler) or the guarded code.

Changing either switch reloads the sketch. Move slider fires one slider input. The line that just ran is highlighted, a status strip reads "Runs" or shows the error, and a log line says whether a statement was recorded. With the runtime off, the unguarded sketch stops at ReferenceError: LRSSim is not defined in setup(), so draw() never starts; a slider move then fails a second time because speedEvidence is still null. The guarded sketch keeps running and records nothing. A grid fills in each outcome as you try it.

The runtime here is simulated inside the sketch, so this page itself runs in the p5.js editor.

Learning objective: The learner will distinguish a guarded from an unguarded instrumentation call by observing what each does when the runtime is present and when it is absent.

Bloom's taxonomy level: Analyze (verb: distinguish)

How to Use

1. With the runtime loaded and the guarded style, press Move slider. One interacted statement is recorded.
2. Switch to the unguarded style and press Move slider again. Nothing changes: with the runtime present, both styles behave the same.
3. Uncheck Runtime loaded. The unguarded sketch stops in setup(). Press Move slider and read the second error.
4. Switch back to the guarded style. The ball runs again and slider moves record nothing.
5. Compare the highlighted lines and fill in all four cells of the outcome grid.

Iframe Embed Code

You can add this MicroSim to any web page by adding this to your HTML:

<iframe src="https://dmccreary.github.io/microsims/sims/guarded-call-runtime-toggle/main.html"
        height="722px"
        width="100%"
        scrolling="no"></iframe>

Lesson Plan

Audience

Teachers, instructional designers, learning-technology developers and learning-analytics practitioners (college undergraduate and professional development).

Duration

10-15 minutes

Prerequisites

• The LRS runtime script and handles such as lrs.slider() (Chapter 17)
• Basic p5.js structure: setup(), draw() and control callbacks

Activities

1. Fill the grid (5 min): Try all four combinations of runtime and call style and press Move slider in each. Record the status and the statement count.
2. Explain the difference (4 min): For the one failing combination, name the exact line that throws and explain why draw() never starts.
3. Guard a new call (5 min): Write the guarded version of a Reset-button press, resetEvidence.press('reset'), and of a call to LRS.conceptId(353), which the chapter says must also not run at the top level of a sketch.

Assessment

• The learner predicts the outcome of each of the four combinations before trying it.
• The learner identifies the line that throws in the unguarded sketch and names the error.
• The learner rewrites an unguarded instrumentation call so that it runs silently without the runtime.

References

1. ReferenceError - MDN. The error thrown when code names a variable that does not exist.
2. Optional chaining (?.) - MDN. A related JavaScript idiom for skipping calls on a null object.
3. p5.js Web Editor - p5.js. The environment the unguarded sketch fails in.
4. p5.js createRadio() reference - p5.js. The control used for the call style.