Github in Disguise• Forking (Hitting “Save as...”)• Versioning (Not yet user exposed)• Runtime Versioning • All runtime changes are versioned to protect against API changes
Graphics• Use Processing.js’ API• Works in all browsers with <canvas>• Import it wholesale, ignore the “Processing language” bits
Real-Time Execution• Code is constantly checked for changes• If a change is detected, code is re-evaluated• Code is not run using a simple “eval”, code is dynamically injected into the current runtime.
How Code is Injected• JSHint (run in worker thread) • Optionally run BabyHint • If Error, stop execution• Cache Images• Run Code in Worker Thread, Looking for long-running code • If error, stop execution• Execute Code • If first time running code, just eval • If later: • Eval code and extract current state • Inject extracted values into runtime • Maintain closures with functions!
Error Messages• All code is run through JSHint• And through an extra layer of error handling (called “BabyHint”)
JSHint• Validation!• Also: Gives us a list of all global variables used in the program• Compare against a master list of properties in Processing.js• Everything else is user-defined!
BabyHint• Handles common mistakes: • Spelling and case: “strokeWeight” vs. “strokeweight” • Gives sane errors about missing semicolons • Provide hints about correct function arguments strokeWeight(); (gives an error asking for more args) • Check for function declaration mistakes and possible spacing mistakes (“vartest”)
Worker Threads• Run JavaScript code asynchronously in the background of a page• Available in Chrome, Firefox, Safari, and IE 10 (Need to make sure it works in IE 9!)• Works by doing a string-only postMessage to the worker and waiting for a response
With Statements• var obj = { name: “John”, city: “Boston” }; with (obj) { name += “ Resig”; city = “Brooklyn”; }• obj.name === “John Resig” obj.city === “Brooklyn”
With Statements• var obj = { name: “John”, city: “Boston” }; with (obj) { var city = “Brooklyn”; var job = “Khan Academy”; }• obj.city === “Brooklyn” obj.job === undefined
With Statements• var obj = { name: “John”, city: “Boston” }; obj.job = undefined; with (obj) { var city = “Brooklyn”; var job = “Khan Academy”; }• obj.city === “Brooklyn” obj.job === “Khan Academy”
Example• var x = 5, y = 1; var draw = function() { x += y; };• JSHint: PASS• Long Run Detection: PASS• First Run: True • Code is evaluated • lastGrab = { x: “5”, y: “1”, draw: “function() { x += y; }” }
Example (cont.)• var x = 50, y = 1; var draw = function() { x += y; };• JSHint: PASS• Long Run Detection: PASS• First Run: False • Code is evaluated • grabAll = { x: “50”, y: “1”, draw: “function() { x += y; }” } • Compare with lastGrab: grabAll.x !== lastGrab.x • Eval: “var x = 50;”
Example (cont.)• var x = 50, y = 1; var draw = function() { x += y * 2; };• JSHint: PASS• Long Run Detection: PASS• First Run: False • Code is evaluated • grabAll = { x: “50”, y: “1”, draw: “function() { x += y * 2; }” } • Compare with lastGrab: grabAll.draw !== lastGrab.draw • Eval: “var draw = function() { x += y * 2; };”
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Takeshi Watanabe
3 months ago
Khan Academy Computer Science
