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Timeouts and the Event Loop

[!INFO]

In Gost-DOM, the event loop is not "running" in the background. Instead, client code must explicitly tell Gost-DOM to run enqueued tasks.

In a browser, the event loop is executed in the main thread of the browser; the thread that also handles user interaction, and updates the DOM accordingly. JavaScript code will not run concurrently with DOM updates.

When using Gost-DOM, you might manipulate the DOM. Either directly, or using the packages to simulate user input. As a consequence, delayed functions, e.g., setTimeout or setInterval callbacks don't execute as time pass. You have to explicitly advance time.

Caution

All interaction with a window must happen in the same goroutine.

Gost-DOM runs in simulated time.

Gost-DOM uses simulated time, allowing tests to fast-forward time. So verifying throttling behaviour with a 200ms delay does not need to wait 200 milliseconds. Client code advance time by calling these two functions.

  • Window.Clock().Advance(time.Duration) advances time for a certain amount of time, running all timeout and interval callbacks that should run in that period.
  • Window.Clock().RunAll() will run until all setTimeout callbacks are called. If a callback adds a new callback, they will be executed too.

Warning

Both Advance() and RunAll() will try to prevent an infinite loop caused by new callbacks continuously being registered. It expects that the number of registered callbacks decrease over time; and will panic if they don't. This will be triggered if a setTimeout callback continuously registers new timeouts. Likewise, RunAll() will effectively panic if there are any intervals registered (that are not cleaned up).^[1] ^[2]

Note

Simulated time only affects execution of setTimeout/setInterval callbacks. The return value of new Date() will return the system clock.

Waiting for async operations

If Gost-DOM client code calls, Click() on a button that triggers a fetch call, the data will be fetched in a new goroutine.

To wait for the code to complete, use Window.Clock().ProcessEvents(). This will wait for all promises to have settled. Alternately, Window.Clock().ProcessEventsWhile() will wait as long as a specific condition is true.

Warning

ProcessEvents()/ProcessEventsWhile() are new functions added as the behaviour was necessary to write predictable test code when using fetch. The entire scenario exposed a problem not properly anticipated originally, and as a consequence, the entire clock/event loop may require some changes.