Trailing State Changes¶
By default, Quickstrom only listens for a single DOM state change after each action it performs. This behavior can be overriden, so that it waits for a configurable number of trailing state changes.
The term trailing refers primarily to asynchronous changes that occur as result of an action. For example:
the user agent clicks a button
a loading indicator is shown immediately
an HTTP request is performed
later, the result of the request is printed
In this example, the loading indicator being shown is the first state change. The result of the HTTP request being shown is the trailing state change.
Some systems change the state of the DOM without any dependence on user action, and do so infinitely. For instance, a clock (hopefully) keeps ticking, no matter what the user is up to. It doesn’t make much sense to think of a clock’s behavior as “trailing”. However, it’s still possible to test a finite subsequence of such a behavior using Quickstrom and trailing state changes.
The command-line options available are:
--max-trailing-state-changes=NUMBER: how many trailing state changes Quickstrom will try to observe.
--trailing-state-change-timeout=MILLISECONDS: maximum time that it will wait for a change of DOM state. The timeout doubles for every subsequent trailing state change that is awaited.
Let’s say we set the following options:
Then the DOM state changes would be observed at the following times:
initial state, immediately
first trailing state, at most 200ms after #1
second trailing state, at most 400ms after #2
third trailing state, at most 800ms after #3
It’s at most, because Quickstrom observes the DOM and can pick up state changes as they happen.