core/dev.fritz2.core/Tag/whenever

whenever

open fun <T> T.whenever(condition: Flow<Boolean>): Flow<T?>

This extension method takes a boolean Flow that controls the forwarding of the initial value: If it is true the value will be passed further on the result flow, if it is false a null will appear instead.

This is especially useful for DOM node attributes, that should only appear if a certain condition is true.

Take the aria-controls attribute as example. This should only be set, if there is an area active / visible to control. Within a dynamic component - like some disclosure based one - the latter is only shown, if a state-flow is true:

// `open`: Flow<Boolean>
button.attr("aria-controls", "panelId".whenever(open))
//                                     ^^^^^^^^^^^^^^
//                                     if open == true -> result flow provides "panelId" String
//                                     if open == false -> result flow provides `null` -> whole attribute is removed
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Parameters

condition

the boolean flow that decides whether to forward T or null

open fun <T> Flow<T>.whenever(condition: Flow<Boolean>): Flow<T?>

This extension method takes a boolean Flow that controls the forwarding of an initial flow: If it is true the current value will be passed further on the result flow, if it is false a null will appear instead.

See also