 value incuding once
when observe is called. f is only ever called when the value changes afterwards.</p><p>If the method shouldn't be called directly, but only when it first changes its value, use <a href="ReactiveLibraryUsage$SignalExtensions.html" class="extype" name="react.core.ReactiveLibraryUsage.SignalExtensions">react.core.ReactiveLibraryUsage#SignalExtensions#</a></p><h4> Usage as Event </h4><p>When used on an event, observe is called each time the event triggers. Thus it's not called directly afte