Using listeners and signals with Nasal: Difference between revisions

The first argument is a property node object (<tt>props.Node()</tt> hash) or a property path. Because the node hash depends on the props.nas module being loaded, <tt>setlistener()</tt> calls need to be deferred when used in an [[$FG_ROOT]]/Nasal/*.nas file, usually by calling them in a <tt>settimer(func {}, 0)</tt> construction. To avoid that, one can use the raw <tt>_setlistener()</tt> function directly, for which <tt>setlistener()</tt> is a wrapper. The raw function does only accept node paths (e.g. "/sim/menubar/visibility"), but not props.Node() objects.

The second argument is a function object (not a function call!). The function you pass here will be called with the target property node as its sole argument as soon as someone writes to the property.

The third argument is optional. If it is non-null, then it causes the listener to be called initially. This is useful to let the callback function pick up the node value at startup.

The fourth argument is optional, and defaults to 1. This means that the callback function will be executed whenever the property is written to, independent of the value.