@can.getWhatIChange(key)

The @can.getWhatIChange symbol is added to make sure can-debug can report all the observables whose values are set by a given observable's key.

This function iterates over the event handlers attached to a given key and collects the result of calling @can.getChangesDependencyRecord on each handler; this symbol allows the caller to tell what observables are being mutated by the event handler when it is executed.

In the following example a can-simple-map instance named me is created and when its age property changes, the value of a can-simple-observable instance is set. The event handler that causes the mutation is then decatorated with @can.getChangesDependencyRecord to register the mutation dependency.

var obs = new SimpleObservable("a");
var me = new SimpleMap({ age: 30 });
var canReflect = require("can-reflect");

var onAgeChange = function onAgeChange() {
canReflect.setValue(obs, "b");
};

onAgeChange[canSymbol.for("can.getChangesDependencyRecord")] = function() {
return {
    valueDependencies: new Set([ obs ]);
}
};

canReflect.onKeyValue(me, "age", onAgeChange);
me[canSymbol.for("can.getWhatIChange")]("age");

The dependency records collected from the event handlers are divided into two categories:

• mutate: Handlers in the mutate/domUI queues
• derive: Handlers in the notify queue

Since event handlers are added by default to the "mutate" queue, calling @can.getWhatIChange on the me instance returns an object with a mutate property and the valueDependencies Set registered on the onAgeChange handler.

Please check out the can-reflect-dependencies docs to learn more about how this symbol is used to keep track of custom observable dependencies.