BETA!!!!

I can give you sample code for the upcoming onChange handler.
It's already built in the version you already have. Feel free to test it: 

Example 1

If the lookup named "patient_id" has changed and if user has selected a lookup item, then show the field named "groups"

// if user selected a patient, show groups field
new AppGiniField("patient_id").onChange(function (value) {
    new AppGiniField("groups").showIf(value != null);
});


Example 2

// example 2 - (long version of example 1)
// if user selected a patient, show groups field
new AppGiniField("patient_id").onChange(function (value) {
    if (value != null) {
        new AppGiniField("groups").show();
    } else {
        new AppGiniField("groups").hide();
    }
});


Example 3

// example 3 - on change of field "groups" change visibility of other fields
new AppGiniField("groups").onChange(updateFieldsVisibility);

function updateFieldsVisibility(value) {
   // hide/show fields here
}


Pro-Tipp:

You can also use AppGiniFields (plural) instead of AppGiniField (singular) to react to changes on multiple fields, for example new AppGiniFields(["a", "b", "c"]).onChange(function() { /* ... */ } );

// example 4
new AppGiniFields(["patient_id", "groups"]).onChange(function (field, value) {
    console.log("Field \"" + field + "\" has been changed:");
    console.log(value);
});


Attention: Lookup fields return id + text of selected item
Usually, the variable value is a simple type like string or integer. But if you react on lookup fields, value-variable will be an object with properties "id" and "text" (both lowercase). You can read the values from value.id and value.text

// example 5 -  lookup field "patient id"
new AppGiniFields(["patient_id"]).onChange(function (field, value) {

    console.log("Field \"" + field + "\" has been changed:");
    
    if (value == null) {
        console.log("Nothing selected");
    } else {
        console.log("Id=" + value.id);
        console.log("Text=" + value.text);
    }
});