Asynchronous update OOB field value with jsLink

Question

In an edit form, I want to generate/update the filename of a document based on another field in the form.

How to set the filename value after the other field changed ? At this point, I don't have a reference to the FileLeafRef field.

Please note that I want to preserve native rendering, so my custom render method only adds validator, before letting the original render do the job.

Here's what I tried (used a custom validator to hook the change):

(function () {
    var registerDocumentTypeToFilename = function () {
        var overrideCtx = {};
        overrideCtx.Templates = {};

        overrideCtx.Templates.Fields = {};

        var myCustomValidator= function (originalFormCtx,ctx) {
            myCustomValidator.prototype.Validate = function (value) {
                console.log(value, originalFormCtx, ctx);

                // This line does nothing (neither set the field nor error)
                originalFormCtx.updateControlValue("FileLeafRef", value);


                return new SPClientForms.ClientValidation.ValidationResult(false, null);
            };
        };

        var myCustom= function (ctx) {
            var formCtx = SPClientTemplates.Utility.GetFormContextForCurrentField(ctx);
            var fieldValidators = new SPClientForms.ClientValidation.ValidatorSet();
            fieldValidators.RegisterValidator(new myCustomValidator(formCtx,ctx));
            formCtx.registerClientValidator(formCtx.fieldName, fieldValidators);

            return SPFieldLookup_Edit(ctx);
        }

        overrideCtx.Templates.Fields = {
            "myCustomLookupField": { 'NewForm': myCustom, 'EditForm': myCustom}
        };

        SPClientTemplates.TemplateManager.RegisterTemplateOverrides(overrideCtx);
    };

    /* MDS and non MDS plumbing*/
    RegisterModuleInit("corp/myCustom.js", registerDocumentTypeToFilename); // CSR-override for MDS enabled site
    registerDocumentTypeToFilename(); //CSR-override for MDS disabled site (because we need to call the entry point function in this case whereas it is not needed for anonymous functions)
})();

Answer 1

As a workaround, I can dynamically find the input field:

var myCustomValidator= function (originalFormCtx,ctx) {
    myCustomValidator.prototype.Validate = function (value) {

        var fileLeafRefInput = document.querySelector("#WebPart" + ctx.FormUniqueId + " input[id^='FileLeafRef']");

        fileLeafRefInput.value = "Mynewvalue";

        return new SPClientForms.ClientValidation.ValidationResult(false, null);
    };
};

This is working, but I feel I'm missing something.