Revert to default option doesn't work in my custom visual

I have developed a simple custom visual following the documentation available. I have created few format options in the capabilities.ts file and have assigned values to them in visual.ts using variables. I want to know how to get the "Revert to default" button to work which is present in the format pane. Note: I haven't used settings.ts. Here is the sample of my code.

Capabilities.ts

----------------

"objects": {
"kpiName": {
"displayName": "KPI Name",
"properties": {
"name": {
"displayName": "Name",
"type":{ "text": true}
},
"color": {
"displayName": "Color",
"type": {
"fill": {
"solid": {
"color": true
}
}
}
},
"fontSize":{
"displayName": "Font size",
"type": {"formatting": {"fontSize": true}}
}
}
}

}

Visual.ts:

---------

module powerbi.extensibility.visual {
"use strict";
export class Visual implements IVisual {
private kpiName: string;
private kpiColor: string;
private kpiTextSize: string;
private kpiFont: string;

constructor(options: VisualConstructorOptions) {

this.kpiName = "KPI Name";
this.kpiColor = "#444444";
this.kpiTextSize = "20";

}

 public update(options: VisualUpdateOptions) {

if (options.dataViews[0].metadata.objects)
{
if (options.dataViews[0].metadata.objects["kpiName"] && options.dataViews[0].metadata.objects["kpiName"]["name"]) {this.kpiName = options.dataViews[0].metadata.objects["kpiName"]["name"].toString();}
if (options.dataViews[0].metadata.objects["kpiName"] && options.dataViews[0].metadata.objects["kpiName"]["color"]) {this.kpiColor = options.dataViews[0].metadata.objects["kpiName"]["color"]["solid"]["color"].toString();}
if (options.dataViews[0].metadata.objects["kpiName"] && options.dataViews[0].metadata.objects["kpiName"]["fontSize"]) {this.kpiTextSize = options.dataViews[0].metadata.objects["kpiName"]["fontSize"].toString();}
}

}

public enumerateObjectInstances(options: EnumerateVisualObjectInstancesOptions): VisualObjectInstanceEnumeration {
let objectName = options.objectName;
let objectEnumeration: VisualObjectInstance[] = [];

switch (objectName) {
case 'kpiName':
objectEnumeration.push({
objectName: objectName,
properties: {
name: this.kpiName,
color: this.kpiColor,
fontSize: this.kpiTextSize,
},
selector: null
});
break;

};

return objectEnumeration;
}