Thanks to Jed Tiotuico that's taking advantage of the impressive PPSSignatureView for iOS and android-signaturepad for Android you can now start capturing signatures in your React Native apps.

react-native-signature-capture on GitHub

React Native library for capturing signature

User would sign on the app and when you press the save button it returns the base64 encoded png

var React = require("react-native");
var SignatureCapture = require("react-native-signature-capture");
var { AppRegistry } = React;

var NPMTest = React.createClass({
  _onSaveEvent: function (result) {
    //result.encoded - for the base64 encoded png
    //result.pathName - for the file path name
    console.log(result);
  },

  render: function () {
    return (
      <SignatureCapture
        rotateClockwise={true}
        square={true}
        onSaveEvent={this._onSaveEvent}
      />
    );
  },
});