Wrap OData v2 model read into a Promise

ListReportExt.controller.js

sap.ui.define(
	[], function(){
	"use strict";
	
	return sap.ui.controller("todo.app.todoapp1.ext.controller.ListReportExt", {
		
		promiseToRead: function(sPath){
			var oModel = this.oModel;
			
			return new Promise(function(resolveCallback, rejectCallback){
				oModel.read(sPath, {
					success: resolveCallback,
					error: rejectCallback
				});	
			});
		},
		
		onClickActionTodo1: function (oEvent) {
			this.oModel = this.getView().getModel();
			
			var oPromisedToRead1 = this.promiseToRead("/Todo"); 	// EntitySet name
			var oPromisedToRead2 = this.promiseToRead("/Subtask");	// EnttiySet name
			
			Promise.all([
				oPromisedToRead1,
				oPromisedToRead2
			]).then(function(aData){
				console.log("all promises are done");
				/**
				 * aData is an array that contains result of both oModel.read() calls
				 * aData[0] -> result of first read() call
				 * aData[1] -> result of second read() call
				 */
				console.log(aData);
			});
		}
	});
});

Notice the function promiseToRead which returns a new Promise every time it is called.

• when the ODATA read call is success, this promise is resolved (calling resolveCallback),
• when it fails, promise is rejected (calling rejectCallback)

To ensure that we do something only when BOTH the calls are successful, use Promise.all( [ ] ).then( ).
aData is an array which contains the response of both OData calls. Callback function on line 27 is called only when both the promises are resolved.