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Sometimes we need to get some data before do something else, each one can be from different sources, like 2 different web services.

Using Retrofit 1.9 and retrolambda

• NewOrder is the object that we need to create from 2 data sources, both getted from an Observable<Address>.

class NewOrder {
	
	...

	public NewOrder(Address dropAddress, Address pickAddress){
		...
	}

}

The Retrofit service that consumes the object and creates the final Observable:

@POST("url")
Observable<Order> create(@Body NewOrder id);

• We will have 2 Observables to wait for result: pickObservable and dropObservable. (can be more! just add each Observable to combineLatest`)

• Address.get(...) is a function that will return an observable with the address for that LatLng.

• Use Observable.combineLatest to combine both pickObservable and dropObservable and wait for it´s results and will end in a new Observable<NewOrder>.

• Use flatMap to apply

public static Observable<Order> create(final OrderData orderData) {
   	// Prerequisites
    Observable<Address> pickObservable = Address.get(new LatLng(1.2. -1,2));
    Observable<Address> dropObservable = Address.get(new LatLng(1.2. -1,2);

    // Create a new Observer that will end until both observables get data.
    Observable<NewOrder> newOrderObservable = 
    	Observable.combineLatest(pickObservable, dropObservable
            (pickAddress, dropAddress,) ->
                    new NewOrder(pickAddress, dropAddress)
    );

    // create new observable from last observable.
    return newOrderObservable.flatMap(order -> create(order));
}

private static Observable<Order> create(OrderService.NewOrder newOrder) {
    return SERVICE.createOrder(newOrder);
}

Thanks to lukaciko. Check this question on stackoverflow.