Exchange by wire is time limit. If you change one variable from Arduino it change to App in a few hundreds milliseconds. Transferring back executed the same milliseconds.
What will happen if the variable will be changed a same time on both side?
For previous example with button and slider. What will happen if press the button and move slider at same time? What event be transfered to the Arduino first?

That you can program you out off on the arduino.
So don't see a problem.
Its better if you can set a value, like from a config page or anything else in you program.

so badly - the app would be much better if it was bidirectional.
I didn't though it was unidirectional - wouldn't have payed for it, if I knew this in first place.
But my fault - I was able to test that - but I never get in mind that this project is only half way done ;-)

:-) Thats way i don't just by a program anymore. But lets see if thay want to make changes to this fine program, as i see it only a things left, to modify, then it vill be a grate program, to use on ESP8266 units.

Yes, I don't understand why it's possible to set color of a LED but not the value of a slider (and other controls) Anyway, already suggested here.

Everything should be bidirectional, IMO.

Hi
Lets say you have a system with at on / off key. 
Then you have a slider to set some speed on a motor.
When you turn off the button all stop.
When you turn it all on, you want to have the slider (speed of a motor) to start at 0.
And you newer know what position the slider was left in, when you turned system off.

Everything should be bidirectional, IMO.

You can not set any values to slider from sketch. Data is transmitted from app to Arduino only. But initial value is transmitted from Arduino to app when start connection.

How do you update a LED then? Way not make et possible to set a slider pos, from arduino then.

I have a slider, and i what to contol that slider by program.

I have a on/off button, i when i set it on i like to set a slider to 0 like this RemoteXY.slider_cutter = 0;