I have bought this MIDI device and It works.
I can control opacity and volume because the Out port is float type.
But how can I control the MediaPlayer? for example, by using the PlayItem node?
There are 2 ways:
One is to connect the MIDI Messages output from the MIDI Controller node to the Messages input of MediaPlayer. Then go to the Media Player, right-click on the item in the playlist, and select “Map item…” - the item becomes yellow signaling that it is ready to be mapped. Now, by pressing a button on the MIDI controller you will map the item. The name of the mapping will be shown in the top left corner of the item.
Another way is to use the Threshold Float node to convert the float value to actions when the float value goes from below 0.5 to above 0.5 and back. Add the PlayItem node, configure it, and connect it to the Threshold Float node.
(MidiController → ThresholdFloat → PlayItem)