Salt the cucumber slices and leave for 30 minutes in a colander.
Rinse, drain and pat the cucumber slices dry with kitchen paper towels.
Heat the oil in a frying pan.
Add the cucumber slices, salt and vinegar and cook for 4 minutes.
Add the pineapple and cook for 2 minutes longer.
Mix together the water, flour, sugar and soy sauce in a small bowl, stirring well to dissolve the flour.
Pour into the frying pan and cook, stirring constantly, for a further 5 minutes or until the liquid has thickened.