Conclusion

This is a handy app for beginners and everyone forever short on space and wanting to carry a versatile audio signal generator in the pocket, for quick deployment. The generator’s dual channel capability and waveform memory should allow less common measurements to be carried out in conjunction with an oscilloscope (whether an app or a ‘real’ one).

On the down side, the rectangle wave is shoddy and has severe limitations even for simple use, where it may create more problems than help you to resolve. Although supported by an impressive and sleek UI, the rectangle part is probably beaten for cleanness and risetime by just about any generator with more than five electronic parts.
All considered I would heartily recommend the Keuwlsoft Dual Channel Function Generator, bearing in mind its limitations compared to a benchtop instrument. Well deserved, the 4.5 star rating this app got on Play Store. Restricted to the sinewave and the audio range the app is great for educational use, quick testing of audio gear, and exploring acoustics. And of course to bargain for lower prices on your e-debris purchases.
 
 

Equipment used

Tektronix 2235 100-MHz oscilloscope; Earphones cable, salvaged; 2 x binding post to BNC adapter; Samsung J5 DUO (2016) smartphone; 2 x 150-ohm resistor.