SPI communication

I’m thinking of using Mbeds SPI communication between 2 devices, where the master would continuously send some random (meaningless) data and the slave would reply. But in some cases, the slave wont have the data I want it to reply ready for another second second or so. Should I in that case just do something like slave.reply(some other meaningless data) or should I just wait until I have it and then say slave.reply(data)?

maybe you should have an extra pin from slave that indicates slave is ready, so the master knows when it can or cannot do something with slave.

I’d prefer to do this software wise. Based on your reply I’d assume I’m supposed to immediately reply to the master?