Question

I’m getting started with Objective C and Cocoa on Mac and i would like to develop a app that will have a trial period of 30 days for example like most of the apps have.
How can this be implemented on Mac OSX? Do i store somewhere the installation date and then on each run i check for that? Or what is the general way of achieving this?

Answer 1

That is certainly one way to check the expiry of a trial, but I’ve found that implementing time based trials can be troublesome when dealing with slightly cleverer users.

If you use a time trial (i.e. 30 days) how do you check the time? You can store the time that the app was installed, and you can read the current time to work out the difference, but how can you guarantee the time(s) you are reading is correct. The user may have edited one or both of these values. (By changing your stored value if it’s not encrypted or by changing the system time).

You could use the Internet to verify the time, but what would you do if the user isn’t connected to the Internet?

I’m sure there will be more sophisticated ways of checking how long the user has had the program installed, but I’m afraid I can’t enlighten you to any.

I’d consider one of the alternatives methods of providing a trial:

• Number of times the app has been started – (take into account that your app may crash, or the user may keep the app open). You may wish to count instead, the number of days on which the app has been opened and allow the user to open the app on 15 different days.
• Restricted Content – Make a separate target for your app with less content. This is a safer way to ensure that the user can’t work around your restrictions, but it’s also more work for them to install a separate version. Consider that you may lose some sales.