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Good ways to test or check the compatibility with Lion is being asked in a different question: What tools are people using to check for Lion compatible software?

What, generally, would make an app—that works with Snow Leopard—incompatibile with Lion? Could I have an educated guess whether app works / doesn't work / works partially with Lion based on some features?

What does, for example, all the apps reported "Not working" or "Has some problems" on RoaringApps Compatibility table have in common?

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up vote 3 down vote accepted

In general all programs call on the operating system to do things. These are the API - application programming interface.

Lion changes some API, adds new ones, and deletes old ones. The biggest problem is when a program needs an API that is now gone. The second issue is when things change. Lastly, when a new feature like Mission Control is introduced, perhaps the older program isn't expecting it's windows to be moved around in that manner.

The assumptions made by a developer can turn out to no longer be true which will cause little or big glitches.

Also - programs that don't use API but just react to things as they are can break when those things move. For example - if a program assumed ~/Library is visible - then it would clearly break when running on a clean version of Lion.

Device drivers change - that can cause errors.

Lastly, entire compatibility layers like Rosetta are dropped.

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Wouldn't apple build their OS in mind that Mission Control will move outdated apps aside? –  Odinulf Jul 20 '11 at 19:54
    
I think Apple demonstrate a pretty strident - we're moving forward attitude. I love it, personally. I don't know of any 10.6 apps that stuck closely to the 10.6 API and have any problems whatsoever with Lion. Apps that reinvented the wheel may not fare so well. It's not a bad thing - I like mac developers having more rope - just don't expect everything to be iOS level curated. Private API are much easier to use on the mac side so they get used. –  bmike Jul 20 '11 at 20:15

My educated guess would be:

1) Any apps that deal with operating system files

2) Any Power PC apps

These guesses are just based upon the information given in the app profile on roaring apps and about what apple has told me.

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