How do the Android Support Libraries work?

Android 4.0.x (ICS) has a lot of added features compared to lets say Android 2.3.x (Gingerbread). The compatibility libraries are there to bridge some of those changes that were added to ICS that -could be- supported by Gingerbread. "could be" being the key phrase here because there are a ton of changes made to ICS that would never work with Gingerbread and those, of course, will not get a compatibility library.

Fragments for example, which you brought up, are actually done slightly different in ICS than in the compatibility library because ICS has more features it can use. If you look at the code for ICS for the Fragments class, they are not the same as those in the compatibility library. Its a whole second set of code to make something "like" the Fragments in ICS be used in an older version like Gingerbread without the programmer noticing much difference.

That is the point of compatibility libraries and the reason why they aren't used to extensively patch Gingerbread to use all of the features in ICS (they just can't). The point of the compatibility libraries is to interface things available in newer versions of android such as ICS, done the ICS way into older versions such as GB, done the GB way.

As far as why they don't just keep the support library growing and leave the same base OS -- the answer to that is compatibility issues. If a user only has v4 and v12 is out, what happens? Android right now uses the Android API version of the OS as a basis for application compatibility and developers have an option to include support libraries (increasing file size of their app, but giving them newer features). Every application that uses support libraries, independently includes them (meaning 4 apps = 4x included).

The idea is, you may only download apps supported by the current API version of your OS (as far as Google Play is concerned) and you may choose to include support libraries to keep the look and feel of your app supported for older APIs, that do not yet have features that you elect to have available for those users on newer APIs. It's really a look and feel consideration more than anything else.

Hope that clears things up :)

The support library does not in fact have everything that is in the newer APIs. It does support parts of the Fragment API, but it does not yet support ActionBar. For that you need another library like ActionBar Sherlock.

Why are there two libraries?

Because part of the problem was Google only back ported some of the stuff, but my understanding is that, additionally, some of the new functionality can't be back ported due to core OS framework limitations and missing APIs deep in the core of the Android UI framework.

What has been said already it is true. Although there are some details missing. In fact, I had the chance to attend to a session in the last Google IO where they specially talked about that. It was actually surprising for me to get to know that support library doesn't host the code for all the posible API versions, rather it's an adaptation of the new features which they think are relevant enough to make them available to older platforms. So the way it works it is generally the following:

• Let's say we need to use the brand new (available from API 16) ConnectivityManager to keep track of network changes
• We include support libraries v4 and we use the class
• The way it works after is that the system will check our API version and will run built-in native code in case we are in API 16 or will run the code of the support library in any other case.

So it's acting like some kind of route gateway. Reason being is that it's generally more efficient (and performing) to use the last code optimized for the last OS and the last system improvements (ie.: hardware acceleration).

Nonetheless there are some elements that weren't translated to the compat library as they are in the native built-in code. For instance fragments weren't meant to be re-written as they are from API13 since they are a huge component that need to run in a wide range of different scenarios within system and devices with less capabilities.

Ultimately and because all this, it is recommended to use compats libraries which is known as a good practice, specially if you intend to make your apps / code available for older versions (which should be the ideal way)

As far as I understood, support libraries may work as alternative of built-in APIs, but they are not supposed to be, because they directly effect the size of the application.

For example, a support library is of 2MB, and to use its functionality, it takes all classes, resources, etc (of 2MB), so now classes.dex (Dalvik executable file of all classes being used in application) of my application also include that support library classes, and same for resources. So, if without support library my app size was 1MB, then now with support library the size is 2MB extra, which means 3MB total.

Now, suppose this support library feature is so common that on single device, if I have 10 apps, then at least 9 are using this same support library, so 9*2 = 18MB on my device is being used by the same support library, which is repeated in every application, which is bad, because for now 18MB might not be so much, but the space required can increase, if you have more applications using that support library.

Thus, the best option is to have that 2MB support library already in your OS for any number of apps, instead of having it for each application. So, support libraries are meant to be used when you really want some efficient features in your app to support older versions.

Another question arise here:

why not this support library is added to the OS as its own update, so that every app without size problems can access that functionality?

The answer is that there could be a lot of errors. Suppose some user doesn't have that update (support library) installed...

There is also the chance that as an update, it may not work as efficient as supposed to be, or may cause problems while integrating with the OS, as we already seen that each OS (windows, Linux, mac) comes with new versions, instead of just giving updates for life time for all new features.