Working within the Managed Memory Model Design Pattern
As powerful as it is, the iPad is limited in resources, and the most critical of these resources is memory. To truly understand how to manage memory correctly in your application, you need to understand how the iOS memory works.
Understanding memory management
Whenever you (or a framework object) create an object using Objective-C, you allocate memory for the object. Although the iPad and the Mac both use what’s known as virtual memory, unlike the Mac, virtual memory in the iPad is limited to the actual amount of physical memory. So when it begins to run low on memory, the Memory Manager frees memory pages that contain read-only content (such as code); this way, all it has to do is load the “originals” back into memory when they’re needed. In contrast to what the Mac does, the iPad doesn’t temporarily store “changeable” memory (such as object data) to the disk to free space and then read the data back later when it’s needed. This state of affairs limits the amount of available memory.
So as you can see, when one object is done using memory, it’s critical that the memory be released for use by other objects.
If memory continues to be limited, the system may also send notifications to the running application, asking it to free additional memory. This is one of the critical events that all applications must respond to, and I explain this process in the section “Observing Low-Memory Warnings,” later in this chapter.
In Objective-C, ...