Changes

That said, however, each operating system layer provides an increasing level of abstraction away from the complexity of working with the hardware. You should, therefore, always look for solutions to your programming goals in the frameworks located in the higher level OS layers before writing code that reaches down to the lower level layers. In general, the higher level of layer you program too, the less effort and fewer lines of code you will have to write to achieve your objective. And as any veteran programmer will tell you, the less code you have to write the less opportunity you have to introduce bugs.

Now that we have identified the various layers that comprise the iPhone OS we can now look in more detail at the services provided by each layer and the corresponding frameworks that make those services available to us as app developers. For details refer to the following chapters: