· Objective-C is a superset of the C programming language, the #import statement is a minor refinement upon C’s #include statement.
· The #include statement is very simple; it copies everything it finds in the included file into your code during compilation. This can sometimes cause significant problems. For example, imagine you have two header files: SomeFileA.h and SomeFileB.h;
· SomeFileA.h includes SomeFileB.h, and SomeFileB.h includes SomeFileA.h. This creates a loop, and can confuse the compiler.
· To deal with this, C programmers have to write guards against this type of event from occurring. When using #import, you don’t need to worry about this issue or write header guards to avoid it. However, #import is still just a glorified copy-and-paste action, causing slow compilation time among a host of other smaller but still very dangerous issues (such as an included file overriding something you have declared elsewhere in your own code.)
· Modules are an attempt to get around this. They are no longer a copy-and-paste into source code, but a serialized representation of the included files that can be imported into your source code only when and where they’re needed. By using modules, code will generally compile faster, and be safer than using either #include or #import.
· #import <SomeLibrary/SomeFile.h>
· To import this library as a module, the code would be changed to:
· This has the added bonus of Xcode linking the SomeLibrary framework into the project automatically. Modules also allow you to only include the components you really need into your project. For example, if you want to use the AwesomeObject component in the AwesomeLibrary framework, normally you would have to import everything just to use the one piece. However, using modules, you can just import the specific object you want to use:
· For all new projects made in Xcode 5, modules are enabled by default. If you want to use modules in older projects (and you really should) they will have to be enabled in the project’s build settings. Once you do that, you can use both #import and @import statements in your code together without any concern.