What is the significance of #pragma marks? Why do we need #pragma marks?

问题

What is the purpose of #pragma marks in Xcode? Does their location in .m files matter? Should some #pragma come before all others?

Do they have to be present?
Can new marks be added? Why would they be? What causes it?
Is there any harm in having a mark removed? Would one ever want to?

回答1:

#pragma mark directives show up in Xcode in the menus for direct access to methods. They have no impact on the program at all.

For example, using it with Xcode 4 will make those items appear directly in the Jump Bar.

There is a special pragma mark - which creates a line.

回答2:

#pragma mark is used to tag the group of methods so you may easily find and detect methods from the Jump Bar. It may help you when your code files reach about 1000 lines and you want to find methods quickly through the category from Jump box.

In a long program it becomes difficult to remember and find a method name. So pragma mark allows you to categorize methods according to the work they do. For example, you tagged some tag for Table View Protocol Methods, AlertView Methods, Init Methods, Declaration etc.

#pragma mark is the facility for XCode but it has no impact on your code. It merely helps to make it easier to find methods while coding.

回答3:

In simple word we can say that #pragma mark - is used for categorizing methods, so you can find your methods easily. It is very useful for long projects.

回答4:

Just to add the information I was looking for: pragma mark is Xcode specific, so if you deal with a C++ project that you open in different IDEs, it does not have any effect there. In Qt Creator, for example, it does not add categories for methods, nor generate any warnings/errors.

EDIT

#pragma is a preprocessor directive which comes from C programming language. Its purpose is to specify implementation-dependent information to the compiler - that is, each compiler might choose to interpret this directive as it wants. That said, it is rather considered an extension which does not change/influence the code itself. So compilers might as well ignore it.

Xcode is an IDE which takes advantage of #pragma and uses it in its own specific way. The point is, #pragma is not Xcode and even Objective-C specific.

回答5:

When we have a big/lengthy class say more than couple 100 lines of code we can't see everything on Monitor screen, hence we can't see overview (also called document items) of our class. Sometime we want to see overview of our class; its all methods, constants, properties etc at a glance. You can press Ctrl+6 in XCode to see overview of your class. You'll get a pop-up kind of Window aka Jump Bar.

By default, this jump bar doesn't have any buckets/sections. It's just one long list. (Though we can just start typing when jump Bar appears and it will search among jump bar items). Here comes the need of pragma mark

If you want to create sections in your Jump Bar then you can use pragma marks with relevant description. Now refer snapshot attached in question. There 'View lifeCycle' and 'A section dedicated ..' are sections created by pragma marks