I am playing around with some of the new iOS 7 features and working with some of the Image Effects as discussed in the WWDC video "Implementing Engaging UI on iOS". For producing a blur effect within the source code for the session, UIImage was extended via a category which imports UIKit like so:
#import UIKit;
I think I saw something about this in another session video but I'm having trouble finding it. I'm looking for any background information on when to use this. Can it only be used with Apple frameworks? Are the benefits of using this compiler directive enough that I should go back and update old code?
It's a new feature called Modules or "semantic import". There's more info in the WWDC 2013 videos for Session 205 and 404. It's kind of a better implementation of the pre-compiled headers. You can use modules with any of the system frameworks in iOS 7 and Mavericks. Modules are a packaging together of the framework executable and its headers and are touted as being safer and more efficient than #import.
One of the big advantages of using #import is that you don't need to add the framework in the project settings, it's done automatically. That means that you can skip the step where you click the plus button and search for the framework (golden toolbox), then move it to the "Frameworks" group. It will save many developers from the cryptic "Linker error" messages.
You don't actually need to use the #import keyword. If you opt-in to using modules, all #import and #include directives are mapped to use #import automatically. That means that you don't have to change your source code (or the source code of libraries that you download from elsewhere). Supposedly using modules improves the build performance too, especially if you haven't been using PCHs well or if your project has many small source files.
Modules are pre-built for most Apple frameworks (UIKit, MapKit, GameKit, etc). You can use them with frameworks you create yourself: they are created automatically if you create a Swift framework in Xcode, and you can manually create a ".modulemap" file yourself for any Apple or 3rd-party library.
You can use code-completion to see the list of available frameworks:
Modules are enabled by default in new projects in Xcode 5. To enable them in an older project, go into your project build settings, search for "Modules" and set "Enable Modules" to "YES". The "Link Frameworks" should be "YES" too:
You have to be using Xcode 5 and the iOS 7 or Mavericks SDK, but you can still release for older OSs (say iOS 4.3 or whatever). Modules don't change how your code is built or any of the source code.
From the WWDC slides:
Imports complete semantic description of a framework
Doesn't need to parse the headers
Better way to import a framework’s interface
Loads binary representation
More flexible than precompiled headers
Immune to effects of local macro definitions (e.g. #define readonly 0x01)
Enabled for new projects by default
To explicitly use modules:
Replace #import <Cocoa/Cocoa.h> with #import Cocoa;
You can also import just one header with this notation:
#import iAd.ADBannerView;
The submodules autocomplete for you in Xcode.
Nice answer you can find in book Learning Cocoa with Objective-C (ISBN: 978-1-491-90139-7)
Modules are a new means of including and linking files and libraries into your projects. To understand how modules work and what benefits they have, it is important to look back into the history of Objective-C and the #import statement
Whenever you want to include a file for use, you will generally have some code that looks like this:
#import "someFile.h"
Or in the case of frameworks:
#import <SomeLibrary/SomeFile.h>
Because Objective-C is a superset of the C programming language, the #import state‐ ment 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 coimpiler. 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 serialised 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.
Returning to the previous example of importing a framework:
#import <SomeLibrary/SomeFile.h>
To import this library as a module, the code would be changed to:
#import SomeLibrary;
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:
#import AwesomeLibrary.AwesomeObject;
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.
#import Module(ObjC) or Semantic import
instead of usual module using
//as example
#include <Foundation/Foundation.h>
#import <UIKit/UIKit.h>
History:
[#include -> #import] -> [Precompiled Headers .pch] -> #import Module(ObjC); -> [import Module(Swift)]
It is a part of LLVM Modules
#import <module_name>; declaration says to compiler to load(instead of compile) a precompiled binary of module which decrease a building time. Previously compiler compiled dependency every time when runt into it but now it should be compiled beforehand and just loaded
//previously
run into dependency -> compile dependency
run into dependency -> compile dependency
//#import
compile dependency
run into dependency -> load compiled binary
run into dependency -> load compiled binary
[Modulemap] - bridge between module and headers
Xcode
Enable Modules(C and Objective-C)(CLANG_ENABLE_MODULES) - CLANG #include, #import directives are automatically converted to #import that brings all advantages. Modulemap allows to do it seamless because contains a map between headers and sub/modules
Pass -fmodules
#include, #import -> #import
Link Frameworks Automatically(CLANG_MODULES_AUTOLINK) - enables system modules auto linking. Requires activated CLANG_ENABLE_MODULES. Auto-linking allows to pass -framework <framework_name> based on #import, #import(Objective-C), import(Swift)
If NO - passes -fno-autolink flag
CLANG_ENABLE_MODULES == NO and CLANG_MODULES_AUTOLINK == NO
If you want to handle system(#import <UIKit/UIKit.h>) linking manually(instead of auto-linking) you have two variants:
Add dependency into General -> Frameworks and Libraries or Frameworks, Libraries, and Embedded Content
Build Settings -> Other Linker Flags(OTHER_LDFLAGS) -> -framework <module_name>
Next error will be thrown if:
Undefined symbol: _OBJC_CLASS_$_UIView
Undefined symbols for architecture x86_64:
"_OBJC_CLASS_$_UIView", referenced from:
objc-class-ref in ClassB.o
ld: symbol(s) not found for architecture x86_64
clang: error: linker command failed with exit code 1
CLANG_ENABLE_MODULES is disabled
CLANG_MODULES_AUTOLINK is disabled and no manual linking
Reverse engineering
otool -l <binary>
//-l print the load commands
//find LC_LINKER_OPTION
//cmd LC_LINKER_OPTION
It currently only works for the built in system frameworks. If you use #import like apple still do importing the UIKit framework in the app delegate it is replaced (if modules is on and its recognised as a system framework) and the compiler will remap it to be a module import and not an import of the header files anyway.
So leaving the #import will be just the same as its converted to a module import where possible anyway
It seems that since XCode 7.x a lot of warnings are coming out when enabling clang module with CLANG_ENABLE_MODULES
Take a look at Lots of warnings when building with Xcode 7 with 3rd party libraries
There is a few benefits of using modules. You can use it only with Apple's framework unless module map is created. #import is a bit similar to pre-compiling headers files when added to .pch file which is a way to tune app the compilation process. Additionally you do not have to add libraries in the old way, using #import is much faster and efficient in fact. If you still look for a nice reference I will highly recommend you reading this article.
Related
i'm trying to add the SDK of google admob in my game but when i try to build the project after adding the admob sdk, i keep getting the error "Duplicate interface definition for class GADInterstitial"
However, when I do the same in a fresh project, I don't run into any issues at all. I have checked all the imports and project settings, they are all the same.
What should I look for, to resolve such an issue ?
You have two interfaces with the same name. Use Xcode's find in project menu option to find instances of "GADInterstitial".
Hope it will be help you.
The Objective-C #import pre-processor statement specifically ignores duplicate header files, unlike the C/C++ #include pre-processor statement (where it's necessary to use guard macros or #pragma once).
So I think you are using #include "GADInterstitial.h", instead of #import "GADInterstitial.h", somewhere in your source tree.
Base on this question
Why don't iOS framework dependencies need to be explicitly linked to a static library
I read the selected answer and still don't understand so I made an example project
Test Project on Github
In the test project, I remove all framework from Link Binary With Libraries and File navigation for both main project and the static library (including Foundation.framework and UIKit.framework too), basically, both project link to 0 frameworks.
Questions are
In static library, it's including MapKit/MapKit.h without referencing the Mapkit.framework to the project, why is its still working?
In main project, I remove UIKit.framework and Foundation.framework from the project, why is it still working?
Since it's working for now, will there be any issue later?
Thank you for your comment.
P.S. By working, I mean I can run on the simulator and I can archive the main project without any error.
Edit 25/07/2014
I tried with the real app that I'm working on, it's the same.
I highlight Foundation, UIKit, CoreData and 10 another frameworks in File Navigation, well, all of them.
Uncheck the target in Utilities Panel --> Target Membership
Build : Pass, Run : Pass
Every functionality of my app is still working as expected. I don't get this.
Check your project build settings. Underneath LLVM 5.1 — Language — Modules you should see the option 'Link Frameworks Automatically'. In your case it sounds like it's set to 'YES', the default.
In that case, instead of producing an error when you reference a class that the compiler doesn't know, it'll figure out which Framework contains that class and link it. In your code it'll be MKMapView or one of the other MapKit classes that triggers the linkage.
EDIT: from the relevant 'What's New?' document:
Auto Linking is enabled for frameworks imported by code modules. When
a source file includes a header from a framework that supports
modules, the compiler generates extra information in the object file
to automatically link in that framework. The result is that, in most
cases, you will not need to specify a separate list of the frameworks
to link with your target when you use a framework API that supports
modules.
Another way of looking at it is that the compiler is smart enough to mutate #import to #import when the framework has been built appropriately. All system frameworks have been.
To elaborate #Tommy's answer, a framework that supports modules satisfies the following 2 conditions:
Under Build Settings > Packaging
Define Modules is set to YES
Module Map File exists.
So, if you're certain that the framework you're using in your code modularizes like that, you can choose to not explicitly add it in the link phase as it will be automatically added as long as in the project file, under Apple Clang - Language - Modules, The option Link Frameworks Automatically is set to YES.
The new syntax in Objective-C in Xcode 5 is #importto import a framework.
See question for details. Advantage is that you don't have to include the framework in project avoiding linker errors, you don't need to add quotes and .h to just the name of the framework, it is faster for precompiled headers, and you have a namespace that protects you from accidentally renaming a symbol. All nice additions.
My question is, for your own files, for example MyFancyViewController.h, do you continue to use #import or does #import completely replace it? Also, can I define my own modules easily? Just looks more messy having both syntaxes in the same file.
for you including of your project files do you continue to use #import or does #import completely replace it?
#import, so far, is for Apple frameworks only, so at the time of writing you still have to use #import for anything else.
The good news is that, if you opt-in, any #import will be implicitly replaced for you by the compiler, so you don't need to convert your previous code to benefit from modules.
Also, can I define my own modules easily?
Yes and no.
Yes, it's easy, but...
...no you cannot, since this feature is currently not supported for non-Apple frameworks.
To define your own module - if you could - you would need to do:
export MyAwesomeModule:
public:
// methods and whatever you want to export
#import is for official frameworks only. For MyFancyViewController.h continue use #import.
Is there a way to import all files from a framework, and not just the specific headers you want? This may seem inefficient, but I was just curious if this was possible?
Apple's convention is to use a header within the framework which has the same name as the framework:
#import <Foundation/Foundation.h> // << imports all public headers of Foundation.framework
If the framework in question does not include such a file, you can always write your own.
Even better, you can use modules, which use the syntax: #import Foundation;. This is similar to #import <Foundation/Foundation.h>, but uses a much improved compilation model.
Note: Modules are presently supported in Xcode for Apple's frameworks, although support for user-defined module maps exists in clang. So, hopefully we can define our own modules soon.
I'm working on an open-source project that can optionally use a closed-source framework. If the closed-source framework is included in the project, there will be additional functionality. But if the framework isn't included in the project, the project should still compile properly.
How do I check at compile-time if the framework is included in the project?
Basically, I want to do something like this:
#ifdef _MY_FRAMEWORK_EXISTS
#import <MyFramework/MyFramework.h>
#endif
I've seen older questions from 2 years ago like this one, but no answer has surfaced so I might be missing something new now.
I DON'T want to check at run-time via NSClassFromString(), because this will fail at compile time when I try to import MyFramework and it doesn't exist.
You can check for the presence of a header file using the __has_include language extension.
http://clang.llvm.org/docs/LanguageExtensions.html#include-file-checking-macros
However, that only tells you if the header file is installed. It can't tell you if "Link Binary With Libraries" has linked to its framework.
I recommend reading the Mac Developer Library : Framework Programming Guide (which includes a section on Weak Linking).
What do you mean by "exists" or "included in the project"? Do you mean added to the "Link Binary With Libraries" build phase (as described by Including Frameworks)? All that does is affect the linking, not the compilation, build phase. To see this, build. Then, search for -framework in the build log of Xcode's Log Navigator.
So, yes, if you want to affect the compilation of the code you provided, you could manually define the macro _MY_FRAMEWORK_EXISTS.
I don't really understand what you are trying to do. Can you explain what you want at a higher level? Perhaps, there's a better way to go about it.
"Minimal overhead" is nice, but too much magic might be confusing. For example, Xcode's magic hides what really happens when including a framework.
I also recommend checking out how the Facebook SDK for iOS works for high & low-level ideas. It might do the kinds of things you want to do.