I am just wondering, is it good practice to include commonly/mostly used header files in .pch file?
e.g Most of files required below headers
#import <UIKit/UIKit.h>
#import <Foundation/Foundation.h>
So instead of each file should have, is it good practice to add those in .pch file?
.pch file
#ifdef __OBJC__
#import <UIKit/UIKit.h>
#import <Foundation/Foundation.h>
#import <MostlyUsed.h>
#endif
I found some disadvantages of this,
1) Dependancies get hidden.
2) Source files can not be copied directly.
It will be great if you guys provide some inputs on this.
Thanks
Yes you can do it.
Even it reduces compile time.
An Extract from In Objective-C, importing same headers in every class make compile time longer?
In general, newly-generated iOS projects come with this functionality,
which is called a precompiled header or prefix header, and is a file
that has the extension .pch.
You can throw all the headers you want in there and Xcode will
pre-compile it before it builds anything else, and use it to compile
the other compilation units in your project (e.g. .m files).
Using a precompiled header may or may not increase compile time; in
general, it reduces compile time, as long as you have a lot of common
headers and/or a lot of source files.
However, it's not necessarily good practice to treat the pre-compiled
header like a big dumping ground, as your compilation units can form
implicit dependencies on all sorts of stuff when you may want to
enforce loose coupling between components.
It is certainly a good practice as it reduces compile time and code is lot more manageable by doing so.
Related
So many developers are adding various convenience macros to the Prefix.pch. But my question is what is that Prefix.pch file.
If i remove that Prefix.pch file from my Xcode, then will my application run? Or will it show any error? Or will it crash during build?
How can i run my application without Prefix.pch file
Precompiled header.
What is it?
A Prefix.pch is a precompiled header. Precompiled headers were invented to make compiling faster. Rather than parsing the same header files over and over, these files get parsed once, ahead of time.
Xcode
In Xcode, you add imports of the header files you want in a “prefix header,” and enabling Precompile Prefix Header so they get precompiled. But the idea behind a prefix header is different from precompiling.
A prefix header is implicitly included at the start of every source file. It’s like each source file adds
#import "Prefix.pch"
at the top of the file, before anything else.
Removing it.
You can remove the precompiled header. This question has been already answered in thread I'm linking below. It contains all the information you need as well as useful comments.
Is it OK to remove Prefix.pch file from the Xcode project?
What is Prefix.pch file?
A .pch is a Pre-Compiled Header.
In the C and C++ programming languages, a header file is a file whose text may be automatically included in another source file by the C preprocessor, usually specified by the use of compiler directives in the source file.
Prefix headers are compiled and stored in a cache, and then automatically included in every file during compilation. This can speed up compilation, and lets you include a file without adding an import statement to every file using it. They are not required, and actually slow compilation whenever you change them.
Yes, you can compile and run the project without .pch file
In Xcode, go to your target's build settings (Command-Option-E, build tab) and uncheck Precompile Prefix Header (GCC_PRECOMPILE_PREFIX_HEADER). You can also remove the value of the Prefix Header setting if you wish.
Also note that,
Don't put macros in a.pch file! A .pch file is, by definition, a project specific precompiled header. It really shouldn't be used beyond the context of the project and it really shouldn't contain anything but #includes and #imports.
If you have some macros and such that you want to share between headers, then stick 'em in a header file of their own — Common.h or whatever — and #include that at the beginning of the .pch
Prefix headers are compiled and stored in a cache, and then automatically included in every file during compilation. This can speed up compilation, and lets you include a file without adding an import statement to every file using it. They are not required, and actually slow compilation whenever you change them.
Generally .pch file naming formation is yourProjectName-Prefix.pch
Prefix Headers and Precompiled Headers (.pch)
History:
[#include -> #import] -> Precompiled Headers .pch -> [#import Module(ObjC);] -> [import Module(Swift)]
Precompiled Headers - prefix.pch
To create new .pch file
File -> New -> File... -> PCH File
PrefixHeader.pch sample:
#ifndef PrefixHeader_pch
#define PrefixHeader_pch
#import <UIKit/UIKit.h>
#import <Foundation/Foundation.h>
#endif
From Xcode 6 you should set .pch manually. Previously it was called <product_name>-Prefix.pch
Xcode:
Prefix Header(GCC_PREFIX_HEADER) - path to .pch. Adds all content from .pch to all source files
SomeFile1.h
//You can skip this section. It will be compiled
//#import <UIKit/UIKit.h>
//#import <Foundation/Foundation.h>
//implicitly next line is used
#import "PrefixHeader.pch"
Precompile Prefix Header(GCC_PRECOMPILE_PREFIX_HEADER) - When Yes then Prefix headers from GCC_PREFIX_HEADER will be used once* to generate precompiled sources and will be stored in a cache which speeds up a build time
//path to cached precompiled headers
/<some_path>/DerivedData/ExperiementsModuleSwift-dsetoksxykdmgvczgtnmaqmjwhzk/Build/Intermediates.noindex/PrecompiledHeaders/SharedPrecompiledHeaders/14385052922615550324
//e.g
/Users/alex/Library/Developer/Xcode/DerivedData/ExperiementsModuleSwift-dsetoksxykdmgvczgtnmaqmjwhzk/Build/Intermediates.noindex/PrecompiledHeaders/SharedPrecompiledHeaders/14385052922615550324
#import vs .pch
This feature improves build time(comparing with #include/#import) because precompiles a prefix headers(global). When some content of prefix headers is changed it means that Compiles must recompile that again. that is why you should use stable(not often changeable) prefix headers here or you dont't see build time improvement. Also you should support it
I am facing an issue, Currently i am using xcode 5.0, when i am importing clasess from prefix.pch then it dose not give suggestions on appdelegate.h file. But it gives suggestion on other view controller's. I have googled about it and find if we define macros in prefix.pch then this problem occurred. Still after removing the macros it did not give suggestion on appdelegate.
Suggestios means if i try to write
#property(strong,nonatomic) UINavigationController *navControler;
After writing #property(strong,nonatomic) UINav xcode did not give suggestion. But if I remove all the import statement and macros from prefix.pch file then xcode give suggestions on appdelegate.
From my point of view, rather than finding a workaround, I want to put some light on the actual problem.
So when you want to add something in .pch (pre compiled headers), it means you want to use the content of files before your program is actually being compiled & build. In simple words the headers included in pch acts as a stub code, a base code or may be a code that is meant for using as it is intact.
So if you have already included appDelegate.h file in .pch & writing some code in appDelegate.h file, then the compiler is trying to use 'appDelegate.h` from .pch as a pre-compiled information. And may be it is resulting in a indefinite sate of Xcode background processing, which is not allowing to show the drop down helps in the file which is already pre-compiled.
To me it looks like app delegate.h in .pch is not the right place. Where ever you want in your program, you can very well include appDelegate.h file. However it is also a signal of bad Class Design approaches.
Also, if you want to add #define statement & other macros in .pch then it is not the right approach. The right approach is :
Make a simple .h file, say constants.h
Add all your #defines & macros here in constants.h file.
Add the #import "constants.h" in your .pch file.
That should solve your problem.
Hope that helps.
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.
Default file:
#import <Foundation/Foundation.h>
#interface MyClass : NSObject
#end
My understanding is pre-compiled headers are included in every file. Why doesn't this import just go into the pre-compiled headers?
There is no guarantee that every project have a .pch file that import Foundation.h so Xcode need to generate it to make sure new files always compile.
Also some people think it is good style to include header even it is already included in .pch file. Because you know exactly what headers are needed by this file. It also allow people to disable prefix header if needed.
My Xcode 5 doesn't appear to be including it in every class (it is included in the .pch file). I agree with you that this makes more sense than including it everywhere. The answer to your question is probably "an oversight on Apple's part" :)
.pch
#ifdef __OBJC__
#import <Foundation/Foundation.h>
#import <UIKit/UIKit.h>
#import "thing.h"
#endif
I have a pretty large project I'm trying to debug and I have a suspicion the problem is in thing.h
From what I understand the .pch file is included in EVERY .m and .h in the project.
Is it possible to have it not included in just one or two files? For debugging I would like to not include thing.h in just one test file, but removing it from the .pch causes a cascading nightmare across the project.
This is a really old question but I just ran into this issue. I was able to solve the problem by wrapping the pch file with a ifndef SOME_NAME. Then in the project settings on each specific file I wanted it to not be included I added a compiler flag of -DSOME_NAME=1. Hope that helps someone in the future.