Difference between revisions of "Using ARC in tweaks"

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(Add some quick info on ARC. Most likely incorrect as I have no idea what I'm talking about.)
 
(Remove warnings against using ARC.)
 
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What is ARC?
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== What is ARC? ==
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ARC (or automatic memory management) is a new way to manage memory in Objective-C, first fully introduced in 2011 with iOS 5. This replaces Objective-C's standard memory model, manual memory management.
 
  
While ARC can be very convenient, it can also be used as a crutch. While ARC can be helpful while learning, it is important not to rely on it, and to still be capable of managing memory manually.
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Quoting from [https://en.wikipedia.org/wiki/Automatic_Reference_Counting the Wikipedia article on Automatic Reference Counting]:
  
How should I use ARC in tweaks?
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<blockquote>In Objective-C and Swift programming, Automatic Reference Counting (ARC) is a memory management enhancement where the burden of keeping track of an object's reference count is lifted from the programmer to the compiler. In traditional Objective-C, the programmer would send retain and release messages to objects in order to mark objects for deallocation or to prevent deallocation. Under ARC, the compiler does this automatically by examining the source code and then adding the retain and release messages in the compiled code.</blockquote>
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As ARC was first introduced with iOS 5, it is very likely that iOS is written with MMR (citation needed), as it seems unlikely that they would rewrite their entire codebase. For this reason, it is not recommended that you use ARC in any hooks, as it could potentially cause problems -- injecting automatically managed code into a manually managed system simply doesn't sound like a good idea.
 
  
However, you probably will not encounter any problems using ARC in any custom classes that you create.
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== Should I use ARC in tweaks? ==
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=== For hooks ===
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Yes, use ARC. Writing hooks within a tweak are ''no different'' than using Objective-C methods to swizzle private APIs within an app. As such, in almost all cases '''it is totally fine to use ARC,''' and you should. However, there ''are'' rare cases where you might get unexpected behavior—such as not correctly marking autoreleasing parameters as autoreleasing, or forming a retain cycle, etc. This is not ARC's fault, and it is not unique to hooks. It really comes down to understanding how ARC works and what all the different keywords mean—but that is beyond the scope of this document.
 +
 
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=== For class declarations ===
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 +
Yes, use ARC.
 +
 
 +
=== Generally speaking ===
 +
 
 +
If you've had someone tell you shouldn't use ARC in tweaks, they are mistaken. Some developers still follow the misguided notion that ARC is unsafe for use in tweaks. Admittedly, some fault lies on this very wiki page for previously advising against ARC. Those people may consider it to be a bad idea to use it ''at all''. Generally, you should just use ARC everywhere unless you have a specific reason not to. Without this approach, cases may arise where the programmer ends up forgetting which files use ARC and which files don't, inevitably leading to memory leaks. One example of this is when you write some code in a file that you thought uses ARC, but actually doesn't.
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In a nutshell, just use ARC.
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==How ''do'' I use ARC in tweaks?==
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 +
 
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=== Using Theos ===
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In your Makefile:
 +
 
 +
<source lang="bash">
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TweakName_CFLAGS = -fobjc-arc
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</source>
 +
 
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This applies ARC to <u>all</u> files you are compiling including your tweak file.
 +
 
 +
If you only want to use ARC on specific files, you can use one of two solutions. The most straightforward is to set the flag specifically for the file(s) in question:
 +
 
 +
<source lang="bash">
 +
Tweak.xm_CFLAGS = -fobjc-arc
 +
</source>
 +
 
 +
For more complex situations, where a large amount of the code uses ARC, it may be beneficial to separate the projects. [https://github.com/hbang/NewTerm NewTerm], for instance, uses ARC but contains external code that isn't ARC. The latter code is split into a library called <code>libvt100.dylib</code>, which the main NewTerm project then links against:
 +
 
 +
<source lang="bash">
 +
LIBRARY_NAME = libvt100
 +
# ...
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libvt100_INSTALL_PATH = /Applications/NewTerm.app
 +
 
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APPLICATION_NAME = NewTerm
 +
# ...
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NewTerm_CFLAGS = -fobjc-arc
 +
NewTerm_LDFLAGS = -L$(THEOS_OBJ_DIR)
 +
NewTerm_LIBRARIES = vt100
 +
 
 +
include $(THEOS_MAKE_PATH)/library.mk
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include $(THEOS_MAKE_PATH)/application.mk
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</source>
 +
 
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You should pick another install path if your project isn't an app; try <code>/Library/Application Support/PackageName</code>.
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 +
=== Without theos ===
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 +
This is how the flags would look like in a typical build process:
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 +
<source lang="bash">
 +
#optional: convert from logos to objective c
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logos.pl tweak.x > tweak.m
 +
#compile tweak without ARC
 +
clang -c tweak.m -isysroot /path/to/sdk
 +
#compile class declaration with ARC
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clang -c YourClass.m -isysroot /path/to/sdk -fobjc-arc
 +
#link to dylib
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clang -o YouTweak.dylib -dynamiclib tweak.o YourClass.o -framework Foundation -framework UIKit -framework OtherFrameworkYoureUsing -isysroot /path/to/sdk
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</source>

Latest revision as of 19:09, 13 June 2019

What is ARC?

Quoting from the Wikipedia article on Automatic Reference Counting:

In Objective-C and Swift programming, Automatic Reference Counting (ARC) is a memory management enhancement where the burden of keeping track of an object's reference count is lifted from the programmer to the compiler. In traditional Objective-C, the programmer would send retain and release messages to objects in order to mark objects for deallocation or to prevent deallocation. Under ARC, the compiler does this automatically by examining the source code and then adding the retain and release messages in the compiled code.

Should I use ARC in tweaks?

For hooks

Yes, use ARC. Writing hooks within a tweak are no different than using Objective-C methods to swizzle private APIs within an app. As such, in almost all cases it is totally fine to use ARC, and you should. However, there are rare cases where you might get unexpected behavior—such as not correctly marking autoreleasing parameters as autoreleasing, or forming a retain cycle, etc. This is not ARC's fault, and it is not unique to hooks. It really comes down to understanding how ARC works and what all the different keywords mean—but that is beyond the scope of this document.

For class declarations

Yes, use ARC.

Generally speaking

If you've had someone tell you shouldn't use ARC in tweaks, they are mistaken. Some developers still follow the misguided notion that ARC is unsafe for use in tweaks. Admittedly, some fault lies on this very wiki page for previously advising against ARC. Those people may consider it to be a bad idea to use it at all. Generally, you should just use ARC everywhere unless you have a specific reason not to. Without this approach, cases may arise where the programmer ends up forgetting which files use ARC and which files don't, inevitably leading to memory leaks. One example of this is when you write some code in a file that you thought uses ARC, but actually doesn't.

In a nutshell, just use ARC.

How do I use ARC in tweaks?

Using Theos

In your Makefile:

TweakName_CFLAGS = -fobjc-arc

This applies ARC to all files you are compiling including your tweak file.

If you only want to use ARC on specific files, you can use one of two solutions. The most straightforward is to set the flag specifically for the file(s) in question:

Tweak.xm_CFLAGS = -fobjc-arc

For more complex situations, where a large amount of the code uses ARC, it may be beneficial to separate the projects. NewTerm, for instance, uses ARC but contains external code that isn't ARC. The latter code is split into a library called libvt100.dylib, which the main NewTerm project then links against:

LIBRARY_NAME = libvt100
# ...
libvt100_INSTALL_PATH = /Applications/NewTerm.app

APPLICATION_NAME = NewTerm
# ...
NewTerm_CFLAGS = -fobjc-arc
NewTerm_LDFLAGS = -L$(THEOS_OBJ_DIR)
NewTerm_LIBRARIES = vt100

include $(THEOS_MAKE_PATH)/library.mk
include $(THEOS_MAKE_PATH)/application.mk

You should pick another install path if your project isn't an app; try /Library/Application Support/PackageName.

Without theos

This is how the flags would look like in a typical build process:

#optional: convert from logos to objective c
logos.pl tweak.x > tweak.m
#compile tweak without ARC
clang -c tweak.m -isysroot /path/to/sdk
#compile class declaration with ARC
clang -c YourClass.m -isysroot /path/to/sdk -fobjc-arc
#link to dylib
clang -o YouTweak.dylib -dynamiclib tweak.o YourClass.o -framework Foundation -framework UIKit -framework OtherFrameworkYoureUsing -isysroot /path/to/sdk