What is the best way of method swizzling?

Transcript

1. What is the best way of
   method swizzling?
   Akira Miki
   Repro Inc.
   iOS meetup@voyage
   Feb 14, 2015
   

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2. Who am I?
   

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3. Akira Miki
   CTO / Repro
   @ae06710
   

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4. Reproduce real user behavior
   

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5. Reproduce User behavior
   

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6. Quantitative Analytics
   

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7. Crash Report
   (with UIView Breadcrumbs and Movie)
   

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8. What’s method swizzling?
   

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9. Replacing the
   implementation of that
   method with another
   > Meta Programming
   

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10. Like that:
    #import
    #import
    @interface GeneralSwz : NSObject
    @end
    @implementation GeneralSwz
    + (void)load
    {
    Method originalMethod = class_getInstanceMethod(self, @selector(foo));
    Method swizzledMethod = class_getInstanceMethod(self, @selector(swz_foo));
    method_exchangeImplementations(originalMethod, swizzledMethod);
    }
    - (void)foo
    {
    NSLog(@"original foo method");
    }
    - (void)swz_foo
    {
    NSLog(@"swizzed foo method");
    [self swz_foo];
    }
    @end
    

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11. Calling `foo method` Flow:
    Call `foo`
    IMP `swz_foo`
    Method `foo`
    execute `swz_foo `
    struct  objc_method  
             SEL  method_name                  OBJC2_UNAVAILABLE;  
             char  *method_types            OBJC2_UNAVAILABLE;  
             IMP  method_imp                    OBJC2_UNAVAILABLE;  
    }
    Dispatch table
    Developer
    iOS
    

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12. > method_exchangeImplementations(original, swizzled);
    It’s very easy.

    But, this has a few
    problems.
    

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13. Replace timing
    D Cannot check user define callback method
    and replace this.
    D Another third-party library replacement same
    method at the same time on another thread
    @implementation GeneralSwz
    + (void)load
    {
    Method originalMethod = class_getInstanceMethod(self, @selector(foo));
    Method swizzledMethod = class_getInstanceMethod(self, @selector(swz_foo));
    method_exchangeImplementations(originalMethod, swizzledMethod);
    }
    @end
    

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14. Target scope
    D Not by super class
    D 

    

    

    

    @interface Bare : NSObject
    @end
    @implementation Bare
    - (void)foo {
    NSLog(@"Bare foo method");
    }
    @end
    @interface InheritedSwz : Bare
    @end
    @implementation InheritedSwz
    + (void)load {
    Method originalMethod = class_getInstanceMethod(self, @selector(foo));
    Method swizzledMethod = class_getInstanceMethod(self, @selector(swz_foo));
    method_exchangeImplementations(originalMethod, swizzledMethod);
    }
    // we got `-[Bare swz_foo]: unrecognized selector`
    - (void)swz_foo {
    NSLog(@"Inherited swz_foo method");
    [self swz_foo];
    }
    @end
    

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15. Conflict
    D Naming conflict can happen
    @implementation CategorySwz
    // swizzling foo method to swz_foo
    - (void)foo
    {
    NSLog(@"original foo method");
    }
    - (void)swz_foo
    {
    NSLog(@"swizzed foo method");
    [self swz_foo];
    }
    @end
    @implementation CategorySwz (category)
    - (void)swz_foo
    {
    // Call this only
    NSLog(@"Cateogry swz_foo method");
    }
    @end
    

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16. Assertion
    D if check about assertion for keep everything
    running smoothly, don’t work
    @implementation AssertSwz
    // Swizzle method foo to swz_foo
    - (void)foo
    {
    assert([NSStringFromSelector(_cmd) isEqualToString:@"foo"]);
    NSLog(@"original foo method");
    }
    - (void)swz_foo
    {
    NSLog(@"swizzed foo method");
    [self swz_foo];
    }
    @end
    

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17. Best swizzling behavior is…
    D Swizzle on demand timing
    D Thread safe
    D Effect to super class
    D Can’t override replacement method
    D _cmd return original method name
    

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18. Prepare
    @interface Bare : NSObject
    @end
    @implementation Bare
    - (void)foo
    {
    NSLog(@"original foo method");
    }
    @end
    @interface General : Bare
    @end
    @implementation General
    @end
    Base Class
    - (void)foo
    General Class
    Inherited Class
    - (void)foo
    Swizzle `foo` method
    

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19. Solution-1
    D Prepare new implementation as function

    D 

    

    D And replace with `method_setImplementation`
    void _replacement_Method(id self, SEL _cmd)
    {
    NSLog(@"swizzed foo method”);
    ((void(*)(id,SEL))__original_Method_Imp)(self, _cmd);
    }
    static IMP __original_Method_Imp;
    @implementation InheritedBare
    + (void)swizzle
    {
    Method m = class_getInstanceMethod( [self class], @selector(foo));
    __original_Method_Imp =
    method_setImplementation( m, (IMP)_replacement_Method );
    }
    @end
    

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20. Solution-2
    D Lock thread and call once

    D 

    

    #import
    

    + (void)swizzle
    {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
    OSSpinLock lock = OS_SPINLOCK_INIT;
    OSSpinLockLock(&lock);
    Method m = class_getInstanceMethod( [self class], @selector(foo));
    __original_Method_Imp =
    method_setImplementation( m, (IMP)_replacement_Method );
    OSSpinLockUnlock(&lock);
    });
    }
    

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21. #import
    #import
    static IMP __original_Method_Imp;

    void _replacement_Method(id self, SEL _cmd)
    {
    // it will pass
    assert([NSStringFromSelector(_cmd) isEqualToString:@"foo"]);
    NSLog(@"swizzed foo method");
    ((void(*)(id,SEL))__original_Method_Imp)(self, _cmd);
    }
    @implementation GoodSwz
    + (void)swizzle
    {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
    OSSpinLock lock = OS_SPINLOCK_INIT;
    OSSpinLockLock(&lock);
    Method m = class_getInstanceMethod( [self class], @selector(foo));
    __original_Method_Imp =
    method_setImplementation( m, (IMP)_replacement_Method );
    OSSpinLockUnlock(&lock);
    });
    }
    @end
    

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22. And class method
    swizzling is…
    D as hard as what we just saw
    

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23. So, you want to implement
    all these yourself?
    

    No, use “rabovik/RSSwizzle”!
    

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24. > These bad practices aren’t really
    a big deal if you’re writing
    standalone applications
    - http://blog.newrelic.com/2014/04/16/right-way-to-swizzle/
    

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25. Sample Code and references
    https://github.com/ae06710/swizzle-sample
    D Method Swizzling - Written by Mattt Thompson on February 17th, 2014
    D http://nshipster.com/method-swizzling/
    D The Right Way to Swizzle in Objective-C
    D http://blog.newrelic.com/2014/04/16/right-way-to-swizzle/
    D What are the Dangers of Method Swizzling in Objective C?
    D http://stackoverflow.com/questions/5339276/what-are-the-dangers-of-method-swizzling-in-
    objective-c/8636521#8636521
    D rabovik/RSSwizzle
    D https://github.com/rabovik/RSSwizzle
    D http://www.opensource.apple.com/source/xnu/xnu-1456.1.26/libkern/libkern/OSAtomic.h
    D Objective-C Runtime Reference
    D https://developer.apple.com/library/mac/documentation/Cocoa/Reference/ObjCRuntimeRef/
    index.html#//apple_ref/c/func/method_getImplementation
    

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26. [Experts wanted]
    || ( && )
    

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27. Thank you :)
    

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