Objective-C: Method Swizzling , why use class_addMethod?












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I am reviewing method-swizzling.



Why add the branch if (didAddMethod) { ?




#import <objc/runtime.h>

@implementation UIViewController (Tracking)

+ (void)load {
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
Class class = [self class];

SEL originalSelector = @selector(viewWillAppear:);
SEL swizzledSelector = @selector(xxx_viewWillAppear:);

Method originalMethod = class_getInstanceMethod(class, originalSelector);
Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);

// When swizzling a class method, use the following:
// Class class = object_getClass((id)self);
// ...
// Method originalMethod = class_getClassMethod(class, originalSelector);
// Method swizzledMethod = class_getClassMethod(class, swizzledSelector);

BOOL didAddMethod =
class_addMethod(class,
originalSelector,
method_getImplementation(swizzledMethod),
method_getTypeEncoding(swizzledMethod));

if (didAddMethod) {
class_replaceMethod(class,
swizzledSelector,
method_getImplementation(originalMethod),
method_getTypeEncoding(originalMethod));
} else {
method_exchangeImplementations(originalMethod, swizzledMethod);
}
});
}

#pragma mark - Method Swizzling

- (void)xxx_viewWillAppear:(BOOL)animated {
[self xxx_viewWillAppear:animated];
NSLog(@"viewWillAppear: %@", self);
}

@end



Why not get Method , then method_exchangeImp ?



I take a breakpoint several times, never runs to the branch if (didAddMethod) {









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    0












    $begingroup$


    I am reviewing method-swizzling.



    Why add the branch if (didAddMethod) { ?




    #import <objc/runtime.h>

    @implementation UIViewController (Tracking)

    + (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
    Class class = [self class];

    SEL originalSelector = @selector(viewWillAppear:);
    SEL swizzledSelector = @selector(xxx_viewWillAppear:);

    Method originalMethod = class_getInstanceMethod(class, originalSelector);
    Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);

    // When swizzling a class method, use the following:
    // Class class = object_getClass((id)self);
    // ...
    // Method originalMethod = class_getClassMethod(class, originalSelector);
    // Method swizzledMethod = class_getClassMethod(class, swizzledSelector);

    BOOL didAddMethod =
    class_addMethod(class,
    originalSelector,
    method_getImplementation(swizzledMethod),
    method_getTypeEncoding(swizzledMethod));

    if (didAddMethod) {
    class_replaceMethod(class,
    swizzledSelector,
    method_getImplementation(originalMethod),
    method_getTypeEncoding(originalMethod));
    } else {
    method_exchangeImplementations(originalMethod, swizzledMethod);
    }
    });
    }

    #pragma mark - Method Swizzling

    - (void)xxx_viewWillAppear:(BOOL)animated {
    [self xxx_viewWillAppear:animated];
    NSLog(@"viewWillAppear: %@", self);
    }

    @end



    Why not get Method , then method_exchangeImp ?



    I take a breakpoint several times, never runs to the branch if (didAddMethod) {









    share









    $endgroup$















      0












      0








      0





      $begingroup$


      I am reviewing method-swizzling.



      Why add the branch if (didAddMethod) { ?




      #import <objc/runtime.h>

      @implementation UIViewController (Tracking)

      + (void)load {
      static dispatch_once_t onceToken;
      dispatch_once(&onceToken, ^{
      Class class = [self class];

      SEL originalSelector = @selector(viewWillAppear:);
      SEL swizzledSelector = @selector(xxx_viewWillAppear:);

      Method originalMethod = class_getInstanceMethod(class, originalSelector);
      Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);

      // When swizzling a class method, use the following:
      // Class class = object_getClass((id)self);
      // ...
      // Method originalMethod = class_getClassMethod(class, originalSelector);
      // Method swizzledMethod = class_getClassMethod(class, swizzledSelector);

      BOOL didAddMethod =
      class_addMethod(class,
      originalSelector,
      method_getImplementation(swizzledMethod),
      method_getTypeEncoding(swizzledMethod));

      if (didAddMethod) {
      class_replaceMethod(class,
      swizzledSelector,
      method_getImplementation(originalMethod),
      method_getTypeEncoding(originalMethod));
      } else {
      method_exchangeImplementations(originalMethod, swizzledMethod);
      }
      });
      }

      #pragma mark - Method Swizzling

      - (void)xxx_viewWillAppear:(BOOL)animated {
      [self xxx_viewWillAppear:animated];
      NSLog(@"viewWillAppear: %@", self);
      }

      @end



      Why not get Method , then method_exchangeImp ?



      I take a breakpoint several times, never runs to the branch if (didAddMethod) {









      share









      $endgroup$




      I am reviewing method-swizzling.



      Why add the branch if (didAddMethod) { ?




      #import <objc/runtime.h>

      @implementation UIViewController (Tracking)

      + (void)load {
      static dispatch_once_t onceToken;
      dispatch_once(&onceToken, ^{
      Class class = [self class];

      SEL originalSelector = @selector(viewWillAppear:);
      SEL swizzledSelector = @selector(xxx_viewWillAppear:);

      Method originalMethod = class_getInstanceMethod(class, originalSelector);
      Method swizzledMethod = class_getInstanceMethod(class, swizzledSelector);

      // When swizzling a class method, use the following:
      // Class class = object_getClass((id)self);
      // ...
      // Method originalMethod = class_getClassMethod(class, originalSelector);
      // Method swizzledMethod = class_getClassMethod(class, swizzledSelector);

      BOOL didAddMethod =
      class_addMethod(class,
      originalSelector,
      method_getImplementation(swizzledMethod),
      method_getTypeEncoding(swizzledMethod));

      if (didAddMethod) {
      class_replaceMethod(class,
      swizzledSelector,
      method_getImplementation(originalMethod),
      method_getTypeEncoding(originalMethod));
      } else {
      method_exchangeImplementations(originalMethod, swizzledMethod);
      }
      });
      }

      #pragma mark - Method Swizzling

      - (void)xxx_viewWillAppear:(BOOL)animated {
      [self xxx_viewWillAppear:animated];
      NSLog(@"viewWillAppear: %@", self);
      }

      @end



      Why not get Method , then method_exchangeImp ?



      I take a breakpoint several times, never runs to the branch if (didAddMethod) {







      objective-c objective-c-runtime





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      asked 4 mins ago









      black_pearlblack_pearl

      1135




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