objective-c - method swizzling 和 isa swizzling 是一回事吗？

Question

method swizzling 和 isa swizzling 是一回事吗？如果不是，那么什么是 swizzling？

Answer 1

方法混搭

Method swizzling在运行时交换一个类的两个方法的实现。这将影响修改后的类的每个已创建或将要创建的实例。

示例：假设您为 NSString 编写了一个类别：

@interface NSString (Swizzling)
@end
@implementation NSString (Swizzling)
- (NSString *)swizzled_uppercaseString {
    //when the method is swizzled, the original method will be called 
    //with swizzled_uppercaseString (so this will not create a stack overflow).
    NSString *result = [self swizzled_uppercaseString];
    // our custom code
    result = [result stringByAppendingString:@" (swizzled)"];
    return result;
}
@end

然后，您可以将 -method 的实现与uppercaseString-method交换swizzled_uppercaseString，以便在调用swizzled_uppercaseString时执行该方法的实现。（并且在调用时执行uppercaseString的原始实现）：uppercaseStringswizzled_uppercaseString

#import <objc/runtime.h>

NSString *sample = @"abc";

// original method is called:
NSLog([sample uppercaseString]); 

//Obtaining original and swizzled method:
original = class_getInstanceMethod([NSString class], @selector(uppercaseString));
swizzled = class_getInstanceMethod([NSString class], @selector(swizzled_uppercaseString));

//Exchange implementations here:
method_exchangeImplementations(original, swizzled);

// swizzled method is called:
NSLog([sample uppercaseString]); //prints "ABC (swizzled)"

---

ISA 调酒

ISA swizzling 修改单个对象的属性，即 ISA ('is a') 属性，它描述了对象的类，因此您可以在运行时将给定单个对象的类型与另一种类型交换。

示例：假设您有这样的类结构：

@interface Sample : NSObject
@property (nonatomic) NSString *sampleStringToLoad;
@end
@implementation Sample
@synthesize sampleStringToLoad;
@end

@interface SampleWithStringLoader :NSObject
@property (nonatomic) NSString *sampleStringToLoad;
-(void)loadString;
@end
@implementation SampleWithStringLoader
@synthesize sampleStringToLoad;
-(void)loadString {
self.sampleStringToLoad = @"abc";
}
@end

然后，您可以将类设置为SampleWithStringLoader，因此sampleStringToLoad-method 变得可用：

#import <objc/runtime.h>

Sample *sample = [Sample new];
// switch isa to new class:
object_setClass(sample, [SampleWithStringLoader class]);

// invoke method that is only in SampleWithStringLoader: 
[sample performSelector:@selector(loadString)]; 

// switch isa back to original class:
object_setClass(sample,[Sample class]); 

// Prints 'abc':
NSLog(sample.sampleStringToLoad);