项目中针对使用NSArray的objectAtIndex:函数时，数组越界问题，可以用category配合runtime的黑魔法 Method Swizzing解决。
#import "NSArray+Checking.h"
#import
@implementation NSArray (Checking)
+ (void)load{
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
Class class = [self class];
Method objAtIndex = class_getInstanceMethod(class, @selector(objectAtIndex:));
Method objAtIndexCheck = class_getInstanceMethod(class, @selector(objAtIndexCheck:));
BOOL didAddMethod = class_addMethod([self class], @selector(objAtIndexCheck:), method_getImplementation(objAtIndexCheck), method_getTypeEncoding(objAtIndexCheck));
if (didAddMethod) {
class_replaceMethod([self class], @selector(objAtIndexCheck:), method_getImplementation(objAtIndex), method_getTypeEncoding(objAtIndex));
} else {
method_exchangeImplementations(objAtIndex, objAtIndexCheck);
}
});
}
- (id)objAtIndexCheck:(NSInteger)index{
if (self.count <= index) {
return nil;
}
return [self objAtIndexCheck:index];
}
@end
这样当调用【NSArray objectAtIndex:】时，就会先先调用objAtIndexCheck:，然后做一些自己的判断，再去调用系统的函数。
但是问题来了
上面的代码函数objAtIndexCheck:并没有被成功调用，检查了一下午，无数次的验证，仍然没有结果。
我怀疑写法问题？还是这种被广大程序猿吹嘘的方法根本就是错的？
然后我进行了尝试
//在一个类里面这样写。
Method objAtIndex = class_getInstanceMethod(self.class, @selector(aa));
Method objAtIndexCheck = class_getInstanceMethod(self.class, @selector(bb));
method_exchangeImplementations(objAtIndex, objAtIndexCheck);
[self aa];
[self bb];
完全没有问题。
我问了朋友，朋友给了我一篇文章
作者遇到了和我一样的问题，他进行了如下尝试，也没有问题。(我没测试)
+(void)load{
static dispatch_once_t onceToken ;
dispatch_once(&onceToken, ^{
Method m1 = class_getInstanceMethod([self class], @selector(lastObject));
Method m2 = class_getInstanceMethod([self class], @selector(My_lastObject));
BOOL isAdd = class_addMethod([self class], @selector(lastObject), method_getImplementation(m2), method_getTypeEncoding(m2));
if (isAdd == YES) {
class_replaceMethod([self class], @selector(My_lastObject), method_getImplementation(m1), method_getTypeEncoding(m1));
}else{
method_exchangeImplementations(m1, m2);
}
Method m3 = class_getInstanceMethod([self class], @selector(firstObject));
Method m4 = class_getInstanceMethod([self class], @selector(My_fistObjcet));
method_exchangeImplementations(m3, m4);
});
}
-(id)My_fistObjcet{
NSLog(@"My_fistObjcet : exchangeSuccess");
return [self My_fistObjcet];
}
-(id)My_lastObject{
NSLog(@"My_lastObject : exchangeSuccess");
return [self My_lastObject];
}
-------结论------
并不是不支持，而是使用错了类名，对于class_getInstanceMethod函数第一个参数的class，使用的不是[self class]，而应该是需要的类alloc init之后的真实类，NSArray对应__NSArrayI，NSMutableArray对应__NSArrayM，而后面的函数class_addMethod、class_replaceMethod使用的则是[self class]。进行如下修改就没问题了
Class class = NSClassFromString(@"__NSArrayI");
Method objAtIndex = class_getInstanceMethod(class, @selector(objectAtIndex:));
Method objAtIndexCheck = class_getInstanceMethod(class, @selector(objAtIndexCheck:));
BOOL didAddMethod = class_addMethod([self class], @selector(objAtIndexCheck:), method_getImplementation(objAtIndexCheck), method_getTypeEncoding(objAtIndexCheck));
if (didAddMethod) {
class_replaceMethod([self class], @selector(objAtIndexCheck:), method_getImplementation(objAtIndex), method_getTypeEncoding(objAtIndex));
} else {
method_exchangeImplementations(objAtIndex, objAtIndexCheck);
}
NSArray Method Swizzing.png
对于OC数组来说，NSArray确实是基类，[NSArray class]是NSArray，但是NSArray *array = [[NSArray alloc] init];之后，这时的array的类就不是NSArray了而是
__NSArrayI。具体为什么不是很清楚，应该是OC的 类簇的问题。
所以先找到实例的具体类，然后再确定参数的class
NSString类.png
/**
* Returns a specified instance method for a given class.
*
* @param cls The class you want to inspect.
* @param name The selector of the method you want to retrieve.
*
* @return The method that corresponds to the implementation of the selector specified by
* \e name for the class specified by \e cls, or \c NULL if the specified class or its
* superclasses do not contain an instance method with the specified selector.
*
* @note This function searches superclasses for implementations, whereas \c class_copyMethodList does not.
*/
OBJC_EXPORT Method class_getInstanceMethod(Class cls, SEL name)
__OSX_AVAILABLE_STARTING(__MAC_10_0, __IPHONE_2_0);
/**
* Adds a new method to a class with a given name and implementation.
*
* @param cls The class to which to add a method.
* @param name A selector that specifies the name of the method being added.
* @param imp A function which is the implementation of the new method. The function must take at least two arguments—self and _cmd.
* @param types An array of characters that describe the types of the arguments to the method.
*
* @return YES if the method was added successfully, otherwise NO
* (for example, the class already contains a method implementation with that name).
*
* @note class_addMethod will add an override of a superclass's implementation,
* but will not replace an existing implementation in this class.
* To change an existing implementation, use method_setImplementation.
*/
OBJC_EXPORT BOOL class_addMethod(Class cls, SEL name, IMP imp,
const char *types)
__OSX_AVAILABLE_STARTING(__MAC_10_5, __IPHONE_2_0);
最后同样测试了一下NSString，同样的情况。
#import "NSString+Check.h"
#import
@implementation NSString (Check)
+ (void)load{
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
Class class = NSClassFromString(@"__NSCFConstantString");
Method objAtIndex = class_getInstanceMethod(class, @selector(isEqualToString:));
Method objAtIndexCheck = class_getInstanceMethod(class, @selector(isEqualToStringCheck:));
BOOL didAddMethod = class_addMethod([self class], @selector(isEqualToStringCheck:), method_getImplementation(objAtIndexCheck), method_getTypeEncoding(objAtIndexCheck));
if (didAddMethod) {
class_replaceMethod([self class], @selector(isEqualToStringCheck:), method_getImplementation(objAtIndex), method_getTypeEncoding(objAtIndex));
} else {
method_exchangeImplementations(objAtIndex, objAtIndexCheck);
}
});
}
- (BOOL)isEqualToStringCheck:(NSString *)aString{
NSLog(@"aaa = %@",aString);
return [self isEqualToStringCheck:aString];
}
@end