NSMutableArray

   为了防止数组添加nil对象和数组越界的问题，通过AOP，用runtime交换系统方法，防止崩溃。

#import "NSMutableArray+Safe.h"

@implementation NSMutableArray (Safe)

+ (void)load {
    static dispatch_once_t onceToken;
    dispatch_once(&onceToken, ^{
        id obj = [[self alloc] init];
        [obj swizzleMethod:@selector(addObject:) withMethod:@selector(safeAddObject:)];
        [obj swizzleMethod:@selector(objectAtIndex:) withMethod:@selector(safeObjectAtIndex:)];
        //        [obj swizzleMethod:@selector(insertObject:atIndex:) withMethod:@selector(safeInsertObject:atIndex:)];
        //        [obj swizzleMethod:@selector(removeObjectAtIndex:) withMethod:@selector(safeRemoveObjectAtIndex:)];
        //        [obj swizzleMethod:@selector(replaceObjectAtIndex:withObject:) withMethod:@selector(safeReplaceObjectAtIndex:withObject:)];
    });
}

- (void)safeAddObject:(id)anObject {
    if (anObject) {
        [self safeAddObject:anObject];
    } else {
        NSLog(@"obj is nil");
    }
}

- (id)safeObjectAtIndex:(NSInteger)index {
    if (index < [self count]) {
        return [self safeObjectAtIndex:index];
    } else {
        NSLog(@"index is beyond bounds ");
    }
    return nil;
}

- (void)swizzleMethod:(SEL)origSelector withMethod:(SEL)newSelector {
    Class class = [self class];

    Method originalMethod = class_getInstanceMethod(class, origSelector);
    Method swizzledMethod = class_getInstanceMethod(class, newSelector);

    BOOL didAddMethod = class_addMethod(class,
                                        origSelector,
                                        method_getImplementation(swizzledMethod),
                                        method_getTypeEncoding(swizzledMethod));
    if (didAddMethod) {
        class_replaceMethod(class,
                            newSelector,
                            method_getImplementation(originalMethod),
                            method_getTypeEncoding(originalMethod));
    } else {
        method_exchangeImplementations(originalMethod, swizzledMethod);
    }
}

@end