iOS如何解决某些方法低版本不支持的问题

#import "NSString+avoidCrash.h"
#import <objc/runtime.h>

@implementation NSString (avoidCrash)

+(void)load
{
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{

SEL systemSel = @selector(containsString:);
Method systemMethod = class_getInstanceMethod([self class], systemSel);

//如果没有找到，说明不包含此方法，需要动态添加此方法
if (systemMethod == NULL) {
SEL customSel = @selector(customContainsString:);
Method customMethod = class_getInstanceMethod([self class], customSel);

BOOL isAdd = class_addMethod(self, systemSel, method_getImplementation(customMethod), method_getTypeEncoding(customMethod));

if (!isAdd) {
NSLog(@"%@ runtime add method 'customContainsString:' failed",NSStringFromClass([self class]));
}
}

});
}

-(BOOL)customContainsString:(NSString *)str
{
NSRange range = [self rangeOfString:str];

if (range.location < self.length && range.length > 0) {
return YES;
}

return NO;
}

@end