RunTime 方法交换 NSURL 空值处理

//
//  NSURL+url.h
//  iOS_study00
//
//  Created by Datacvg on 2019/9/6.
//  Copyright © 2019 Datacvg. All rights reserved.
//

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface NSURL (url)
+(instancetype)SL_URLWithString:(NSString*)URLString;
@end

NS_ASSUME_NONNULL_END

//
//  NSURL+url.m
//  iOS_study00
//
//  Created by Datacvg on 2019/9/6.
//  Copyright © 2019 Datacvg. All rights reserved.
//

#import "NSURL+url.h"
#import <objc/runtime.h>

@implementation NSURL (url)
+(void)load
{
    /**
        在类加载方法里面使用RunTime实现方法交换(URLWithString与SL_URLWithString)
        load加载比init alloc都早
        没有使用NSURL类也会加载(程序运行就会加载)
        NSURL+url.m在Build Phases -> Compile Sources 里面原文件参与编译
     */
    //class_getInstanceMethod :获取对象方法
    //class_getClassMethod: 获取类方法

    Method URLWithString = class_getClassMethod([NSURL class], @selector(URLWithString:));
    Method SLURLWithString =  class_getClassMethod([NSURL class], @selector(SL_URLWithString:));
    //交换
    method_exchangeImplementations(URLWithString, SLURLWithString);
}
+(instancetype)SL_URLWithString:(NSString *)URLString
{
    //方法交换后SL_URLWithStringy对应系统方法URLWithString
    NSURL * url = [NSURL SL_URLWithString:URLString];
    if (url == nil) {
        NSLog(@"url为空");
    }
    return url;
}
@end