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方法源码

https://github.com/FlashHand/BuffKit/tree/master/BuffKit/NullBuff

常常会从服务器拿到蛋疼的null值,说不定就导致闪退了,考虑到数据解析后的结果大不了是NSNumber,NSString,NSDictionary,NSArray这四种,我就写了个NSNull的Extension.

像这样:

-(NSInteger)length
{
return 0;
}
-(NSInteger)count
{
return 0;
}
//...
添加了各种NSNumber,NSString,NSDictionary,NSArray里的对象方法。

这样可以足够覆盖实际需求了,但总归是有限的,细想了下决定用消息转发处理:

//
// Created by BoWang on 16/5/16.
// Copyright (c) 2016 BoWang. All rights reserved.
//

#import <Foundation/Foundation.h>
//NSString,NSArray,NSDictionary
@interface NSNull (NullBuff)

@end

@interface NullBuff : NSObject
@end

//
// Created by BoWang on 16/5/16.
// Copyright (c) 2016 BoWang. All rights reserved.
//

#import "NullBuff.h"
#import <objc/runtime.h>

@implementation NSNull (NullBuff)
- (id)forwardingTargetForSelector:(SEL)aSelector {
//函数说明:class_getInstanceMethod用来获取某个类的某个selector的Method结构体
//method_copyReturnType用来获取该方法的返回值类型,如果没有该方法则返回空。
//处理NSNumber,NSString,NSArray,NSDictionary,这里会遍历这四个类,当returnType为真得时候,说明当前遍历到的类含有该selector.
//由于OC良好的命名机制,同样的selector不会有不同类的返回值。
NSArray *supporttedTypes = @[@"NSNumber" , @"NSString" , @"NSArray" , @"NSDictionary"];
for (int i = 0; i < 4; ++i) {
Method m = class_getInstanceMethod(NSClassFromString(supporttedTypes[i]) , aSelector);
const char *returnType = method_copyReturnType(m);
if (returnType) {
NSString *returnTypeStr = [[NSString alloc] initWithCString:returnType encoding:NSUTF8StringEncoding];
free(returnType);
switch (i) {
case 0:
return @(0);
break;
case 1:
return @"";
break;
case 2:
return @[];
break;
case 3:
return @{};
break;
default:
break;
}
}

}
return [super forwardingTargetForSelector:aSelector];
}
@end

@implementation NullBuff {

}
@end

原理很简单:NSNull是继承于NSObject,而NSObject的

  • (id)forwardingTargetForSelector:(SEL)aSelector 可以被重写。

该方法的可以将消息转发给它的return value.

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