sortUsingSelector使用方法,以及原理
在IOS软件开发中我们需要依据NSArray中对象的成员进行排序,我们知道对象中成员比较多,因此如果我们写了多种方法根据不同的成员进行排序,代码量很大,实现起来不是很方便,因此苹果向我们提供了一种快速排序的方法,叫做
- (void)sortUsingSelector:(SEL)comparator;
本文重视讲诉原理:
我们先来看一个例子:
#import <Foundation/Foundation.h>
//Dog头文件
@interface Dog : NSObject
{
NSString * _name;
NSUInteger _age;
}
-(NSComparisonResult)nameCompae:(Dog *)dog;
-(NSComparisonResult)ageCompae:(Dog *)dog;
-(id)initWithName:(NSString *)name andWithAge:(NSUInteger)age;
@property (copy,readwrite,nonatomic) NSString * name;
@property (assign,readwrite,nonatomic)NSUInteger age;
@end
//Dog函数实现体
@implementation Dog
-(NSComparisonResult)nameCompae:(Dog *)dog
{
if ([_name compare:[dog name]]>0)
{
return NSOrderedDescending;
}
return NSOrderedAscending;
}
-(NSComparisonResult)ageCompae:(Dog *)dog
{
NSLog(@"%ld %ld",_age,[dog age]);
if (_age > [dog age]) {
return NSOrderedAscending;
}
return NSOrderedDescending;
}
-(void)setAge:(NSUInteger)age
{
_age = age;
NSLog(@"dasdsd");
}
-(id)initWithName:(NSString *)name andWithAge:(NSUInteger)age
{
if (self = [super init]) {
_name = name;
_age = age;
}
return self;
}
@end
//main函数
int main(int argc, const char * argv[])
{
@autoreleasepool {
NSMutableArray * dogArray = [[NSMutableArray alloc]init];
Dog * dog1 = [[Dog alloc]initWithName:@"xiaobao" andWithAge:12];
Dog * dog2 = [[Dog alloc]initWithName:@"zhansan" andWithAge:8];
Dog * dog3 = [[Dog alloc]initWithName:@"lisi" andWithAge:16];
Dog * dog4 = [[Dog alloc]initWithName:@"wangwu" andWithAge:7];
[dogArray addObject:dog1];
[dogArray addObject:dog2];
[dogArray addObject:dog3];
[dogArray addObject:dog4];
for (Dog * temp in dogArray) {
NSLog(@"%@ %ld",temp.name,temp.age);
}
[dogArray sortUsingSelector:@selector(nameCompae:)];
for (Dog * temp in dogArray)
{
NSLog(@"%@ %ld",temp.name,temp.age);
}
[dogArray sortUsingSelector:@selector(ageCompae:)];
for (Dog * temp in dogArray)
{
NSLog(@"%@ %ld",temp.name,temp.age);
}
}
return 0;
}
在上面的这个例子中,我将小狗对象存入dogArray中,而该对象中又有两个成员变量,一个是狗的名字,另一个是狗的年龄,我们通过这两个属性对这个dogArray中得对象进行排序,在测试中我发现在调用sortUsingSelector函数时,选择器(即ageCompae:和nameCompae)是被多次调用的,因此我猜测,该函数的伪代码如下:
- (void)mysortUsingSelector:(SEL)comparator:①
{
int temp = 0;
for (int i=0; i<[self count]-1; i++)②
{ temp = i;
for (int j=i+1; j<[self count]; j++)
{
if ([[self objectAtIndex:temp] comparator:[self objectAtIndex:j]]==NSOrderedDescending)③
{
temp = j;
}
}
if (i!=temp)
{
[dogArray exchangeObjectAtIndex:i withObjectAtIndex:temp];
}
}
}
上述代码中又三点需要理解,只要理解了这个伪代码,你就知道sortUsingSelector这个函数是怎么工作的了.
①:(SEL)comparator:这个东西其实就是模仿C语言中的函数指针,我们想要使用什么函数对NSArray的对象只要把函数入口地址传给它就好了,例如上述代码中想使用dog名字进行排序,只要把nameCompae:传给它就好了,而想要按照年龄排序就传ageCompare:如果你理解函数指针,这个对你就很简单了,呵呵
②:这个self其实就是数组的地址,对应与上面代码中的dogArray
③:使用传递过来的函数进行比较,也就是C语言中函数指针的回调函数,上述代码中有一点很重要,也就是
if ([[self objectAtIndex:temp] comparator:[self objectAtIndex:j]]==NSOrderedDescending)这有在这个语句为真的时候才会进行交换,我测试多次sortUsingSelector这个函数在回调(SEL)comparator:函数时,只有在(SEL)comparator:函数返回值为NSOrderedDescending:才会进行排序,对于其它值则不会进行如何处理,这也很好的解释了我上面的if语句的书写方式是正确的.
总结:相信到了这里你一定明白了sortUsingSelector的工作方式了,如果不明白建议看看关于函数指针的视频或者书籍,推荐在51CTO上面找找;