迭代器模式（Iterator Pattern）

迭代器模式:提供一个方法能迅速的访问一个聚合对象中的各个元素。而不暴露内部的表示。

迭代器模式是行为型模式之一。意在提供一种公开的，便捷的，可以迅速访问聚合对象中各个元素的方法。这样方便对不同的聚合对象统一调度。

我们开发的过程中，聚合的情况可能有多种多样，数组，hashtable, 普通数组，自定义的聚合形式，等等。如果我们在整理代码的时候如何统一的去管理这些数据形式的调用？或者说我们如何在设计架构的时候把这种情况考虑进去？

显然可以选择创建一个迭代器封装“遍历集合中每一个对象的过程”。

迭代器模式

举个栗子
我们有两个画板用来展示英文的原音字母和辅音字母。
展示原音字母的画板由张三开发
展示辅音字母的画板由李四开发
他们处理相同的展示对象。但是张三用了系统提供的数组，而李四使用了公司的数组框架。
如何改造，让访问数据更加友好？

迭代器协议（java的接口）

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@protocol Iterator <NSObject>

-(BOOL)hasNext;

-(id)next;

-(void)remove:(id)obj;

@end

NS_ASSUME_NONNULL_END

展示字符的类（逻辑很简单，只做内存级数据存取，这里不贴实现）

#import <Foundation/Foundation.h>

NS_ASSUME_NONNULL_BEGIN

@interface DisplayCharactor : NSObject

@property (nonatomic ,assign)NSString * charactor;

@end

NS_ASSUME_NONNULL_END

原音字符的迭代器实现，处理系统数组

#import "OriginalSoundIterator.h"
@interface OriginalSoundIterator ()

@property (nonatomic ,strong)NSMutableArray * array;

@property (nonatomic ,assign)NSInteger index;

@end

@implementation OriginalSoundIterator

-(instancetype)initWithCharactorArray:(NSMutableArray *)array {
    
    if (self = [super init]) {
        
        _index = 0;
        _array = array;
    }return self;
}

-(id)next {
    
    NSInteger index = _index;
    _index ++;
    return _array[index];
}

-(void)remove:(id)obj {
    _index--;
    [_array removeObject:obj];
}

-(BOOL)hasNext {
    
    if (_index >= _array.count) {
        _index = 0;
        return NO;
    }
    return YES;
}

@end

辅音字符的迭代器实现，采用数组框架

#import "ConsonantIterator.h"
#import "CharactorArray.h"

@interface ConsonantIterator ()

@property (nonatomic ,strong)CharactorArray * array;

@property (nonatomic ,assign)NSInteger index;

@end

@implementation ConsonantIterator

-(instancetype)initWithCharactorArray:(CharactorArray *)array {
    
    if (self = [super init]) {
        
        _index = 0;
        _array = array;
    }return self;
}

-(id)next {
    
    NSInteger index = _index;
    _index ++;
    return [_array objectAtIndex:index];
}

-(void)remove:(id)obj {
    
    _index--;
    [_array removeObject:obj];
}

-(BOOL)hasNext {
    
    if (_index >= _array.length) {
        _index = 0;
        return NO;
    }
    return YES;
}

@end

原音字符画板实现

#import "OriginalSoundBoard.h"
#import "DisplayCharactor.h"
#import "OriginalSoundIterator.h"

@interface OriginalSoundBoard ()

@property (nonatomic ,strong)NSMutableArray * charactorArray;

@property (nonatomic ,strong)id <Iterator> iterator;
@end

@implementation OriginalSoundBoard



-(instancetype)init {
    
    if (self = [super init]) {
        
        _charactorArray = [NSMutableArray array];
        NSArray <NSString *>* charactors  = @[@"a",@"e",@"i",@"o",@"u"];
        for (int i = 0; i < charactors.count; i++) {
            
            [self addCharactor:charactors[i]];
        }
    }return self;
}

-(void)addCharactor:(NSString *)charactor{
    
    DisplayCharactor * display = [[DisplayCharactor alloc] init];
    display.charactor = charactor;
    [_charactorArray addObject:display];
}

-(id<Iterator>)creatIterator {
    
    if (!_iterator) {
        _iterator = [[OriginalSoundIterator alloc] initWithCharactorArray:_charactorArray];
    }
    return _iterator;

}

@end

辅音字符画板实现

#import "ConsonantBoard.h"
#import "CharactorArray.h"
#import "DisplayCharactor.h"
#import "ConsonantIterator.h"

@interface ConsonantBoard ()

@property (nonatomic ,strong)CharactorArray * charactorArray;

@property (nonatomic ,strong)id <Iterator> iterator;

@end

@implementation ConsonantBoard

-(instancetype)init {
    
    if (self = [super init]) {
        
        _charactorArray = [[CharactorArray alloc] init];
        
        NSArray <NSString *> * charactors  = @[@"b",@"c",@"d",@"f",@"g",@"h",@"j",@"k",@"l",@"m",@"n",@"p",@"q",@"r",@"s",@"t",@"v",@"w",@"x",@"y",@"z"];
        for (int i = 0; i < charactors.count; i++) {
            [self addCharactor:charactors[i]];
        }
        
    }return self;
}

-(void)addCharactor:(NSString *)charactor{
    
    DisplayCharactor * display = [[DisplayCharactor alloc] init];
    display.charactor = charactor;
    [_charactorArray addObject:display];
}

- (id<Iterator>)creatIterator {
    
    if (!_iterator) {
        
        _iterator = [[ConsonantIterator alloc] initWithCharactorArray:_charactorArray];
    }
    return _iterator;
}

@end

公司数组框架（随便写的一个，充当下自定义的聚合实现）

#import "CharactorArray.h"

@interface CharactorArray ()

@property (nonatomic ,strong)NSMutableArray * charactorArray;
@end

@implementation CharactorArray

-(instancetype)init {
    
    if (self = [super init]) {
        
        _charactorArray = [NSMutableArray array];
    }return self;
}

-(void)addObject:(id)charactor {
    
    [_charactorArray addObject:charactor];
}

-(void)removeObject:(id)charactor {
    
    [_charactorArray removeObject:charactor];
}

-(id)objectAtIndex:(NSInteger)index {
    
   return [_charactorArray objectAtIndex:index];
}

-(NSInteger)length {
    
    return _charactorArray.count;
}

@end

迭代器的具体使用

#import <Foundation/Foundation.h>
#import "Iterator.h"
#import "OriginalSoundBoard.h"
#import "ConsonantBoard.h"
#import "DisplayCharactor.h"

void showOnBoard(id<Iterator> iterator);

int main(int argc, const char * argv[]) {
    
    
    @autoreleasepool {
        
        id <Iterator> origin = [[[OriginalSoundBoard alloc] init] creatIterator];
        id <Iterator> consonant = [[[ConsonantBoard alloc] init] creatIterator];
        NSLog(@"原音之母:");
        showOnBoard(origin);
        NSLog(@"辅音之母:");
        showOnBoard(consonant);
    }
    return 0;
}

void showOnBoard(id<Iterator> iterator) {
    
    while ([iterator hasNext]) {
        DisplayCharactor * charactor = [iterator next];
        NSLog(@"%@",charactor.charactor);
    }
}

优点

1.支持以不同的方式遍历一个聚合的对象
2.简化聚合类
3.实现了类之间的松耦合

缺点

将遍历聚合对象的逻辑抽离出来，导致类增多，逻辑在一定程度变得复杂

iOS用自己的命名规则“枚举器/枚举”改写了迭代器模式。

---

iOS中的枚举器/枚举有多种
1.NSENumrator
NSMutableArray * array = [[NSMutableArray alloc] initWithCapacity:5];
NSEnumerator * rator = [array objectEnumerator];
id item ;
while (item = [rator nextObject]) {
}

2.块枚举
[array enumerateObjectsUsingBlock:^(id _Nonnull obj, NSUInteger idx, BOOL * _Nonnull stop) {

}];

3.快速枚举
forin

4.内部枚举
NSArray 有一个[array makeObjectsPerformSelector:@selector(selector)]; 该方法提供一个给数组中所有元素发送一个消息的功能，但是如果元素如果不能不能响应就会抛出异常。主要适用于不需要进行太多的运行时检查的简单操作

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