Iterator（迭代器模式）

提供一種方法順序訪問一個聚合對象中各個元素，而又不暴露該對象的內部表示。

iOS的Block迭代、數組迭代都是迭代器模式的典型實現。

設計迭代器之前需要搞清楚線性表中的順序表與鏈表的相關內容，參考：順序表與鏈表的區別

系統的迭代器

//創建集合對象
NSArray *datas = @[@"A", @"B", @"C", @"D"];
//從集合對象創建迭代器
NSEnumerator *iterator = [datas objectEnumerator];
//從集合對象中訪問元素
id arrayObj = nil;
while (arrayObj = [iterator nextObject]) {
    NSLog(@"arrayObj:%@", arrayObj);
}

打印結果：A,B,C,D

自定義迭代器

1. 創建節點類和鏈表類
   Node.h

@interface Node : NSObject
// 指向下一個節點
@property (strong, nonatomic) Node *nextNode;
// 節點的對象
@property (strong, nonatomic) id item;
// 類構造方法
+ (instancetype)nodeWithItem:(id)item;
@end

Node.m

@implementation Node
+ (instancetype)nodeWithItem:(id)item {
    // 這里之所以用self關鍵字來開辟對象，是考慮到有繼承問題
    Node *node = [[[self class] alloc] init];
    node.item = item;
    return node;
}
@end

LinkedList.h

@interface LinkedList : NSObject
// 頭節點
@property (strong, nonatomic, readonly) Node *headNode;
// 有幾個節點
@property (readonly, nonatomic) NSInteger numberOfNodes;
// 節點掛載的對象
- (void)addItem:(id)item;
@end

LinkedList.m

@interface LinkedList()
@property (strong, nonatomic) Node *headNode;
@property (assign, nonatomic) NSInteger numberOfNodes;
@end
@implementation LinkedList
- (instancetype)init {
    self = [super init];
    if (self) {
        self.headNode = [Node new];
    }
    return self;
}
- (void)addItem:(id)item {
    if (self.headNode == nil) {
        self.headNode = [Node nodeWithItem:item];
    } else {
        [self addItem:item node:self.headNode];
    }
}
- (void)addItem:(id)item node:(Node *)node {
    if (node.nextNode == nil) {
        node.nextNode = [Node nodeWithItem:item];
    } else {
        [self addItem:item node:node.nextNode];
    }
}
@end

2. 創建一個迭代器協議類
   IteratorProtocol.h

@protocol IteratorProtocol <NSObject>
@required
//下一個對象
- (id)nextObject;
@end

3. 創建迭代器
   LinkedListIterator.h

@interface LinkedListIterator : NSObject <IteratorProtocol>
// 返回一個鏈表迭代器的構造器
+ (instancetype)linkedListIteratorWithLinkedList:(LinkedList *)linkedList;
@end

LinkedListIterator.m

@interface LinkedListIterator()
@property (strong, nonatomic) LinkedList *linkedList;
@property (strong, nonatomic) Node *currentNode;
@end
@implementation LinkedListIterator
+ (instancetype)linkedListIteratorWithLinkedList:(LinkedList *)linkedList {
    LinkedListIterator *linkedListIterator = [LinkedListIterator new];
    linkedListIterator.linkedList = linkedList;
    linkedListIterator.currentNode = linkedList.headNode;
    return linkedListIterator;
}
- (id)nextObject {
    self.currentNode = self.currentNode.nextNode;
    return self.currentNode;
}
@end

4. 實現
   VC.m

LinkedList *linkList = [[LinkedList alloc] init];
[linkList addItem:@"A"];
[linkList addItem:@"B"];
[linkList addItem:@"C"];
[linkList addItem:@"D"];

// 從集合對象創建迭代器
LinkedListIterator *iterator = [LinkedListIterator linkedListIteratorWithLinkedList:linkList];

// 從集合對象中訪問元素
Node *node = nil;
while (node = [iterator nextObject]) {
    NSLog(@"---- %@", node.item);
}

設計圖