設計模式之迭代器模式

迭代器模式 Iterator

Intro

迭代器模式,提供一種方法順序訪問一個聚合對象中的各個元素,而又不暴露該對象的內部表示。

迭代器模式是分離了集合對象的遍歷行為,抽象出一個迭代器類來負責,這樣既可以做到不暴露集合的內部結構,又可以讓外部程式碼透明地訪問集合內部的數據。

使用場景

當你需要訪問一個聚集對象,而且不管這些對象是什麼都需要遍歷的時候,你就應該考慮用迭代器模式。

當你需要對聚集有多種方式遍歷時,可以考慮用迭代器模式。

實現方式

如果要自己實現需要有下面幾個類:

一個迭代器抽象類(或介面)

一個聚集抽象類(或介面)

具體實現的迭代器類

具體實現的聚集類

Sample

public abstract class Iterator
{
    public abstract object First();

    public abstract object Next();

    public abstract bool IsDone();

    public abstract object CurrentItem();
}

public class ConcreteIterator : Iterator
{
    private readonly ConcreteAggregate _aggregate;
    private int _current = 0;

    public ConcreteIterator(ConcreteAggregate aggregate) => _aggregate = aggregate;

    public override object First()
    {
        return _aggregate[0];
    }

    public override object Next()
    {
        _current++;
        return _current >= _aggregate.TotalCount ? null : _aggregate[_current];
    }

    public override bool IsDone() => _current >= _aggregate.TotalCount;

    public override object CurrentItem() => _aggregate[_current];
}

public abstract class Aggregate
{
    /// <summary>
    /// 創建迭代器
    /// </summary>
    /// <returns></returns>
    public abstract Iterator CreateIterator();
}

public class ConcreteAggregate : Aggregate
{
    private readonly IList<object> _items = new List<object>();

    public override Iterator CreateIterator()
    {
        return new ConcreteIterator(this);
    }

    public int TotalCount => _items.Count;

    public object this[int index]
    {
        get => _items[index];
        set => _items.Insert(index, value);
    }
}


var aggregate = new ConcreteAggregate
{
    [0] = "大鳥",
    [1] = "小菜",
    [2] = "行李",
    [3] = "老外",
    [4] = "公交員工",
    [5] = "小偷"
};

Iterator iterator = new ConcreteIterator(aggregate);
do
{
    Console.WriteLine($"{iterator.CurrentItem()} 請買車票");
    iterator.Next();
} while (!iterator.IsDone());

More

需要注意的是,使用迭代器模式遍歷集合時,不要對集合進行增加元素或者刪除元素操作

在 C# 中實現 IEnumerable 介面就可以比較方便的實現一個迭代器,foreach 就是迭代器的一個語法糖

來看一下 IEnumerabe 的定義:

// 聚集抽象
public interface IEnumerable
{
    /// <summary>Returns an enumerator that iterates through a collection.</summary>
    /// <returns>An <see cref="T:System.Collections.IEnumerator" /> object that can be used to iterate through the collection.</returns>
    IEnumerator GetEnumerator();
}

// 迭代器抽象
public interface IEnumerator
{
    /// <summary>Advances the enumerator to the next element of the collection.</summary>
    /// <returns>
    /// <see langword="true" /> if the enumerator was successfully advanced to the next element; <see langword="false" /> if the enumerator has passed the end of the collection.</returns>
    /// <exception cref="T:System.InvalidOperationException">The collection was modified after the enumerator was created.</exception>
    bool MoveNext();

    /// <summary>Gets the element in the collection at the current position of the enumerator.</summary>
    /// <returns>The element in the collection at the current position of the enumerator.</returns>
    object Current { get; }

    /// <summary>Sets the enumerator to its initial position, which is before the first element in the collection.</summary>
    /// <exception cref="T:System.InvalidOperationException">The collection was modified after the enumerator was created.</exception>
    void Reset();
}

最後留個思考題給你,List 在 foreach 的時候如果刪除一個元素會發生什麼?內部是怎麼樣實現的呢? 可以自己實踐一下

Reference