访问者模式的目的与示例 [英] Visitor pattern's purpose with examples

问题描述

我对访问者模式及其用途感到非常困惑.我似乎无法想象使用这种模式或其目的的好处.如果可能的话，如果有人能用例子来解释那就太好了.

I'm really confused about the visitor pattern and its uses. I can't really seem to visualize the benefits of using this pattern or its purpose. If someone could explain with examples if possible that would be great.

推荐答案

从前...

class MusicLibrary {
    private Set<Music> collection ...
    public Set<Music> getPopMusic() { ... }
    public Set<Music> getRockMusic() { ... }
    public Set<Music> getElectronicaMusic() { ... }
}

然后您意识到您希望能够按其他类型过滤图书馆的收藏.您可以不断添加新的 getter 方法.或者您可以使用访客.

Then you realize you'd like to be able to filter the library's collection by other genres. You could keep adding new getter methods. Or you could use Visitors.

interface Visitor<T> {
    visit(Set<T> items);
}

interface MusicVisitor extends Visitor<Music>;

class MusicLibrary {
    private Set<Music> collection ...
    public void accept(MusicVisitor visitor) {
       visitor.visit( this.collection );
    }
}

class RockMusicVisitor implements MusicVisitor {
    private final Set<Music> picks = ...
    public visit(Set<Music> items) { ... }
    public Set<Music> getRockMusic() { return this.picks; }
}
class AmbientMusicVisitor implements MusicVisitor {
    private final Set<Music> picks = ...
    public visit(Set<Music> items) { ... }
    public Set<Music> getAmbientMusic() { return this.picks; }
}

您将数据与算法分开.您将算法卸载到访问者实现.您可以通过创建更多访问者来添加功能，而不是不断修改(和膨胀)保存数据的类.

You separate the data from the algorithm. You offload the algorithm to visitor implementations. You add functionality by creating more visitors, instead of constantly modifying (and bloating) the class that holds the data.

这篇关于访问者模式的目的与示例的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！