开发该选择Blocks还是Delegates

约定不等于承诺〃 2022-08-09 09:48 49阅读 0赞

原文链接：[http://www.cnblogs.com/qiyer/p/4844564.html][http_www.cnblogs.com_qiyer_p_4844564.html]

有人问了我一个很棒的问题，我把这个问题总结为：“开发过程中该选择 blocks or delegates？当我们需要实现回调的时候，使用哪一种方式比较合适呢？”

一般在这种情况下，我喜欢问我自己：“如果问题交给Apple，他会怎么做呢？”当然，我们都知道Apple肯定知道怎么做，因为从某一层面上看，Apple的文档就是一本用来指导我们如何使用设计模式的指导书。

因此我们需要去研究一下Apple分别是在什么情况下使用delegate和block，如果我们发现了Apple做这种选择的套路，我们就可以构建出一些规则，可以帮助在我们在自己的代码中做相同选择。

要找出Apple使用delegate的场景很简单，我们只要搜索官方文档中的“delegate”，就会获取到很多使用delegation的类。

但是搜索Apple中有关使用blocks的文档就有点困难了，因为我们不能直接搜索文档中的“^” 。然而，Apple声明方法时有很好的命名习惯（这也是我们精通iOS开发的一项必备技能）。例如：一个以NSString为参数的方法，方法的selector就会有String字眼，像initWithString;dateFromString;StartSpeaingString。

当Apple的方法使用block，这个方法将会有“Handler”,“Completion”或者简单的“Block”作为selector;因此我们可以在标准的iOS API文档中搜索这些关键词，用以构建一个可信任的block用例列表。

1.大多数delegate protocols 都拥有几个消息源。

以我正在看的GKMatch为例(A GKMatch object provides a peer-to-peer network between a group of devices that are connected to Game Center，是iOS API中用来提供一组设备连接到Game Center点对点网络的对象)。从这个类中可以看到消息的来源分别是：当从其他玩家那接收到数据、当玩家切换了状态、当发生错误或者当一个玩家应该被重新邀请。这些都是不同的事件。如果Apple在这里使用block，那么可能会有以下两种解决方式：

*  可以对应每一个事件注册相应的block，显然这种方式是不合理的。（ If someone writes a class that does this in Objective-C, they are probably an asshole.）
 *  创建一个可以接受任何可能输入的block

很明显这种方式既不简便又不易读，所以你可能从未看过这样的解决方案。如果你看过这样的解决方式，但是这显然是一个糟糕至极的代码行，你不会有精力去维护这个。

因此，我们可以得出一个结论：如果对象有超过一个以上不同的事件源，使用delegation。

2.一个对象只能有一个delegate

由于一个对象只能有一个delegate，而且它只能与这个delegate通信。让我们看看CLLocationManager 这个类，当发现地理位置后，location manager 只会通知一个对象（有且只有一个）。当然，如果我们需要更多的对象去知道这个更新，我们最好创建其他的location manager。

这里有的人可能想到，如果CLLocationManager是个单例呢？如果我们不能创建CLLocationManager的其他实例，就必须不断地切换delegate指针到需要地理数据的对象上（或者创建一个只有你理解的精密的广播系统）。因此，这样看起来，delegatetion在单例上没有多大意义。

关于这点，最好的印证例子就是UIAccelerometer。在早期版本的iOS中，单例的 accelerometer 实例有一个delegate，导致我们必须偶尔切换一下。这个愚蠢的问题在之后的IOS版本被修改了，现在，任意一个对象都可以访问CMMotionManager block，而不需要阻止其他的对象来接收更新。

因此，我们可以得出另一个结论：“如果一个对象是单例，不要使用delegation”。

3.一般的delegate方法会有返回值

如果你观察一些delegate方法（几乎所有的dataSource方法）都有一个返回值。这就意味着delegating对象在请求某些东西的state(对象的值，或者对象本身)，而一个block则可以合理地包含state或者至少是推断state，因此block真正是对象的一个属性。

让我们思考一下一个有趣的场景，如果向一个block提问：“What do you think about Bob？”。block可能会做两件事情：发送一个消息去捕获对象并询问这个对象怎么看待Bob，或者直接返回一个捕获的值。如果返回了一个对象的响应，我们应该越过这个block直接获取这个对象。如果它返回了一个捕获的值，那么这应该是一个对象的属性。

从以上的观察，我们可以得出结论：如果对象的请求带有附加信息，更应该使用delegation

4.过程 vs 结果（Process vs. Results）

如果查看NSURLConnectionDelegate 以及 NSURLConnectionDataDelegate，我们在可以protocol中看到这样的消息：我将要做什么（如： willSendRequest，将要发送请求）、到目前为止我知道的信息（如：canAuthenticateAgainstProtectionSpace）、我已经完成这些啦（ didReceiveResponse，收到请求的回复，即完成请求）。这些消息组成一个流程，而那些对流程感兴趣的delegate将会在每一步得到相应的通知。

当我们观察handler和完整的方法时，我们发现一个block包含一个响应对象和一个错误对象。显然这里没有任何有关“我在哪里，我正在做什么的”的交互。

因此我们可以这样认为，delegate的回调更多的面向过程，而block则是面向结果的。如果你需要得到一条多步进程的通知，你应该使用delegation。而当你只是希望得到你请求的信息（或者获取信息时的错误提示），你应该使用block。（如果你结合之前的3个结论，你会发现delegate可以在所有事件中维持state，而多个独立的block确不能）

从上面我们可以得出两个关键点。首先，如果你使用block去请求一个可能失败的请求，你应当只使用一个block。我们可以看到如下的代码：

上面代码的可读性明显比下面block的可读性差（作者说这个是他不谦虚的观点，其实个人认为没有那么严重）

<table style="border-collapse:collapse; border-spacing:0px; border:1px solid silver; width:848px; word-break:break-word; margin:0px!important; padding:0px!important; bottom:auto!important; float:none!important; height:auto!important; left:auto!important; line-height:1.1em!important; outline:0px!important; overflow:visible!important; position:static!important; right:auto!important; top:auto!important; vertical-align:baseline!important; font-family:Consolas,'Bitstream Vera Sans Mono','Courier New',Courier,monospace!important; font-size:12px!important; min-height:auto!important; background:none!important"> 
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      7 
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