将NSPredicate子类化为add运算符 [英] Subclassing NSPredicate to add operator

问题描述

Cocoa定义了谓词类（ NSPredicate ， NSExpression 等）在Cocoa中指定查询 ^谓词编程 。

NSComparisonPredicate 已处理14个运算符（ NSPredicateOperatorType ），但我想添加，说，时间运算符...或运算符来表示诸如：

• 的值 连续天（三元运算符）

$ b b

显然，我需要自己实现这些，并且执行这些查询的数据模型将必须支持这些操作符。但是，有没有一种方法来实现它，并从现有的NSPredicate类中受益？由于运算符被定义为枚举，我怀疑我可以在这方面延伸。

解决方案

有 支出 a lot of 时间 播放 周围 与 NSPredicate ，我不确定这是最好的想法。 / p>

理论上，您将子类 NSPredicate ，创建新的初始化器和属性，然后覆盖 -evaluateWithObject：substitutionVariables：方法进行自定义比较。

实际上，

您可以考虑使用 FUNCTION（）。 我写了一篇博客文章 FUNCTION ，以及如何使用 NSExpression ，因此使用 NSPredicate 。个人而言，我可能会用这个，因为那时你仍然可以使用 + predicateWithFormat：语法创建 NSPredicate 。创建子类以添加运算符必然会阻止您使用内置的解析器。

Cocoa defines predicate classes (NSPredicate, NSExpression, etc.) which "provide a general means of specifying queries in Cocoa" ^{Predicate Programming}. This set of classes describes what I need but with one little short-coming : I'd like additional operators.

NSComparisonPredicate already handles 14 operators (NSPredicateOperatorType) but I would like to add, say, temporal operators... or operators to represent things such as:

• " variable has at least n entries" (binary operator)
• " variable has value for, at most, n consecutive days" (ternary operator)

Obviously, I would need to implement these myself and the data model on which such queries are performed will have to support these operators. But, is there a way to implement it and benefit from the existing NSPredicate classes? Since operators were defined as an enum, I doubt I can extend on that front. Or am I completely missing the boat on this?!

解决方案

Having spent a lot of time playing around with NSPredicate, I'm not sure this is the greatest idea.

Theoretically, you'd subclass NSPredicate, create your new initializer and properties, and then override the -evaluateWithObject:substitutionVariables: method to do your custom comparison.

Practically, it's probably a lot more difficult than that.

You might consider using FUNCTION() instead. I wrote a blog post about FUNCTION a while ago and how it plays with NSExpression and therefore with NSPredicate. Personally, I'd probably go with this, because then you could still use the +predicateWithFormat: syntax to create the NSPredicate. Creating a subclass to add an operator would necessarily prevent you from using the built-in parser.

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