NSPredicate 表达式中 SUBQUERY 的快速解释

发布于 2024-09-25 08:33:55 字数 192 浏览 9 评论 0原文

关于 Apple 的 SUBQUERY 关键字的文档似乎为零，我在 SO 或 Google 上找不到关于它的简单解释。这是一个阴谋！ ;)

拜托，内部圈子里的人可以快速解释一下它的语法，以便我可以使用它吗？

SUBQUERY(Bs, $x, $x IN %@)

谢谢

原文

There appears to be zero documentation about the SUBQUERY keyword from Apple and I can't find a simple explanation about it on SO or on Google. It's a conspiracy! ;)

Please, could someone from the inner-circle please just provide a quick explanation of its syntax so I can use it?

SUBQUERY(Bs, $x, $x IN %@)

Thanks

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屋顶上的小猫咪 2024-10-02 08:33:55

对于不太明白文档内容的人来说，SUBQUERY 本质上是这样的：

SUBQUERY(collection, variableName, predicateFormat)

并且可以（简单地）像这样实现：

id resultingCollection = ...; //a new collection, either a mutable set or array
NSMutableDictionary * substitutions = [NSMutableDictionary dictionary];
NSPredicate * p = [NSPredicate predicateWithFormat:predicateFormat];
for (id variable in collection) {
  [substitutions setObject:variable forKey:variableName];
  NSPredicate * filter = [p predicateWithSubstitutionVariables:substitutions];
  if ([filter evaluateWithObject:collection] == YES) {
    [resultingCollection addObject:variable];
  }
}
return resultingCollection;

所以简而言之，SUBQUERY code> 基本上是获取一个对象集合，并根据 SUBQUERY 的谓词表达式过滤出各种对象，然后返回结果集合。（谓词本身可以包含其他 SUBQUERY）

示例：

NSArray * arrayOfArrays = [NSArray arrayWithObjects:
                           [NSArray arrayWithObjects:....],
                           [NSArray arrayWithObjects:....],
                           [NSArray arrayWithObjects:....],
                           [NSArray arrayWithObjects:....],
                           [NSArray arrayWithObjects:....],
                           [NSArray arrayWithObjects:....],
                           nil];
NSPredicate * filter = [NSPredicate predicateWithFormat:@"SUBQUERY(SELF, $a, $a.@count > 42)"];
NSArray * filtered = [arrayOfArrays filteredArrayUsingPredicate:filter];
//"filtered" is an array of arrays
//the only arrays in "filtered" will have at least 42 elements each

And for people who don't quite get what the documentation is saying, a SUBQUERY is essentially this:

SUBQUERY(collection, variableName, predicateFormat)

And could (simplistically) be implemented like this:

id resultingCollection = ...; //a new collection, either a mutable set or array
NSMutableDictionary * substitutions = [NSMutableDictionary dictionary];
NSPredicate * p = [NSPredicate predicateWithFormat:predicateFormat];
for (id variable in collection) {
  [substitutions setObject:variable forKey:variableName];
  NSPredicate * filter = [p predicateWithSubstitutionVariables:substitutions];
  if ([filter evaluateWithObject:collection] == YES) {
    [resultingCollection addObject:variable];
  }
}
return resultingCollection;

So in a nutshell, a SUBQUERY is basically taking a collection of objects and filtering out various objects based on the predicate expression of the SUBQUERY, and returning the resulting collection. (And the predicate itself can contain other SUBQUERYs)

Example:

NSArray * arrayOfArrays = [NSArray arrayWithObjects:
                           [NSArray arrayWithObjects:....],
                           [NSArray arrayWithObjects:....],
                           [NSArray arrayWithObjects:....],
                           [NSArray arrayWithObjects:....],
                           [NSArray arrayWithObjects:....],
                           [NSArray arrayWithObjects:....],
                           nil];
NSPredicate * filter = [NSPredicate predicateWithFormat:@"SUBQUERY(SELF, $a, $a.@count > 42)"];
NSArray * filtered = [arrayOfArrays filteredArrayUsingPredicate:filter];
//"filtered" is an array of arrays
//the only arrays in "filtered" will have at least 42 elements each

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