在predicateWithBlock中的块的bindings参数是什么？ [英] What is the bindings parameter for the block in predicateWithBlock: used for?

问题描述

+ [NSPredicate predicateWithBlock：] 的声明如下所示：

  +（NSPredicate *）predicateWithBlock：（BOOL（^）（id calculatedObject，NSDictionary * bindings））块
  
 
 $ b b 
 Apple的文档中对块的第二个参数 bindings 表示它是：
 
 
替换变量字典。字典必须包含接收器中所有变量的键值对。
 
 
我不知道为什么需要这个参数 -  - 也没有我看到它被用在任何地方。为什么会有？
 
 
 
此外，当使用基于块的谓词时，我需要查看 bindings  code>  -  [NSArray filteredArrayUsingPredicate：] ？
解决方案
 / p> 
 
 
 
 
您还可以创建包含变量的谓词，以便可以在将具体值替换为$ b之前预定义
谓词$ b运行时。在Mac OS X v10.4中，对于使用变量的谓词，
求值是一个两步过程（参见
 predicateWithSubstitutionVariables：和evaluateWithObject :)。在Mac 
 OS X v10.5及更高版本中，您可以使用
 evaluateWithObject：substitutionVariables :,它结合了这些步骤。
 
 
 
 $ b b 
然后查看谓词语法文档。
 
 
 
 
 
 
如果您以后调用 evaluateWithObject：substitutionVariables：，所述字典将被传递给你的块。这使得相当通用的谓词创建，其中可以传递得到的谓词，并且可以使用一致的替换语言进行评估。
 
The declaration for +[NSPredicate predicateWithBlock:] looks like this:
+ (NSPredicate *)predicateWithBlock:(BOOL (^)(id evaluatedObject, NSDictionary *bindings))block
Apple's documentation for the second parameter to the block, bindings, says that it is:

  
The substitution variables dictionary. The dictionary must contain key-value pairs for all variables in the receiver.
I can't figure out why this parameter is needed -- nor have I seen it being used anywhere. Why is it there?

Also, do I need to look inside bindings when using a block based predicate with -[NSArray filteredArrayUsingPredicate:]?
解决方案
See the class documentation:

  
You can also create predicates that include variables, so that the
  predicate can be pre-defined before substituting concrete values at
  runtime. In Mac OS X v10.4, for predicates that use variables,
  evaluation is a two step process (see
  predicateWithSubstitutionVariables: and evaluateWithObject:). In Mac
  OS X v10.5 and later, you can use
  evaluateWithObject:substitutionVariables:, which combines these steps.
Then check out the predicate syntax docs.



If you were to subsequently invoke evaluateWithObject:substitutionVariables:, said dictionary would be passed to your block.   This enables rather generic predicate creation where the resulting predicate can be passed around and a consistent substitution language can be used for evaluation.

                        
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