问题描述
我可以重用NSPredicate作为新的变数替代品吗？我的NSPredicate是相当简单：
Can I reuse NSPredicate for new variable substitute? My NSPredicate is rather simple:
NSPredicate *userPredicate = [NSPredicate predicateWithFormat:@"id == %@",userID];
userID是在运行时生成不同的值，现在对于每个userID我需要创建一个NSPredicate。是的，我可以重用我的NSPredicate吗？
The userID is generated at run-time with different value, right now for each userID I need to create a NSPredicate. So is it possible I can reuse my NSPredicate ?
推荐答案
是的，你可以做一个变量：
Yep, you can do this with a variable:
NSPredicate *userPredicate = [NSPredicate predicateWithFormat:@"id == $user"];
将该谓词保存在某个位置，然后在需要实际开始过滤内容时执行此操作：
Save that predicate somewhere, then do this when you need to actually start filtering stuff:
NSDictionary *sub = [NSDictionary dictionaryWithObject:user forKey:@"user"];
NSPredicate *filter = [userPredicate predicateWithSubstitutionVariables:sub];
这是一种更有效的重用谓词的方法，因为解析格式字符串相当昂贵）只发生一次。
This is a much more efficient way to reuse a predicate, because parsing the format string (which can be pretty expensive) only happens once.
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