NSPredicate和CoreData？ [英] NSPredicate and CoreData?

问题描述

如果直接添加到谓词，查询就可以正常工作

  NSPredicate * predicate = [NSPredicate predicateWithFormat：@author == ％@，author]; 
 [request setPredicate：predicate]; 
 [self.managedObjectContext executeFetchRequest：request error：nil]; 
  

如果创建并且传递到谓词

有没有解决方案？我不是将谓词本身传递给方法

作者是NSManagedObject的子类

  ***由于未捕获异常而终止应用程序'NSInvalidArgumentException'，原因：'无法解析格式字符串％@'
 
 [self fetchObjectsWithQuery ：[NSString stringWithFormat：@author ==％@，author]; 
 
  - （void）fetchObjectsWithQuery：（NSString *）query 
 {
 NSPredicate * predicate = [NSPredicate predicateWithFormat：@％@，query]; 
 [request setPredicate：predicate]; 
 [self.managedObjectContext executeFetchRequest：request error：nil]; 
} 
  

解决方案

/ p>

  NSString * query = [NSString stringWithFormat：@author ==％@，author] //（1）
  

和

  NSPredicate * predicate = [NSPredicate predicateWithFormat：@author ==％@，author] 
  

特别是占位符％@和％K有不同的含义。

（1）产生

 author ==<作者对象的文本描述> 
 

您不能在

中使用

  NSPredicate * predicate = [NSPredicate predicateWithFormat：@％@，query]; 
  

因此，您不能将谓词预先格式化为字符串。另一个例子来说明问题：

  [NSPredicate predicateWithFormat：@author == nil] 
 
  
 NSPredicate predicateWithFormat：％@，@author == nil] 
  
 p> 
The query works fine if directly added to a predicate
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"author == %@", author];
[request setPredicate:predicate];
[self.managedObjectContext executeFetchRequest:request error:nil];
The query doesn't work if created and then passed to a predicate

Is there a solution? I rather not pass the predicate itself to the method

Author is a subclass of NSManagedObject
*** Terminating app due to uncaught exception 'NSInvalidArgumentException', reason: 'Unable to parse the format string "%@"'

[self fetchObjectsWithQuery:[NSString stringWithFormat:@"author == %@", author];

- (void)fetchObjectsWithQuery:(NSString *)query
{
   NSPredicate *predicate = [NSPredicate predicateWithFormat:@"%@", query];
   [request setPredicate:predicate];
   [self.managedObjectContext executeFetchRequest:request error:nil];
}

解决方案
Format strings work differently in
NSString *query = [NSString stringWithFormat:@"author == %@", author] // (1)
and
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"author == %@", author]
In particular the placeholders "%@" and "%K" have different meanings.

(1) produces a string of the form
"author == <textual description of author object>"
which you cannot use in
NSPredicate *predicate = [NSPredicate predicateWithFormat:@"%@", query];
So you cannot pre-format predicates as strings. Another example to demonstrate the problem:
[NSPredicate predicateWithFormat:@"author == nil"]
works, but
[NSPredicate predicateWithFormat:"%@", @"author == nil"]
does not.

                        
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