解析和更改 NSPredicate

Question

我必须将数据从以前的应用程序版本迁移到新版本。这也会影响用户保存的一些谓词（NSPredicate实例），这意味着我必须以编程方式更改它们。

目前，我尝试解析我用[NSPredicate predicateFormat]得到的字符串并手动更改一些表达式。例如oldKeyPath == "something" newKeyPath == "something"。但这感觉就像一个黑客，我很好奇是否有更好的方法？

我阅读了Apple关于使用NSPredicate和NSExpression编程的文档。有很多方法可以从NSExpressions中组合NSPredicate对象。我希望找到相反的方法从NSPredicate中获取NSExpression对象。我错过了什么吗？

谢谢你的任何提示。

溶液

多亏了Martin R，我才能在NSPredicate上创建一个类别，允许我为表达式注入修改。

@implementation NSPredicate (ExtractComparisions)
- (NSPredicate *)predicateByChangingComparisionsWithBlock:(NSPredicate *(^)(NSComparisonPredicate *))block {
  if ([self isKindOfClass: [NSCompoundPredicate class]]) {
    NSCompoundPredicate *compPred = (NSCompoundPredicate *)self;
    NSMutableArray *predicates = [NSMutableArray array];
    for (NSPredicate *predicate in [compPred subpredicates]) {
      NSPredicate *newPredicate = [predicate predicateByChangingComparisionsWithBlock: block];
      if (newPredicate != nil)
        [predicates addObject: newPredicate];
    }
    return [[[NSCompoundPredicate alloc] initWithType: compPred.compoundPredicateType
                                       subpredicates: predicates] autorelease];
  } if ([self isKindOfClass: [NSComparisonPredicate class]]) {
    return block((NSComparisonPredicate *)self);
  }
  return self;
}
@end

这是如何使用它的示例代码

- (void)applicationDidFinishLaunching:(NSNotification *)aNotification
{
  NSPredicate *predicate = [NSPredicate predicateWithFormat: @"key.path like %@ and count > 17", @"Hello"];
  NSPredicate *newPredicate = [predicate predicateByChangingComparisionsWithBlock:
                               ^NSPredicate *(NSComparisonPredicate *cp) {
    NSExpression *left = [cp leftExpression];
    NSExpression *right = [cp rightExpression];
    if ([[cp leftExpression] expressionType] == NSKeyPathExpressionType) {
      NSString *keyPath = [[cp leftExpression] keyPath];
      if ([keyPath isEqualToString: @"key.path"])
        left = [NSExpression expressionForKeyPath: @"key.new.path"];
      return [NSComparisonPredicate predicateWithLeftExpression: left
                                                rightExpression: right
                                                       modifier: cp.comparisonPredicateModifier
                                                           type: cp.predicateOperatorType
                                                        options:cp.options];
    }
    return cp;
  }];
  NSLog(@"Before: %@", predicate);
  NSLog(@"After: %@", newPredicate);
}

Answer 1

（这只是一个想法。每个谓词都是子类的实例，例如 NSCompoundPredicate或NSComparisonPredicate.所以你可以检查实际谓词的类，将其强制转换为该类的对象，然后检查其属性。如有必要，使用子谓词或重复该过程表达 式。

以下只是一个示例，一个简单的复合谓词可能是以这种方式"解剖"。它不是检查所有情况的通用解决方案，但可能会为你指明正确的方向。

NSPredicate *p0 = [NSPredicate predicateWithFormat:@"foo = 'bar' AND count > 17"];
if ([p0 isKindOfClass:[NSCompoundPredicate class]]) {
    NSCompoundPredicate *p0a = (NSCompoundPredicate *)p0;
    NSCompoundPredicateType type0 = p0a.compoundPredicateType;  // NSAndPredicateType
    NSPredicate *p1 = p0a.subpredicates[0];                     // foo = 'bar'
    NSPredicate *p2 = p0a.subpredicates[1];                     // count > 17
    if ([p1 isKindOfClass:[NSComparisonPredicate class]]) {
        NSComparisonPredicate *p1a = (NSComparisonPredicate *)p1;
        NSPredicateOperatorType type1 = p1a.predicateOperatorType;  // NSEqualToPredicateOperatorType
        NSExpression *e3 = p1a.leftExpression;                  // foo, NSKeyPathExpression
        NSExpression *e4 = p1a.rightExpression;                 // "bar", NSConstantValueExpression
    }
    if ([p2 isKindOfClass:[NSComparisonPredicate class]]) {
        NSComparisonPredicate *p2a = (NSComparisonPredicate *)p2;
        NSPredicateOperatorType type2 = p2a.predicateOperatorType;  // NSGreaterThanPredicateOperatorType
        NSExpression *e5 = p2a.leftExpression;                  // count, NSKeyPathExpression
        NSExpression *e6 = p2a.rightExpression;                 // 17, NSConstantValueExpression
    }
}