Question

以下来自互联网的代码似乎表现不正常。 NSPredicate返回对象值而不是该对象的属性值，该对象是一个字符串。（见下面的输出）。有没有人见过这个？

如果您认为这是正确的，那么我该如何打印字符串值？

// Person.h

#import <Foundation/Foundation.h>

@interface Person : NSObject
@property NSString *firstName;
@property NSString *lastName;
@property NSNumber *age;
@end

// ViewControll.m

#import "ViewController.h"
#import "Person.h"

@interface ViewController ()
@end

@implementation ViewController

- (void)viewDidLoad
{
    [super viewDidLoad];
    // Do any additional setup after loading the view, typically from a nib.

    NSArray *firstNames = @[ @"Alice", @"Bob", @"Charlie", @"Quentin" ];
    NSArray *lastNames = @[ @"Smith", @"Jones", @"Smith", @"Alberts" ];
    NSArray *ages = @[ @24, @27, @33, @31 ];

    NSMutableArray *people = [NSMutableArray array];
    [firstNames enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        Person *person = [[Person alloc] init];
        person.firstName = firstNames[idx];
        person.lastName = lastNames[idx];
        person.age = ages[idx];
        [people addObject:person];
    }];

    NSPredicate *bobPredicate = [NSPredicate predicateWithFormat:@"firstName = 'Bob'"];
    NSPredicate *smithPredicate = [NSPredicate predicateWithFormat:@"lastName = %@", @"Smith"];
    NSPredicate *thirtiesPredicate = [NSPredicate predicateWithFormat:@"age >= 30"];

    // ["Bob Jones"]
    NSLog(@"Bobs: %@", [people filteredArrayUsingPredicate:bobPredicate]);

    // ["Alice Smith", "Charlie Smith"]
    NSLog(@"Smiths: %@", [people filteredArrayUsingPredicate:smithPredicate]);

    // ["Charlie Smith", "Quentin Alberts"]
    NSLog(@"30's: %@", [people filteredArrayUsingPredicate:thirtiesPredicate]);
}

@end

输出

2014-08-12 17:51:31.526 NSPredicate[41013:60b] Bobs: (
    "<Person: 0x10931c720>"
)
2014-08-12 17:51:31.527 NSPredicate[41013:60b] Smiths: (
    "<Person: 0x10931cf60>",
    "<Person: 0x10931c110>"
)
2014-08-12 17:51:31.527 NSPredicate[41013:60b] 30's: (
    "<Person: 0x10931c110>",
    "<Person: 0x10931ca30>"
)

Answer 1

NSPredicate的行为应该如此。

NSPredicate将返回符合谓词规范的每个对象。它将不返回您正在寻找的属性值。

如果您认为这是正确的，那么我该如何打印字符串值？

简单：NSPredicate返回具有所需属性的对象，因此只需询问每个对象的属性！

NSPredicate *thirtiesPredicate = [NSPredicate predicateWithFormat:@"age >= 30"];

// ["Charlie Smith", "Quentin Alberts"]
NSArray *peopleOver30 = [people filteredArrayUsingPredicate:thirtiesPredicate];
for (int i = 0; i < peopleOver30.count; i++) {
    Person *person = peopleOver30[i];
    NSLog(@"First Name: %@, Last Name: %@, Age: %@", person.firstName, person.lastName, person.age);
}

预期产出：

 2014-08-12 17:51:31.526 NSPredicate[41013:60b] First Name: Charlie Last Name: Smith Age: 33
 2014-08-12 17:51:31.526 NSPredicate[41013:60b] First Name: Quentin Last Name: Alberts Age: 31

Answer 2

当您使用自定义类来保存数据时。当您尝试使用NSLog打印值时，它将打印person对象的地址您必须在Person类中使用description方法来打印所有属性

-(NSString *)description{

return @"FirstName: %@, LastName: %@, E-mail: %@", 
                    _firstName, _lastName, _email;

}

以下是相同的参考问题点击here！

NSPredicate返回对象而不是属性字符串值

2 个答案: