objective-c - 键/值观察未触发定义为 : @property (readwrite, 的属性 retain) __attribute__((NSObject)) CGImageRef thumbnailImage

Question

我有一个属性定义如下的类:

// Interface
@property (retain) __attribute__((NSObject)) CGImageRef thumbnailImage;

// Implementation
@synthesize thumbnailImage;

问题是 self.thumbnailImage = newCGImageRef; 不会触发键/值观察事件，并且永远不会调用 observeValueForKeyPath:...。

我用同一个类的另一个属性 (BOOL) 测试了代码，它对那个属性工作得很好。这不应该也适用于上面的属性吗？

注意:我尝试手动设置该属性([self setValue:newValue forKey:@"thumbnailImage"]，导致以下错误:

setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key thumbnailImage.

Answer 1

__attribute__((NSObject))告诉编译器将结构指针视为对象，因此这允许您保留/释放对象(通常用于 block )。此属性对 CFImageRef 是一个结构指针这一事实没有影响，尽管 KVC 会包装结构，但它不会自动包装结构指针。要解决您的问题，您需要拦截未定义键上的 KVC 以设置和返回正确的对象，或者只使用 UIImage 代替。

KVC Documentation:

Automatic wrapping and unwrapping is not confined to NSPoint, NSRange, NSRect, and NSSize—structure types (that is, types whose Objective-C type encoding strings start with {) can be wrapped in an NSValue object.

测试CF_KVC.h

#import <Foundation/Foundation.h>

typedef struct
{
    int x, y, z;
} AStruct;

@interface TestCF_KVC : NSObject {}

@property (nonatomic, retain) NSString *text;
@property (nonatomic, assign) AStruct aStruct;
@property (nonatomic, retain) __attribute__((NSObject)) CFStringRef cftext;

-(void)test;

@end

测试CF_KVC.m

#import "TestCF_KVC.h"

@implementation TestCF_KVC

@synthesize text, aStruct, cftext;

-(void)setValue:(id)value forUndefinedKey:(NSString *)key
{
    NSLog(@"undefined key set for %@", key);
    if([key isEqualToString:@"cftext"])
    {
        self.cftext = (CFStringRef)value;
    }
    else
    {
        [super setValue:value forUndefinedKey:key];
    }
}

-(id)valueForUndefinedKey:(NSString *)key
{
    NSLog(@"undefined key get for %@", key);
    if([key isEqualToString:@"cftext"])
    {
        return (NSString*)self.cftext;
    }

    return [super valueForUndefinedKey:key];
}

-(void)test
{
    NSString *txt = @"text worked";
    AStruct astr = { .x=1, .y=5, .z=10 };
    CFStringRef cftxt = (CFStringRef)@"cftext worked";

    //Test a normal NSString for KVC
    [self setValue:txt forKey:@"text"];
    txt = [self valueForKey:@"text"];
    NSLog(@"text[%s]: %@", @encode(NSString), [self valueForKey:@"text"]);

    //Test a struct for KVC
    NSValue *value = [NSValue value:&astr withObjCType:@encode(AStruct)];
    [self setValue:value forKey:@"aStruct"];
    [[self valueForKey:@"aStruct"] getValue:&astr];
    NSLog(@"aStruct[%s]: %d %d %d", @encode(AStruct), aStruct.x, aStruct.y, aStruct.z);

    //Test a Core Foundation type for KVC
    [self setValue:(NSString*)cftxt forKey:@"cftext"];
    cftxt = (CFStringRef)[self valueForKey:@"cftext"];
    NSLog(@"cftext[%s]: %@", @encode(CFStringRef), (NSString*)cftxt);
}

@end

记录调用 -test 的输出:

text[{NSString=#}]: text worked
aStruct[{?=iii}]: 1 5 10
undefined key set for cftext
undefined key get for cftext
cftext[^{__CFString=}]: cftext worked