当你合成一个子类的变量时会发生什么？

Question

我有一个超类和一个子类。我可以通过子类中的some_property (在超类中声明)访问变量self.some_property。

但是，如果我尝试使用_some_property直接访问实例变量，我将得到错误'Use of undeclared identifier _some_property...'。

使用@synthesize some_property = _some_property可以消除此警告。

当我重新合成这个属性的时候发生了什么？

Answer 1

您正在创建，另一个名为_some_property - and的 ivar也重写了getter方法，以返回这个新ivar的值。如果基类的@implementation (即它的_some_property ivar的隐式声明)在子类中的@synthesize站点上可见，编译器会给出一个错误。

(顺便说一句，别这样！)

您可以通过检查Obj运行时来演示：

#import <Foundation/Foundation.h>
#import <objc/runtime.h>

@interface Base : NSObject
@property id foo;
@end


@interface Derived : Base
@end

@implementation Derived

@synthesize foo=_foo; // the compiler doesn't know about Base's _foo yet, so this is OK...

- (instancetype)init {
    if ((self = [super init])) {
        _foo = @"I'm derived";
    }
    return self;
}
@end

@implementation Base  // after Derived to avoid the error
- (instancetype)init {
    if ((self = [super init])) {
        _foo = @"I'm base";
    }
    return self;
}
@end

int main(int argc, const char * argv[]) {
    @autoreleasepool {

        Derived *obj = [Derived new];
        NSLog(@"getter returns %@", obj.foo);

        unsigned int count = 0;

        // Examine Base ivars
        NSLog(@"Base ivars:");
        Ivar *ivars = class_copyIvarList([Base class], &count);
        for (unsigned int i = 0; i < count; i++) {
            NSLog(@" %s = %@", ivar_getName(ivars[i]), object_getIvar(obj, ivars[i]));
        }

        // Examine Derived ivars
        NSLog(@"Derived ivars:");
        ivars = class_copyIvarList([Derived class], &count);
        for (unsigned int i = 0; i < count; i++) {
            NSLog(@" %s = %@", ivar_getName(ivars[i]), object_getIvar(obj, ivars[i]));
        }
    }
    return 0;
}

输出：

getter returns I'm derived
Base ivars:
 _foo = I'm base
Derived ivars:
 _foo = I'm derived