我从苹果文档中读到的内容保留将保留计数增加 1 ,发布将减少 1 。这对我来说非常清楚。
但是在复制和保留的情况下,我有点困惑。
让我用我正在尝试的代码解释一下。
属性(property) - -
@property(nonatomic, retain) NSMutableString *a;
@property(nonatomic, copy) NSMutableString *b;
@synthesize a = _a ,b = _b
a=[[NSMutableString alloc]initWithString:@"Hello Ankit"];
NSLog(@"a memory location A - %p", &a );
b=[[NSMutableString alloc]initWithString:@"Hello Nigam"];
NSLog(@"a memory location B- %p", &b );
c= [[NSMutableString alloc]initWithString:@"Ankit Nigam"];
NSLog(@"a memory location C %p",&c);
NSMutableString *temp =[[NSMutableString alloc]initWithString:@"hey"];
NSLog(@"temp = %@ %p",temp,&temp);
self.b = temp;
NSLog(@"B is now %@ %p",self.b,&b);
self.a = temp;
NSLog(@"A is now %@ %p",self.a,&a);
And i get the output as -- - -
2012-05-10 03:24:34.756 retainCountTest[2655:f803] a memory location A - 0x6d314fc
2012-05-10 03:24:34.757 retainCountTest[2655:f803] a memory location B- 0x6d31500
2012-05-10 03:24:34.764 retainCountTest[2655:f803] a memory location C 0x6d31504
2012-05-10 03:24:34.764 retainCountTest[2655:f803] temp = hey 0xbfffdd04
2012-05-10 03:24:34.764 retainCountTest[2655:f803] B is now hey 0x6d31500
2012-05-10 03:24:34.765 retainCountTest[2655:f803] A is now hey 0x6d314fc
但据我从文档中了解到,保留对象必须具有相同的内存地址,其中复制对象将创建一个具有不同内存位置的新对象。
再次当我将日志更改为 ---
self.b = temp;
NSLog(@"B is now %@ %p",self.b,&_b);
self.a = temp;
NSLog(@"A is now %@ %p",self.a,&_a);
It return me a complete different memory location for both the object.
2012-05-10 03:28:49.905 retainCountTest[2688:f803] a memory location A - 0x6d4a4ac
2012-05-10 03:28:49.906 retainCountTest[2688:f803] a memory location B- 0x6d4a4b0
2012-05-10 03:28:49.907 retainCountTest[2688:f803] a memory location C 0x6d4a4b4
2012-05-10 03:28:49.907 retainCountTest[2688:f803] temp = hey 0xbfffdd04
2012-05-10 03:28:49.908 retainCountTest[2688:f803] B is now hey 0x6d4a4c0
2012-05-10 03:28:49.908 retainCountTest[2688:f803] a is now hey 0x6d4a4bc
任何人都可以帮助我理解这些保留和复制的完整概念。也是为什么我会得到这些意想不到的结果。
非常感谢。
最佳答案
属性只是一个声明,允许使用 setter、getter 和点语法访问器(接口(interface)变量隐藏)。
它本身绝对没有任何作用,但允许您使用 -[myInstance myProperty]
获取变量或使用 -[myInstance setMyProperty:]
设置它(是的,方法名称自动分配给 -setProperty:
和 -property
)。
声明属性时,您分为三类 - 线程锁定、访问控制和内存管理。您只能为每个类别选择一个修饰符,如果您不选择一个,它会自动分配给一个。@property (<thread locking>, <access control>, <memory management>) id property;
第一类可以是 atomic
或 nonatomic
. atomic
修饰符强制对变量使用 @synchronized(myInstance) 锁,以实现线程安全。 nonatomic
不使用同步块(synchronized block),并且不是线程安全的。如果你都不使用,它会自动设置为 atomic
.
第二类可以是 readonly
或 readwrite
. readwrite
修饰符也允许修改属性,并允许自动生成 -setProperty: 方法。当readonly
使用了修饰符,不能使用 -setProperty:
方法。您必须使用对象内部的内部变量来直接设置变量。
第三类可以是 assign
, retain
, 和 copy
. assign
修饰符表示内部对象指针设置为传递给 -setProperty:
的指针信息。 retain
修饰符分配传递的指针并传递 -retain
到对象。copy
修改器对对象进行直接克隆 - 指向内存中新地址处的新对象的新指针。这通过调用 -copy
将内部对象指针设置为传递对象的副本。在传递的对象上。默认修饰符是 assign
,如果您没有在对象上设置内存管理类别修饰符,编译器会警告您 - 因为 assign
对象上的修饰符不受欢迎(除非明确声明)。
有关 -copy 的示例,请查看:
- (void)setProperty:(GXMyObject *)property {
// This points to the original passed object.
GXMyObject *original = property;
// This points to a copy of the passed object.
CGMyObject *copied = [property copy];
// This points to yet another copy of the passed object-
// Independent of the other copies and original.
_property = [property copy];
// The anotherProperty is now different on this copy
// than on the original and the other copies.
_property.anotherProperty = 4;
// This will prove that they are all individual objects.
NSLog(@"%p, %p, %p", original, copied, _property);
}
有一个可选的方法名称声明修饰符,用法如下:
getter = myCustomPropertyGetter
和 setter = myCustomPropertySetter:
(setter 方法名称末尾的冒号 :
是必需的,因为它表示必须传递一个参数)。这的后半部分是属性合成器或动态生成器。一旦声明了一个属性(例如,
myView
),如下所示:@property (nonatomic, retain) NSView *myView;
您可以:自己定义 setter 和 getter;
@synthesize
二传手和 setter/getter ; @dynamic
属性表示它存在于一个类别或主类中,或者可以在运行时添加(请注意,这不是一个有趣的想法,并且可能导致糟糕的运行时异常)。第一个示例,您自己编写方法将如下所示:
// In Apple's LLVM 3.1 Compiler, instance variables can be added
// within {} below the @implementation as well as the @interface,
// and in private categories (@interface GXMyClass ()) like before.
@implementation GXMyClass {
// The internal object pointer is prefixed with an _ to avoid name confusions.
NSView *_myView;
}
- (NSView *)myView {
return _myView;
}
- (void)setMyView:(NSView *)myView {
_myView = [myView retain];
}
@end
第二个例子是使用
@synthesize
自动合成它指示:@implementation GXMyClass
// In the new Apple LLVM 3.1 Clang compiler, the = operator when used
// next to the @synthesize directive declares an internal private
// variable and automatically sets to that variable.
@synthesize myView = _myView;
// The internal variable name is now myOtherView, because we did not use the
// = operator to assign an internal variable name to the property.
@synthesize myOtherView;
@end
在最后一个示例中,也许是最令人困惑的,因为它需要使用 @dynamic 指令,您需要添加一些类别或运行时方法:
@interface GXMyClass (InternalMethods)
@end
@implementation GXMyClass
// The = assignment operator does not work here.
@dynamic myView;
@end
@implementation GXMyClass (InternalMethods)
- (NSView *)myView {
return [self methodThatReturnsAnNSView];
}
- (void)setMyView:(NSView *)myView {
[self methodThatAcceptsAnNSViewArgument:myView];
}
@end
@property
声明要求存在上述三个声明之一-它自己不做任何事情。然而,它允许的是点语法访问器(用于设置和获取属性的类 Java 访问器)。例如,
@property (copy) NSString *myName;
可以使用 -[myObject myName]
访问并使用 -[myObject setMyName:]
设置.现在可以使用
myObjectInstance.myName = @"Bob";
进行设置并开始使用 myObjectInstance.myName
.利用上述所有概念,可以创建一个这样的对象:// The GXBufferQueue is a queue which buffers all requests, till they are read
// asynchronously later. The backing store is an NSMutableArray to which all
// buffer writes are appended to, and from which the first object is pulled and
// returned when the buffer is read to.
@interface GXBufferQueue
@property (nonatomic, readwrite, copy, setter = write:, getter = read) id buffer;
+ (GXBufferQueue *)queue;
@end
@implementation GXBufferQueue {
// This queue is an internal array and is 'tacked on' to the @implementation
// so no others can see it, and it can be marked @private so subclasses cannot
// use it. It is also good code practice to have @interfaces composed of only
// @properties, setters, and getters, rather than expose internal variables.
NSMutableArray *_internalQueue;
}
+ (GXBufferQueue *)queue {
return [[[GXBufferQueue alloc] init] autorelease];
}
- (id)init {
if((self = [super init])) {
_internalQueue = [[NSMutableArray alloc] init];
}
}
- (void)write:(id)buffer {
[_internalQueue addObject:buffer];
}
- (id)read {
if(!(_internalQueue.count > 0)) return nil;
id buffer = [_internalQueue objectAtIndex:0];
[_internalQueue removeObjectAtIndex:0];
return buffer;
}
@end
注意:此代码未经过任何测试。
既然你有一个 GXBufferQueue,下面所有的工作:
GXBufferQueue *queue = [GXBufferQueue queue];
// Option One: using the traditional message syntax:
[queue write:@"This will be now added to the buffer."];
NSLog(@"Now the string written to the queue will be read \
and removed from the queue, like a stack pop. ", [queue read]);
// Option Two: using the new dot-syntax accessors:
queue.buffer = @"As clunky as this looks, it works the same as above.";
NSLog(@"These lines work just the same as the ones above: ", queue.buffer);
正如您所看到的,属性有很多可能,而且除了变量声明之外,它们还可以做更多的事情。如果有任何问题或社区希望我在帖子中添加/纠正的任何内容,请发表评论! :D
关于iphone - objective-c 中的属性。复制并保留,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/10524772/