7.Runtime官方文档学习--声明属性

官方文档

When the compiler encounters property declarations (see Declared Properties in The Objective-C Programming Language), it generates descriptive metadata that is associated with the enclosing class, category or protocol. You can access this metadata using functions that support looking up a property by name on a class or protocol, obtaining the type of a property as an @encode string, and copying a list of a property's attributes as an array of C strings. A list of declared properties is available for each class and protocol.

当编译器遇到属性声明（参阅The Objective-C Programming Language中的 Declared Properties）时，它会生成和封闭类，类别或协议相关联的描述性元数据。你可以使用支持在类或协议中通过名称查找属性，获取属性类型作为@encode()字符串，复制属性列表作为C字符串数组的函数来访问这个元数据。每个类和协议都有一个声明的属性列表。

1. 属性类型和函数

The Property structure defines an opaque handle to a property
descriptor.

Property结构体定义了属性描述符的不透明句柄。

typedef struct objc_property *Property;

You can use the functions class_copyPropertyList and protocol_copyPropertyList to retrieve an array of the properties associated with a class (including loaded categories) and a protocol respectively:

你可以使用函数class_copyPropertyList和protocol_copyPropertyList来分别获取类（包括加载的类别）和协议相关联的属性数组。

objc_property_t *class_copyPropertyList(Class cls, unsigned int *outCount)
objc_property_t *protocol_copyPropertyList(Protocol *proto, unsigned int *outCount)

例如，给出以下类的声明：

@interface Lender : NSObject {
    float alone;
}
@property float alone;
@end

你可以这样获取属性列表：

id LenderClass = objc_getClass("Lender");
unsigned int outCount;
objc_property_t *properties = class_copyPropertyList(LenderClass, &outCount);

你可以使用property_getName查出属性名称：

const char *property_getName(objc_property_t property)

你可以使用class_getProperty和protocol_getProperty通过给出的名称分别获取一个类和协议中的属性的引用。

objc_property_t class_getProperty(Class cls, const char *name)
objc_property_t protocol_getProperty(Protocol *proto, const char *name, BOOL isRequiredProperty, BOOL isInstanceProperty)

You can use the property_getAttributes function to discover the name and the @encode type string of a property. For details of the encoding type strings, see Type Encodings; for details of this string, see Property Type String and Property Attribute Description Examples.

你可以使用property_getAttributes函数来查出一个属性的名称和@encode类型字符串。有关编码类型字符串的详细信息，请参考 Type Encodings；这个字符串的详细信息，请参考
Property Type String and Property Attribute Description Examples。

const char *property_getAttributes(objc_property_t property)

Putting these together, you can print a list of all the properties associated with a class using the following code:

把这些放在一起，你可以使用以下代码打印和类相关的所有属性的列表：

id LenderClass = objc_getClass("Lender");
unsigned int outCount, i;
objc_property_t *properties = class_copyPropertyList(LenderClass, &outCount);
for (i = 0; i < outCount; i++) {
    objc_property_t property = properties[i];
    fprintf(stdout, "%s %s\n", property_getName(property), property_getAttributes(property));
}

2.属性类型字符串

You can use the property_getAttributes function to discover the name, the @encode type string of a property, and other attributes of the property.

你可以使用property_getAttributes函数来查找一个属性的名称，@encode类型字符串以及其他的属性。

The string starts with a T followed by the @encode type and a comma, and finishes with a V followed by the name of the backing instance variable. Between these, the attributes are specified by the following descriptors, separated by commas:

该字符串以T开头，后跟@encode类型和一个逗号，以V结尾并跟着后续实例变量的名称。在这些之间，属性由以下描述符指定，以逗号分隔：

Table 7-1 Declared property type encodings

Code	Meaning
R	The property is read-only (readonly).
C	The property is a copy of the value last assigned (copy).
&	The property is a reference to the value last assigned (retain).
N	The property is non-atomic (nonatomic).
G<name>	The property defines a custom getter selector name. The name follows the G (for example, GcustomGetter,).
S<name>	The property defines a custom setter selector name. The name follows the S (for example, ScustomSetter:,).
D	The property is dynamic (@dynamic).
W	The property is a weak reference (__weak).
P	The property is eligible for garbage collection.
t<encoding>	Specifies the type using old-style encoding.

更多示例请参考 Property Attribute Description Examples。

3.属性的属性描述符示例

有以下定义：

enum FooManChu { FOO, MAN, CHU };
struct YorkshireTeaStruct { int pot; char lady; };
typedef struct YorkshireTeaStruct YorkshireTeaStructType;
union MoneyUnion { float alone; double down; };

下表展示了property_getAttributes返回的属性描述符和对应字符串的示例：

Property declaration	Property description
@property char charDefault;	Tc,VcharDefault
@property double doubleDefault;	Td,VdoubleDefault
@property enum FooManChu enumDefault;	Ti,VenumDefault
@property float floatDefault;	Tf,VfloatDefault
@property int intDefault;	Ti,VintDefault
@property long longDefault;	Tl,VlongDefault
@property short shortDefault;	Ts,VshortDefault
@property signed signedDefault;	Ti,VsignedDefault
@property struct YorkshireTeaStruct structDefault;	T{YorkshireTeaStruct="pot"i"lady"c},VstructDefault
@property YorkshireTeaStructType typedefDefault;	T{YorkshireTeaStruct="pot"i"lady"c},VtypedefDefault
@property union MoneyUnion unionDefault;	T(MoneyUnion="alone"f"down"d),VunionDefault
@property unsigned unsignedDefault;	TI,VunsignedDefault
@property int (*functionPointerDefault)(char *);	T^?,VfunctionPointerDefault
@property id idDefault; Note: the compiler warns: "no 'assign', 'retain', or 'copy' attribute is specified - 'assign' is assumed"	T@,VidDefault
@property int *intPointer;	T^i,VintPointer
@property void *voidPointerDefault;	T^v,VvoidPointerDefault
@property int intSynthEquals;	In the implementation block:
@synthesize intSynthEquals=_intSynthEquals;	Ti,V_intSynthEquals
@property(getter=intGetFoo, setter=intSetFoo:) int intSetterGetter;	Ti,GintGetFoo,SintSetFoo:,VintSetterGetter
@property(readonly) int intReadonly;	Ti,R,VintReadonly
@property(getter=isIntReadOnlyGetter, readonly) int intReadonlyGetter;	Ti,R,GisIntReadOnlyGetter
@property(readwrite) int intReadwrite;	Ti,VintReadwrite
@property(assign) int intAssign;	Ti,VintAssign
@property(retain) id idRetain;	T@,&,VidRetain
@property(copy) id idCopy;	T@,C,VidCopy
@property(nonatomic) int intNonatomic;	Ti,VintNonatomic
@property(nonatomic, readonly, copy) id idReadonlyCopyNonatomic;	T@,R,C,VidReadonlyCopyNonatomic
@property(nonatomic, readonly, retain) id idReadonlyRetainNonatomic;	T@,R,&,VidReadonlyRetainNonatomic