[OC Runtime编程指南

[OC Runtime编程指南_翻译]一、介绍 & 二、运行时版本和平台 [OC Runtime编程指南_翻译]三、与运行时交互 [OC Runtime编程指南_翻译]四、消息传递 [OC Runtime编程指南_翻译]五、动态方法解析 [OC Runtime编程指南_翻译]六、消息转发 [OC Runtime编程指南_翻译]七、类型编码 [OC Runtime编程指南_翻译]八、声明属性

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当编译器遇到属性声明（请参阅 _Objective-C _编程语言中 声明属性）时，它会生成与封闭类、类别或协议相关联的描述性元数据。您可以使用支持在类或协议上按名称查找属性、以**@encode**字符串形式获取属性类型以及以C字符串数组形式复制属性属性列表的函数来访问此元数据。声明的属性列表可用于每个类和协议。

属性类型和功能

**Property**结构定义属性描述符的不透明句柄。

typedef struct objc_property *Property;

可以使用函数**class_copyPropertyList和protocol_copyPropertyList**分别检索与类（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，给定以下类声明：

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

可以使用以下方法获取属性列表：

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

您可以使用 **property_getName**函数来发现属性的名称：

const char *property_getName(objc_property_t property)

可以使用函数**class_getProperty和protocol_getProperty**分别获取对类和协议中给定名称的属性的引用：

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

可以使用**property_getAttributes函数来发现属性的名称和@encode** type字符串。有关编码类型字符串的详细信息，请参阅 类型编码；有关此字符串的详细信息，请参阅 Property Type String和 Property Attribute Description Examples。

const char *property_getAttributes(objc_property_t property)

将这些组合在一起，可以使用以下代码打印与类关联的所有属性的列表：

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)); }

属性类型字符串

可以使用**property_getAttributes函数来发现属性的名称、@encode** type字符串以及属性的其他属性。

字符串以**T开头，后跟@encode类型和逗号，以V**结尾，后跟支持实例变量的名称。其中，属性由以下描述符指定，用逗号分隔：

Table 7-1 声明的属性类型编码

CodeMeaningRThe property is read-only (readonly).属性为只读（只读）。CThe property is a copy of the value last assigned (copy).&The property is a reference to the value last assigned (retain).NThe property is non-atomic (nonatomic).GThe property defines a custom getter selector name. The name follows the G (for example, GcustomGetter,).SThe property defines a custom setter selector name. The name follows the S (for example, ScustomSetter:,).DThe property is dynamic (@dynamic).WThe property is a weak reference (__weak).PThe property is eligible for garbage collection.tSpecifies the type using old-style encoding.

示例, 请参见 Property Attribute Description Examples.

Property Attribute 描述示例

根据这些定义：

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

下表显示了示例属性声明和属性**property_getAttributes**返回的相应字符串：

Property declarationProperty 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