> For the complete documentation index, see [llms.txt](https://linsyorozuya.gitbook.io/objective-c-runtime-programming-guide/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://linsyorozuya.gitbook.io/objective-c-runtime-programming-guide/sheng-ming-de-shu-xing.md). # 声明的属性 当编译器遇到属性声明时（请参阅[*Objective-C编程语言*](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ObjectiveC/Introduction/introObjectiveC.html#//apple_ref/doc/uid/TP30001163)中[*的*](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ObjectiveC/Introduction/introObjectiveC.html#//apple_ref/doc/uid/TP30001163)[声明属性](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ObjectiveC/Chapters/ocProperties.html#//apple_ref/doc/uid/TP30001163-CH17)），它会生成与封闭类，类别或协议相关联的描述性元数据。您可以在类或协议上按名称查找属性，将属性类型作为`@encode`字符串获取，以及将属性列表作为C字符串数组复制的函数来访问此元数据。每个类和协议都有一个声明的属性列表。 ### 属性类型和功能 该`Property`结构定义了属性描述符的不透明句柄。 ``` typedef struct objc_property *Property; ``` 您可以使用这些函数`class_copyPropertyList 和 protocol_copyPropertyList`分别检索与类关联的属性数组（包括已加载的类别）和协议： ``` 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 ``` 您可以使用以下方式获取属性列表： ``` id LenderClass = objc_getClass("Lender"); unsigned int outCount; objc_property_t *properties = class_copyPropertyList(LenderClass, &outCount); ``` 您可以使用该`property_getName`函数来发现属性的名称： ``` const char *property_getName(objc_property_t property) ``` 您可以使用这些函数`class_getProperty`并`protocol_getProperty`分别获取对类和协议中具有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) ``` 您可以使用该`property_getAttributes`函数来发现`@encode`属性的名称和类型字符串。有关编码类型字符串的详细信息，请参阅[类型编码](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtTypeEncodings.html#//apple_ref/doc/uid/TP40008048-CH100-SW1) ; 有关此字符串的详细信息，请参阅[属性类型字符串](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtPropertyIntrospection.html#//apple_ref/doc/uid/TP40008048-CH101-SW6)和[属性属性描述示例](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtPropertyIntrospection.html#//apple_ref/doc/uid/TP40008048-CH101-SW5)。 ``` const char *property_getAttributes(objc_property_t property) ``` 将这些放在一起，您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 您可以使用该`property_getAttributes`函数来发现名称，`@encode`属性的类型字符串以及属性的其他属性。 该字符串以a开头，`T`后跟`@encode`类型和逗号，并以a 结尾，后跟`V`后备实例变量的名称。在这些属性之间，属性由以下描述符指定，用逗号分隔： | **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` | The property defines a custom getter selector name. The name follows the `G` (for example, `GcustomGetter,`). | | `S` | 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` | Specifies the type using old-style encoding. | 有关示例，请参见[属性属性描述示例](https://developer.apple.com/library/archive/documentation/Cocoa/Conceptual/ObjCRuntimeGuide/Articles/ocrtPropertyIntrospection.html#//apple_ref/doc/uid/TP40008048-CH101-SW5)。 ### 属性属性描述示例 鉴于这些定义： ``` 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返回的相应字符串`property_getAttributes`： | 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` |