Objective-C 运行时编程指南之 Declared Properties

2019-11-06 09:48:31

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当编译器遇到属性声明时（参见《The Objective-C PRogramming Language》中的《Declared Properties》），它会产生与封闭类、类别或协议相关联的描述性元数据。你可以使用方法访问该元数据，支持通过类或协议中的名字查找属性，获得以 @encode 字符串表示的属性类型，以及拷贝属性的标志列表作为C字符串数组。已声明的属性的列表对于每个类和协议都是可用的。

7.1 Property Type and Functions 属性类型和方法

Property 结构体定义了属性描述符的不透明句柄。

typedef struct objc_property *Property;

你可以使用 class_copyPropertyList 函数和 protocol_copyPropertyList 函数分别取回关联到类（包括已加载的类别）和协议的属性数组：

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

你可以通过以下方法获得属性列表：

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

你可以使用 property_getName 函数发现属性的名称：

const char *property_getName(objc_property_t property)

你可以使用 class_getProperty 和 protocol_getProperty 分别获得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)

你可以使用 property_getAttributes 函数发现名称和属性的@encode类型字符串属性。编码类型字符串的详情，参见 Type Encodings；这个字符串的详情，见 Property Type String 和 Property Attribute Description Examples。

const char *property_getAttributes(objc_property_t property)

把这些放到一起，你可以使用以下代码打印一份关联到类的所有属性的列表：

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));}

7.2 Property Type String 属性类型字符串

你可以使用 property_getAttributes 函数发现属性的名称、 @encode 类型字符串以及属性的其他特征。

该字符串以跟着 @encode 类型和逗号的 T 开头，最后以跟着后台实例变量的名称的 V 结束。在它们之间，属性由以下描述符说明，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	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.

例子参见《Property Attribute Description Examples》。

7.3 Property Attribute Description Examples 属性描述实例

给出这些定义：

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

下表展示了属性声明范例和 property_getAttributes 方法的响应字符串：

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