Objective-C 運行時編程指南之 Declared Properties

2019-11-06 09:48:31

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當編譯器遇到屬性聲明時（參見《The Objective-C PRogramming Language》中的《Declared Properties》），它會產生與封閉類、類別或協議相關聯的描述性元數據。你可以使用方法訪問該元數據，支持通過類或協議中的名字查找屬性，獲得以 @encode 字符串表示的屬性類型，以及拷貝屬性的標志列表作為C字符串數組。已聲明的屬性的列表對于每個類和協議都是可用的。

7.1 Property Type and Functions 屬性類型和方法

Property 結構體定義了屬性描述符的不透明句柄。

typedef struct objc_property *Property;

你可以使用 class_copyPropertyList 函數和 protocol_copyPropertyList 函數分別取回關聯到類（包括已加載的類別）和協議的屬性數組：

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

例如，給定以下類聲明：

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

你可以通過以下方法獲得屬性列表：

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

你可以使用 property_getName 函數發現屬性的名稱：

const char *property_getName(objc_property_t property)

你可以使用 class_getProperty 和 protocol_getProperty 分別獲得類和協議中給定名稱的屬性的引用：

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 函數發現名稱和屬性的@encode類型字符串屬性。編碼類型字符串的詳情，參見 Type Encodings；這個字符串的詳情，見 Property Type String 和 Property Attribute Description Examples。

const char *property_getAttributes(objc_property_t property)

把這些放到一起，你可以使用以下代碼打印一份關聯到類的所有屬性的列表：

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 屬性類型字符串

你可以使用 property_getAttributes 函數發現屬性的名稱、 @encode 類型字符串以及屬性的其他特征。

該字符串以跟著 @encode 類型和逗號的 T 開頭，最后以跟著后臺實例變量的名稱的 V 結束。在它們之間，屬性由以下描述符說明，以逗號分隔：

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.

例子參見《Property Attribute Description Examples》。

7.3 Property Attribute Description Examples 屬性描述實例

給出這些定義：

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

下表展示了屬性聲明范例和 property_getAttributes 方法的響應字符串：

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