如何在 Objective-C 中声明类级属性？
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How do I declare class-level properties in Objective-C?
提问by mamcx
Maybe this is obvious, but I don't know how to declare class properties in Objective-C.
也许这很明显，但我不知道如何在 Objective-C 中声明类属性。
I need to cache per-class a dictionary and wonder how put it in the class.
我需要为每个班级缓存一个字典，并想知道如何将它放在班级中。
采纳答案by Andrew Grant
properties have a specific meaning in Objective-C, but I think you mean something that's equivalent to a static variable? E.g. only one instance for all types of Foo?
属性在 Objective-C 中具有特定含义，但我认为您的意思是等效于静态变量？例如，所有类型的 Foo 只有一个实例？
To declare class functions in Objective-C you use the + prefix instead of - so your implementation would look something like:
要在 Objective-C 中声明类函数，您可以使用 + 前缀而不是 - 这样您的实现将类似于：
// Foo.h
@interface Foo {
}
+ (NSDictionary *)dictionary;
// Foo.m
+ (NSDictionary *)dictionary {
static NSDictionary *fooDict = nil;
if (fooDict == nil) {
// create dict
}
return fooDict;
}
回答by spooki
I'm using this solution:
我正在使用这个解决方案：
@interface Model
+ (int) value;
+ (void) setValue:(int)val;
@end
@implementation Model
static int value;
+ (int) value
{ @synchronized(self) { return value; } }
+ (void) setValue:(int)val
{ @synchronized(self) { value = val; } }
@end
And i found it extremely useful as a replacement of Singleton pattern.
我发现它作为单例模式的替代非常有用。
To use it, simply access your data with dot notation:
要使用它，只需使用点表示法访问您的数据：
Model.value = 1;
NSLog(@"%d = value", Model.value);
回答by Alex Nolasco
As seen in WWDC 2016/XCode 8 (what's new in LLVM session@5:05). Class properties can be declared as follows
如 WWDC 2016/XCode 8 中所见（LLVM 会话中的新内容@5:05）。类属性可以声明如下
@interface MyType : NSObject
@property (class) NSString *someString;
@end
NSLog(@"format string %@", MyType.someString);
Note that class properties are never synthesized
请注意，永远不会合成类属性
@implementation
static NSString * _someString;
+ (NSString *)someString { return _someString; }
+ (void)setSomeString:(NSString *)newString { _someString = newString; }
@end
回答by Jim Puls
If you're looking for the class-level equivalent of @property, then the answer is "there's no such thing". But remember, @propertyis only syntactic sugar, anyway; it just creates appropriately-named object methods.
如果您正在寻找类级别的 等价物@property，那么答案是“没有这样的东西”。但请记住@property，无论如何，这只是语法糖；它只是创建适当命名的对象方法。
You want to create class methods that access static variables which, as others have said, have only a slightly different syntax.
您想创建访问静态变量的类方法，正如其他人所说，这些静态变量的语法略有不同。
回答by Quentin
Here's a thread safe way of doing it:
这是一种线程安全的方法：
// Foo.h
@interface Foo {
}
+(NSDictionary*) dictionary;
// Foo.m
+(NSDictionary*) dictionary
{
static NSDictionary* fooDict = nil;
static dispatch_once_t oncePredicate;
dispatch_once(&oncePredicate, ^{
// create dict
});
return fooDict;
}
These edits ensure that fooDict is only created once.
这些编辑确保 fooDict 只创建一次。
From Apple documentation: "dispatch_once - Executes a block object once and only once for the lifetime of an application."
来自Apple 文档：“dispatch_once - 在应用程序的生命周期内只执行一次块对象。”
回答by berbie
As of Xcode 8 Objective-C now supports class properties:
从 Xcode 8 Objective-C 开始，现在支持类属性：
@interface MyClass : NSObject
@property (class, nonatomic, assign, readonly) NSUUID* identifier;
@end
Since class properties are never synthesised you need to write your own implementation.
由于类属性永远不会被合成，因此您需要编写自己的实现。
@implementation MyClass
static NSUUID*_identifier = nil;
+ (NSUUID *)identifier {
if (_identifier == nil) {
_identifier = [[NSUUID alloc] init];
}
return _identifier;
}
@end
You access the class properties using normal dot syntax on the class name:
您可以使用类名上的普通点语法访问类属性：
MyClass.identifier;
回答by mouviciel
Properties have values only in objects, not classes.
属性仅在对象中具有值，而不在类中。
If you need to store something for all objects of a class, you have to use a global variable. You can hide it by declaring it staticin the implementation file.
如果你需要为一个类的所有对象存储一些东西，你必须使用一个全局变量。您可以通过static在实现文件中声明它来隐藏它。
You may also consider using specific relations between your objects: you attribute a role of master to a specific object of your class and link others objects to this master. The master will hold the dictionary as a simple property. I think of a tree like the one used for the view hierarchy in Cocoa applications.
您还可以考虑使用对象之间的特定关系：您将主人的角色归因于您的类的特定对象，并将其他对象链接到该主人。主人将把字典作为一个简单的属性。我想到了一种类似于 Cocoa 应用程序中用于视图层次结构的树。
Another option is to create an object of a dedicated class that is composed of both your 'class' dictionary and a set of all the objects related to this dictionary. This is something like NSAutoreleasePoolin Cocoa.
另一种选择是创建一个专用类的对象，该对象由您的“类”字典和与该字典相关的所有对象组成。这有点像NSAutoreleasePool可可。
回答by Jean-Marie D.
Starting from Xcode 8, you can use the classproperty attribute as answered by Berbie.
从 Xcode 8 开始，您可以使用Berbie 回答的class属性属性。
However, in the implementation, you need to define both class getter and setter for the class property using a static variable in lieu of an iVar.
但是，在实现中，您需要使用静态变量代替 iVar 为类属性定义类 getter 和 setter。
Sample.h
样本.h
@interface Sample: NSObject
@property (class, retain) Sample *sharedSample;
@end
Sample.m
样本.m
@implementation Sample
static Sample *_sharedSample;
+ ( Sample *)sharedSample {
if (_sharedSample==nil) {
[Sample setSharedSample:_sharedSample];
}
return _sharedSample;
}
+ (void)setSharedSample:(Sample *)sample {
_sharedSample = [[Sample alloc]init];
}
@end
回答by Pedro Borges
If you have many class level properties then a singleton pattern might be in order. Something like this:
如果您有许多类级别的属性，那么可能适合使用单例模式。像这样的东西：
// Foo.h
@interface Foo
+ (Foo *)singleton;
@property 1 ...
@property 2 ...
@property 3 ...
@end
And
和
// Foo.m
#import "Foo.h"
@implementation Foo
static Foo *_singleton = nil;
+ (Foo *)singleton {
if (_singleton == nil) _singleton = [[Foo alloc] init];
return _singleton;
}
@synthesize property1;
@synthesize property2;
@synthesise property3;
@end
Now access your class-level properties like this:
现在访问您的类级属性，如下所示：
[Foo singleton].property1 = value;
value = [Foo singleton].property2;
回答by jawad
[Try this solution it's simple] You can create a static variable in a Swift class then call it from any Objective-C class.
[试试这个解决方案，它很简单] 您可以在 Swift 类中创建一个静态变量，然后从任何 Objective-C 类中调用它。
