blocks|key|1346707|text|需要在您的TestProtocolsViewController类定义中定义变量anObject，协议只是通知您它应该在那里。|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1346708|编译器错误告诉你事实--变量不存在。@properties毕竟只是帮助器。|1346709|entityMap^0|0|0^^$0|@$1|2|3|4|5|6|7|F|8|@]|9|@]|A|$]]|$1|B|3|C|5|6|7|G|8|@]|9|@]|A|$]]|$1|D|3|-4|5|6|7|H|8|@]|9|@]|A|$]]]|E|$]]

The variable, anObject, needs to be defined in your TestProtocolsViewController class definition, the protocol is just informing you that it should be there. 

The compiler errors are telling you the truth - the variable doesn't exist. @properties are just helpers after all.

blocks|key|1562319|text|该协议只是告诉通过该协议了解您的类的每个人，属性anObject将在那里。协议不是真实的，它们本身没有变量或方法-它们只描述了一组关于您的类的特定属性，以便持有对它们的引用的对象可以以特定的方式使用它们。|type|unstyled|depth|inlineStyleRanges|offset|length|style|CODE|entityRanges|data|1562320|这意味着在符合协议的类中，您必须尽一切努力确保anObject正常工作。|1562321|@property和@synthesize本质上是为您生成代码的两种机制。@property只是说该属性名称将有一个getter+(和/或setter)方法。如今，仅靠@property就足以让系统为您创建方法和存储变量(您过去必须添加@sythesize)。但是你必须有一些东西来访问和存储变量。|1562322|entityMap^0|O|8|0|0|0|9|A|B|11|9|2C|9|3A|A|0^^$0|@$1|2|3|4|5|6|7|L|8|@$9|M|A|N|B|C]]|D|@]|E|$]]|$1|F|3|G|5|6|7|O|8|@]|D|@]|E|$]]|$1|H|3|I|5|6|7|P|8|@$9|Q|A|R|B|C]|$9|S|A|T|B|C]|$9|U|A|V|B|C]|$9|W|A|X|B|C]|$9|Y|A|Z|B|C]]|D|@]|E|$]]|$1|J|3|-4|5|6|7|10|8|@]|D|@]|E|$]]]|K|$]]

The protocol is just telling everyone that knows about your class through the protocol, that the property <code>anObject</code> will be there. Protocols are not real, they have no variables or methods themselves - they only describe a specific set of attributes that is true about your class so that objects holding references to them can use them in specific ways.

That means in your class that conforms to your protocol, you have to do everything to make sure anObject works. 

<code>@property</code> and <code>@synthesize</code> are at heart two mechanisms that generate code for you. <code>@property</code> is just saying there will be a getter (and/or setter) method for that property name. These days <code>@property</code> alone is enough to also have methods and a storage variable created for you by the system (you used to have to add <code>@sythesize</code>). But you have to have something to access and store the variable.

blocks|key|1346770|text|这是我的一个完美工作的例子，首先是协议定义：|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1346771|@class+ExampleClass;

@protocol+ExampleProtocol

@required

//+Properties
@property+(nonatomic,+retain)+ExampleClass+*item;

@end|code-block|syntax|javascript|1346772|下面是一个支持该协议的类的工作示例：|1346773|#import+<UIKit/UIKit.h>
#import+"Protocols.h"

@class+ExampleClass;

@interface+MyObject+:+NSObject+<ExampleProtocol>+{

++++//+Property+backing+store
++++ExampleClass++++++++*item;

}


@implementation+MyObject

//+Synthesize+properties
@synthesize+item;

@end|1346774|entityMap^0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|M|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|N|8|@]|9|@]|A|$E|F]]|$1|G|3|H|5|6|7|O|8|@]|9|@]|A|$]]|$1|I|3|J|5|D|7|P|8|@]|9|@]|A|$E|F]]|$1|K|3|-4|5|6|7|Q|8|@]|9|@]|A|$]]]|L|$]]

Here's an example of mine that works perfectly, the protocol definition first of all:

<pre><code>@class ExampleClass;

@protocol ExampleProtocol

@required

// Properties
@property (nonatomic, retain) ExampleClass *item;

@end
</code></pre>

Below is a working example of a class supporting this protocol:

<pre><code>#import &lt;UIKit/UIKit.h&gt;
#import "Protocols.h"

@class ExampleClass;

@interface MyObject : NSObject &lt;ExampleProtocol&gt; {

 // Property backing store
 ExampleClass *item;

}


@implementation MyObject

// Synthesize properties
@synthesize item;

@end
</code></pre>

blocks|key|1562401|text|​|type|unstyled|depth|inlineStyleRanges|entityRanges|data|1562402|📷|atomic|offset|length|1562403|1562404|示例:2个类(Person和Serial)要使用Viewer的服务...并且必须符合ViewerProtocol。viewerTypeOfDescription是订阅服务器类必须符合的强制属性。|1562405|typedef+enum+ViewerTypeOfDescription+{
++++ViewerDataType_NSString,
++++ViewerDataType_NSNumber,
}+ViewerTypeOfDescription;

@protocol+ViewerProtocol
@property+ViewerTypeOfDescription+viewerTypeOfDescription;
-+(id)initConforming;
-+(NSString*)nameOfClass;
-+(id)dataRepresentation;
@end

@interface+Viewer+:+NSObject
%2B+(void)+printLargeDescription:(id+<ViewerProtocol>)object;
@end

@implementation+Viewer
%2B+(void)+printLargeDescription:(id+<ViewerProtocol>)object+{
++++NSString+*data;
++++NSString+*type;
++++switch+([object+viewerTypeOfDescription])+{
++++++++case+ViewerDataType_NSString:+{
++++++++++++data=[object+dataRepresentation];
++++++++++++type=@"String";
++++++++++++break;
++++++++}
++++++++case+ViewerDataType_NSNumber:+{
++++++++++++data=[(NSNumber*)[object+dataRepresentation]+stringValue];
++++++++++++type=@"Number";
++++++++++++break;
++++++++}
++++++++default:+{
++++++++++++data=@"";
++++++++++++type=@"Undefined";
++++++++++++break;
++++++++}
++++}
++++printf("%25s+[%25s(%25s)]\n",[data+cStringUsingEncoding:NSUTF8StringEncoding],
+++++++++++[[object+nameOfClass]+cStringUsingEncoding:NSUTF8StringEncoding],
+++++++++++[type+cStringUsingEncoding:NSUTF8StringEncoding]);
}
@end


/*+A+Class+Person+*/

@interface+Person+:+NSObject+<ViewerProtocol>
@property+NSString+*firstname;
@property+NSString+*lastname;
@end

@implementation+Person
//+>>
@synthesize+viewerTypeOfDescription;
//+<<
@synthesize+firstname;
@synthesize+lastname;
//+>>
-+(id)initConforming+{
++++if+(self=[super+init])+{
++++++++viewerTypeOfDescription=ViewerDataType_NSString;
++++}
++++return+self;
}
-+(NSString*)nameOfClass+{
++++return+[self+className];
}
-+(NSString*)+dataRepresentation+{
++++if+(firstname!=nil+&&+lastname!=nil)+{
++++++++return+[NSString+stringWithFormat:@"%25@+%25@",+firstname,+lastname];
++++}+else+if+(firstname!=nil)+{
++++++++return+[NSString+stringWithFormat:@"%25@",+firstname];
++++}
++++return+[NSString+stringWithFormat:@"%25@",+lastname];
}
//+<<
@end



/*+A+Class+Serial+*/

@interface+Serial+:+NSObject+<ViewerProtocol>
@property+NSInteger+amount;
@property+NSInteger+factor;
@end

@implementation+Serial
//+>>
@synthesize+viewerTypeOfDescription;
//+<<
@synthesize+amount;
@synthesize+factor;
//+>>
-+(id)initConforming+{
++++if+(self=[super+init])+{
++++++++amount=0;+factor=0;
++++++++viewerTypeOfDescription=ViewerDataType_NSNumber;
++++}
++++return+self;
}
-+(NSString*)nameOfClass+{
++++return+[self+className];
}
-+(NSNumber*)+dataRepresentation+{
++++if+(factor==0)+{
++++++++return+[NSNumber+numberWithInteger:amount];
++++}+else+if+(amount==0)+{
++++++++return+[NSNumber+numberWithInteger:0];
++++}
++++return+[NSNumber+numberWithInteger:(factor*amount)];
}
//+<<
@end




int+main(int+argc,+const+char+*+argv[])
{

++++@autoreleasepool+{

++++++++Person+*duncan=[[Person+alloc]initConforming];
++++++++duncan.firstname=@"Duncan";
++++++++duncan.lastname=@"Smith";

++++++++[Viewer+printLargeDescription:duncan];

++++++++Serial+*x890tyu=[[Serial+alloc]initConforming];
++++++++x890tyu.amount=1564;

++++++++[Viewer+printLargeDescription:x890tyu];

++++++++NSObject+*anobject=[[NSObject+alloc]init];

++++++++//[Viewer+printLargeDescription:anobject];
++++++++//<<+compilator+claim+an+issue+the+object+does+not+conform+to+protocol

++++}
++++return+0;
}|code-block|syntax|javascript|1562406|通过subClassing进行协议继承的另一个示例|1562407|typedef+enum+{
++++LogerDataType_null,
++++LogerDataType_int,
++++LogerDataType_string,
}+LogerDataType;

@protocol+LogerProtocol
@property+size_t+numberOfDataItems;
@property+LogerDataType+dataType;
@property+void**+data;
@end

@interface+Loger+:+NSObject
%2B+(void)+print:(id<LogerProtocol>)object;
@end

@implementation+Loger
%2B+(void)+print:(id<LogerProtocol>)object+{
++++if+([object+numberOfDataItems]==0)+return;
++++void+**data=[object+data];
++++for+(size_t+i=0;+i<[object+numberOfDataItems];+i%2B%2B)+{
++++++++switch+([object+dataType])+{
++++++++++++case+LogerDataType_int:+{
++++++++++++++++printf("%25d\n",(int)data[i]);
++++++++++++break;
++++++++++++}
++++++++++++case+LogerDataType_string:+{
++++++++++++++++printf("%25s\n",(char*)data[i]);
++++++++++++++++break;
++++++++++++}
++++++++++++default:
++++++++++++break;
++++++++}
++++}
}
@end


//+A+Master+Class

@interface+ArrayOfItems+:+NSObject++<LogerProtocol>
@end

@implementation+ArrayOfItems
@synthesize+dataType;
@synthesize+numberOfDataItems;
@synthesize+data;
-+(id)init+{
++++if+(self=[super+init])+{
++++++++dataType=LogerDataType_null;
++++++++numberOfDataItems=0;
++++}
++++return+self;
}
@end

//+A+SubClass

@interface+ArrayOfInts+:+ArrayOfItems
@end

@implementation+ArrayOfInts
-+(id)init+{
++++if+(self=[super+init])+{
++++++++self.dataType=LogerDataType_int;
++++}
++++return+self;
}
@end

//+An+other+SubClass

@interface+ArrayOfStrings+:+ArrayOfItems
@end

@implementation+ArrayOfStrings
-+(id)init+{
++++if+(self=[super+init])+{
++++++++self.dataType=LogerDataType_string;
++++}
++++return+self;
}
@end


int+main(int+argc,+const+char+*+argv[])
{

++++@autoreleasepool+{

++++++++ArrayOfInts+*arr=[[ArrayOfInts+alloc]init];
++++++++arr.data=(void*[]){(int*)14,(int*)25,(int*)74};
++++++++arr.numberOfDataItems=3;

++++++++[Loger+print:arr];

++++++++ArrayOfStrings+*arrstr=[[ArrayOfStrings+alloc]init];
++++++++arrstr.data=(void*[]){(char*)"string1",(char*)"string2"};
++++++++arrstr.numberOfDataItems=2;

++++++++[Loger+print:arrstr];

++++}
++++return+0;
}|1562408|entityMap|0|IMAGE|mutability|IMMUTABLE|imageUrl|https://ask.qcloudimg.com/http-save/yehe-900000/d728f5ec4f9079a60102301073d5cceb.jpeg|imageAlt^0|0|0|1|0|0|0|0|0|0|0^^$0|@$1|2|3|4|5|6|7|11|8|@]|9|@]|A|$]]|$1|B|3|C|5|D|7|12|8|@]|9|@$E|13|F|14|1|15]]|A|$]]|$1|G|3|4|5|6|7|16|8|@]|9|@]|A|$]]|$1|H|3|I|5|6|7|17|8|@]|9|@]|A|$]]|$1|J|3|K|5|L|7|18|8|@]|9|@]|A|$M|N]]|$1|O|3|P|5|6|7|19|8|@]|9|@]|A|$]]|$1|Q|3|R|5|L|7|1A|8|@]|9|@]|A|$M|N]]|$1|S|3|-4|5|6|7|1B|8|@]|9|@]|A|$]]]|T|$U|$5|V|W|X|A|$Y|Z|10|-4]]]]

<img src="https://i.stack.imgur.com/nYLvJ.jpg" alt="Protocol Architecture">

Example: 2 classes (Person and Serial) want use service of Viewer... and must conform to ViewerProtocol. viewerTypeOfDescription is a mandatory property subscriber classes must conform.

<pre><code>typedef enum ViewerTypeOfDescription {
 ViewerDataType_NSString,
 ViewerDataType_NSNumber,
} ViewerTypeOfDescription;

@protocol ViewerProtocol
@property ViewerTypeOfDescription viewerTypeOfDescription;
- (id)initConforming;
- (NSString*)nameOfClass;
- (id)dataRepresentation;
@end

@interface Viewer : NSObject
+ (void) printLargeDescription:(id &lt;ViewerProtocol&gt;)object;
@end

@implementation Viewer
+ (void) printLargeDescription:(id &lt;ViewerProtocol&gt;)object {
 NSString *data;
 NSString *type;
 switch ([object viewerTypeOfDescription]) {
 case ViewerDataType_NSString: {
 data=[object dataRepresentation];
 type=@"String";
 break;
 }
 case ViewerDataType_NSNumber: {
 data=[(NSNumber*)[object dataRepresentation] stringValue];
 type=@"Number";
 break;
 }
 default: {
 data=@"";
 type=@"Undefined";
 break;
 }
 }
 printf("%s [%s(%s)]\n",[data cStringUsingEncoding:NSUTF8StringEncoding],
 [[object nameOfClass] cStringUsingEncoding:NSUTF8StringEncoding],
 [type cStringUsingEncoding:NSUTF8StringEncoding]);
}
@end


/* A Class Person */

@interface Person : NSObject &lt;ViewerProtocol&gt;
@property NSString *firstname;
@property NSString *lastname;
@end

@implementation Person
// &gt;&gt;
@synthesize viewerTypeOfDescription;
// &lt;&lt;
@synthesize firstname;
@synthesize lastname;
// &gt;&gt;
- (id)initConforming {
 if (self=[super init]) {
 viewerTypeOfDescription=ViewerDataType_NSString;
 }
 return self;
}
- (NSString*)nameOfClass {
 return [self className];
}
- (NSString*) dataRepresentation {
 if (firstname!=nil &amp;&amp; lastname!=nil) {
 return [NSString stringWithFormat:@"%@ %@", firstname, lastname];
 } else if (firstname!=nil) {
 return [NSString stringWithFormat:@"%@", firstname];
 }
 return [NSString stringWithFormat:@"%@", lastname];
}
// &lt;&lt;
@end



/* A Class Serial */

@interface Serial : NSObject &lt;ViewerProtocol&gt;
@property NSInteger amount;
@property NSInteger factor;
@end

@implementation Serial
// &gt;&gt;
@synthesize viewerTypeOfDescription;
// &lt;&lt;
@synthesize amount;
@synthesize factor;
// &gt;&gt;
- (id)initConforming {
 if (self=[super init]) {
 amount=0; factor=0;
 viewerTypeOfDescription=ViewerDataType_NSNumber;
 }
 return self;
}
- (NSString*)nameOfClass {
 return [self className];
}
- (NSNumber*) dataRepresentation {
 if (factor==0) {
 return [NSNumber numberWithInteger:amount];
 } else if (amount==0) {
 return [NSNumber numberWithInteger:0];
 }
 return [NSNumber numberWithInteger:(factor*amount)];
}
// &lt;&lt;
@end




int main(int argc, const char * argv[])
{

 @autoreleasepool {

 Person *duncan=[[Person alloc]initConforming];
 duncan.firstname=@"Duncan";
 duncan.lastname=@"Smith";

 [Viewer printLargeDescription:duncan];

 Serial *x890tyu=[[Serial alloc]initConforming];
 x890tyu.amount=1564;

 [Viewer printLargeDescription:x890tyu];

 NSObject *anobject=[[NSObject alloc]init];

 //[Viewer printLargeDescription:anobject];
 //&lt;&lt; compilator claim an issue the object does not conform to protocol

 }
 return 0;
}
</code></pre>

An other Example with Protocol inheritance over subClassing

<pre><code>typedef enum {
 LogerDataType_null,
 LogerDataType_int,
 LogerDataType_string,
} LogerDataType;

@protocol LogerProtocol
@property size_t numberOfDataItems;
@property LogerDataType dataType;
@property void** data;
@end

@interface Loger : NSObject
+ (void) print:(id&lt;LogerProtocol&gt;)object;
@end

@implementation Loger
+ (void) print:(id&lt;LogerProtocol&gt;)object {
 if ([object numberOfDataItems]==0) return;
 void **data=[object data];
 for (size_t i=0; i&lt;[object numberOfDataItems]; i++) {
 switch ([object dataType]) {
 case LogerDataType_int: {
 printf("%d\n",(int)data[i]);
 break;
 }
 case LogerDataType_string: {
 printf("%s\n",(char*)data[i]);
 break;
 }
 default:
 break;
 }
 }
}
@end


// A Master Class

@interface ArrayOfItems : NSObject &lt;LogerProtocol&gt;
@end

@implementation ArrayOfItems
@synthesize dataType;
@synthesize numberOfDataItems;
@synthesize data;
- (id)init {
 if (self=[super init]) {
 dataType=LogerDataType_null;
 numberOfDataItems=0;
 }
 return self;
}
@end

// A SubClass

@interface ArrayOfInts : ArrayOfItems
@end

@implementation ArrayOfInts
- (id)init {
 if (self=[super init]) {
 self.dataType=LogerDataType_int;
 }
 return self;
}
@end

// An other SubClass

@interface ArrayOfStrings : ArrayOfItems
@end

@implementation ArrayOfStrings
- (id)init {
 if (self=[super init]) {
 self.dataType=LogerDataType_string;
 }
 return self;
}
@end


int main(int argc, const char * argv[])
{

 @autoreleasepool {

 ArrayOfInts *arr=[[ArrayOfInts alloc]init];
 arr.data=(void*[]){(int*)14,(int*)25,(int*)74};
 arr.numberOfDataItems=3;

 [Loger print:arr];

 ArrayOfStrings *arrstr=[[ArrayOfStrings alloc]init];
 arrstr.data=(void*[]){(char*)"string1",(char*)"string2"};
 arrstr.numberOfDataItems=2;

 [Loger print:arrstr];

 }
 return 0;
}
</code></pre>

I've seen usage of Objective-C protocols get used in a fashion such as the following:

<pre><code>@protocol MyProtocol &lt;NSObject&gt;

@required

@property (readonly) NSString *title;

@optional

- (void) someMethod;

@end
</code></pre>

I've seen this format used instead of writing a concrete superclass that subclasses extend. The question is, if you conform to this protocol, do you need to synthesize the properties yourself? If you're extending a superclass, the answer is obviously no, you do not need to. But how does one deal with properties that a protocol requires to conform to?

To my understanding, you still need to declare the instance variables in the header file of an object that conforms to a protocol that requires these properties. In that case, can we assume that they're just a guiding principle? CLearly the same isn't the case for a required method. The compiler will slap your wrist for excluding a required method that a protocol lists. What's the story behind properties though?

Here's an example that generates a compile error (Note: I've trimmed the code which doesn't reflect upon the problem at hand):

MyProtocol.h

<pre><code>@protocol MyProtocol &lt;NSObject&gt;

@required
@property (nonatomic, retain) id anObject;

@optional
</code></pre>

TestProtocolsViewController.h

<pre><code>- (void)iDoCoolStuff;

@end

#import &lt;MyProtocol.h&gt;

@interface TestProtocolsViewController : UIViewController &lt;MyProtocol&gt; {

}

@end
</code></pre>

TestProtocolsViewController.m

<pre><code>#import "TestProtocolsViewController.h"

@implementation TestProtocolsViewController
@synthesize anObject; // anObject doesn't exist, even though we conform to MyProtocol.

- (void)dealloc {
 [anObject release]; //anObject doesn't exist, even though we conform to MyProtocol.
 [super dealloc];
}

@end 
</code></pre>

How to handle Objective-C protocols that contain properties?

Objective-C

服务器

编译器

我见过Objective-C协议的使用，如下所示：@protocol MyProtocol <NSObject>@required@property (readonly) NSString *title;@optional- (void) someMethod;@end我见过使用这种格式，而不是编写一个子类扩展的具体超...

问如何处理包含属性的Objective-C协议？
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