KVC profileThe official documentation
- Key-Value Coding
Key-value encoding is a mechanism enabled by the NSKeyValueCoding informal protocol that objects use to provide indirect access to their properties. When an object conforms to a key-value encoding, its properties can be addressed by string arguments through a compact, uniform messaging interface. This indirect access mechanism complements the direct access provided by instance variables and their associated access methods.
The use of KVC
KVC is often used in development, such as assigning object attributes, adding User Defined Runtime view attributes to StoryBoard, etc
- (nullable id)valueForKey:(NSString *)key;
- (void)setValue:(nullable id)value forKey:(NSString *)key;
Copy the code1. KVC – Basic type
LGPerson *person = [[LGPerson alloc] init];
person.name = @"17";
person.age = 18;
person->myName = @"19";
NSLog(@"%@ - %d - %@",person.name,person.age,person->myName);
[person setValue:@"16" forKey:@"name"];
NSLog(@"%@ - %d - %@",person.name,person.age,person->myName);
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2. Kvc-collection type
- Methods a
person.array = @[@"1",@"2",@"3"];
NSLog(@"%@",[person valueForKey:@"array"]);
NSArray *array = [person valueForKey:@"array"];
array = @[@"100",@"2",@"3"];
[person setValue:array forKey:@"array"];
NSLog(@"%@",[person valueForKey:@"array"]);
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- Method 2
person.array = @[@"1",@"2",@"3"];
NSMutableArray *mArray = [person mutableArrayValueForKey:@"array"];
mArray[0] = @"200";
NSLog(@"%@",[person valueForKey:@"array"]);
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3. Kvc-set operator
- Aggregate operator
// @avg, @count, @max, @min, @sum - (void)aggregationOperator{NSMutableArray *personArray = [NSMutableArray array]; for (int i = 0; i < 6; i++) { LGStudent *p = [LGStudent new]; NSDictionary* dict = @{ @"name":@"Tom", @"age":@(18+i), @"nick":@"Cat", @"length":@(175 + 2*arc4random_uniform(6)), }; [p setValuesForKeysWithDictionary:dict]; [personArray addObject:p]; } NSLog(@"%@", [personArray valueForKey:@"length"]); Float avg = [[personArray valueForKeyPath:@"@avg. Length "] floatValue]; NSLog(@"%f", avg); int count = [[personArray valueForKeyPath:@"@count.length"] intValue]; NSLog(@"%d", count); int sum = [[personArray valueForKeyPath:@"@sum.length"] intValue]; NSLog(@"%d", sum); int max = [[personArray valueForKeyPath:@"@max.length"] intValue]; NSLog(@"%d", max); int min = [[personArray valueForKeyPath:@"@min.length"] intValue]; NSLog(@"%d", min); }Copy the code- Array operator
@distinctUnionofObjects @unionofObjects - (void)arrayOperator{NSMutableArray *personArray = [NSMutableArray array]; for (int i = 0; i < 6; i++) { LGStudent *p = [LGStudent new]; NSDictionary* dict = @{ @"name":@"Tom", @"age":@(18+i), @"nick":@"Cat", @"length":@(175 + 2*arc4random_uniform(6)), }; [p setValuesForKeysWithDictionary:dict]; [personArray addObject:p]; } NSLog(@"%@", [personArray valueForKey:@"length"]); NSArray* arr1 = [personArray valueForKeyPath:@" @unionofObjects.length "]; NSLog(@"arr1 = %@", arr1); / / return a collection of objects specified attribute to heavy NSArray * arr2 = [personArray valueForKeyPath: @ "@ distinctUnionOfObjects. Length"]. NSLog(@"arr2 = %@", arr2) }Copy the code- The nested collection operator
@distinctUnionofArrays @unionofArrays @distinctUnionofSets - (void)arrayNesting{NSMutableArray *personArray1 = [NSMutableArray array]; for (int i = 0; i < 6; i++) { LGStudent *student = [LGStudent new]; NSDictionary* dict = @{ @"name":@"Tom", @"age":@(18+i), @"nick":@"Cat", @"length":@(175 + 2*arc4random_uniform(6)), }; [student setValuesForKeysWithDictionary:dict]; [personArray1 addObject:student]; } NSMutableArray *personArray2 = [NSMutableArray array]; for (int i = 0; i < 6; i++) { LGPerson *person = [LGPerson new]; NSDictionary* dict = @{ @"name":@"Tom", @"age":@(18+i), @"nick":@"Cat", @"length":@(175 + 2*arc4random_uniform(6)), }; [person setValuesForKeysWithDictionary:dict]; [personArray2 addObject:person]; } NSArray* nestArr = @[personArray1, personArray2]; NSArray* arr = [nestArr valueForKeyPath:@"@distinctUnionOfArrays.length"]; NSLog(@"arr = %@", arr); NSArray* arr1 = [nestArr valueForKeyPath:@"@unionOfArrays.length"]; NSLog(@"arr1 = %@", arr1); }Copy the code4. KVC – Access non-object attributes
ThreeFloats floats = {1.,2.,3.};
NSValue *value = [NSValue valueWithBytes:&floats objCType: @encode(ThreeFloats)];
[person setValue:value forKey:@"threeFloats"];
NSValue *value1 = [person valueForKey:@"threeFloats"];
NSLog(@"%@",value1);
ThreeFloats th;
[value1 getValue:&th];
NSLog(@"%f-%f-%f",th.x,th.y,th.z);
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5.:KVC – Layer access – keyPath
LGStudent *student = [LGStudent alloc];
student.subject = @"1234";
person.student = student;
[person setValue:@"Swift" forKeyPath:@"student.subject"];
NSLog(@"%@",[person valueForKeyPath:@"student.subject"]);
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KVC sets the value and evaluates the processThe official documentation
Set the value principle
The search module for the basic Setter
The default implementation of setValue:forKey: attempts to set a property named key as value(or, for non-object properties, an unwrapped version of value, as represented by non-object values) inside the object receiving the call, using the following procedure:
Find the first accessor named set
: or _set
, in that order. If it is found, it is called with the input value (or unwrapped value as needed) and done.
If you don’t find a simple accessor, and if the class methods accessinstancevariablesdirect returns YES, please find an instance variable, the name of _ < key >, _is < key >, < the key >, or is < key >. If found, set the variable directly with the input value (or unwrapped value) and finish.
When had not found the accessor or instance variables, called setValue: forUndefinedKey:. By default, this throws an exception, but subclasses of NSObject may provide key-specific behavior.
Set value flow chart
The principle of value
Search mode for basic getters
The default implementation of valueForKey:, given a key argument as input, performs the following operations from within the class instance that receives valueForKey: Call.
Search the instance for accessor methods whose first name is GET
,
, is
, or _< Key>. If it is found, it is called and the results are used to proceed to Step 5. Otherwise, go to the next step.
If no simple accessor method is found, Search the instance for the methods of matching patterns countOf
and objectIn
AtIndex (corresponding to the original method defined by the NSArray class) and
AtIndexes (corresponding to the objectsAtIndexes:) of the NSArray method. If the first and at least one of the other two are found, create a collection proxy object that responds to all NSArray methods and returns it. Otherwise, go to Step 3. The proxy object then converts all NSArray messages it receives into a combination of countOf
, objectIn
AtIndex:, and
AtIndexes: messages and into the key-value encoded object that created it. If the original object also implements an optional method named GET
:range:, the proxy object uses that method when appropriate. In effect, the proxy object works with an object that conforms to the key-value encoding, allowing the underlying property to behave like NSArray even if it is not.
If no simple access method or array access method group is found, look for three methods named countOf
, enumeratorOf
, and memberOf
(corresponding to the base method defined by the NSSet class). If all three methods are found, create a collection proxy object that responds to all NSSet methods and returns it. Otherwise, go to Step 4. This proxy object then converts any NSSet messages it receives to a combination of countOf
, enumeratorOf
, memberOf
: Messages, and to the object that created it. In effect, the proxy object works with objects that conform to the key-value encoding, allowing the underlying property to behave as if it were an NSSet, even though it is not.
If you don’t find a simple way to access or set a set of access methods, and if the recipient’s class methods accessinstancevariablesdirect returns YES, search instance variables _ < key >, _is < key >, < the key >, or is < key >, in turn. If found, simply get the value of the instance variable and proceed to Step 5. Otherwise, go to Step 6.
If the retrieved property value is an object pointer, simply return the result. If the value is a scalar type supported by NSNumber, it is stored in an NSNumber instance and returned. If the result is a scalar type not supported by NSNumber, it is converted to an NSValue object and returned.
If all other methods fail, call valueForUndefinedKey:. By default, this throws an exception, but subclasses of NSObject may provide key-specific behavior.
Value flow chart
KVC exception handling
If there is no key or keyPath (e.g. dictionary value, object assignment), NSUnknownKeyException will be thrown and the program will Crash
Set value of Crash
LGPerson *p = [[LGPerson alloc] init];
[p setValue:@"1" forKey:@"name111"];
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Set value of null
LGPerson *p = [[LGPerson alloc] init];
NSString *string = nil;
[p setValue:string forKey:@"name"];
NSLog(@"%@",[p valueForKey:@"name"]);
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Value of Crash
LGPerson *p = [[LGPerson alloc] init];
[p valueForKey:@"name111"];
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Exception handling
Apple provides corresponding exception handling methods
/* Given that an invocation of -valueForKey: would be unable to get a keyed value using its default access mechanism, return the keyed value using some other mechanism. The default implementation of this method raises an NSUndefinedKeyException. You can override it to handle properties that are dynamically defined at run-time.*/
- (**nullable** **id**)valueForUndefinedKey:(NSString *)key;
/* Given that an invocation of -setValue:forKey: would be unable to set the keyed value using its default mechanism, set the keyed value using some other mechanism. The default implementation of this method raises an NSUndefinedKeyException. You can override it to handle properties that are dynamically defined at run-time.*/
- (**void**)setValue:(**nullable** **id**)value forUndefinedKey:(NSString *)key;
/* Given that an invocation of -setValue:forKey: would be unable to set the keyed value because the type of the parameter of the corresponding accessor method is an NSNumber scalar type or NSValue structure type but the value is nil, set the keyed value using some other mechanism. The default implementation of this method raises an NSInvalidArgumentException. You can override it to map nil values to something meaningful in the context of your application.*/
- (**void**)setNilValueForKey:(NSString *)key;
Copy the codeRecorresponding methods in the target class can avoid crashes, but this can be tedious and can be added to a unified parent class or added classification
