Objective-c Diary-The KVC

KVC (key-value-coding?) 1, overview

A method that sends or obtains a value as a string to an instance variable or property of an object.

2, usage

A, take the value

@property (readwrite,copy) nsstring * name; Car.h

@synthesize NAME;//CAR.M

Now, I want to get the value of name, you can use the KVC method

NSString *name=[car valueforkey:@ "name"];

b, set the value

[Car setvalue:@ "Harlod" forkey:@ "name"];

3, extension usage

A, use a path (string expression form dot expression)-(void) Setvalue:forkeypath

@interface Engine:nsobject <nscopying>{

int horsepower;

}

@end//engine

[Car Setvalue:[nsnumber numberwithint:155]

forkeypath:@ "Engine.horsepower"];

b, using a key value for Nsarray, the key value is found for each object in the array, and the query results are packaged back.

As follows:

Nsarray *pressures=[car valueforkeypath:@ "Tires.pressure"];

NSLog (@ "pressures%@", pressures);

Will output the following results:

Pressures (

34,

34,

34,

34

)

4, progressive usage

A, fast arithmetic, that is, using the escape operator, such as @count, @max, @min, @avg calculate the result on the left side of the key path.

As follows:

@interface garge:nsobject{

NSString *name;

Nsmutablearray *cars;

}

@property (readwrite,copy) NSString *name;

-(void) Addcar: (car *) car;

-(void) print;

@end//garage

NSNumber *count;

Count=[garage valueforkeypath:@ "[email protected]"];

NSLog (@ "We have%@ cars", count);

NSNumber *sum;

Sum=[garage valueforkeypath:@ "[email protected]"];

NSLog (@ "We have a grand total of%@ miles", sum);

b, Quick get or set (using dictionary)

-dictionarywithvaluesforkeys and-setvalueforkeyswithdictionary

The following (Get values):

Car=[[garage valueforkeypath:@ "Cars"] lastobject];//in the path of the key value is not a property in garage, but can also be used as a property path

First, pick a car from the garage, as above

Nsarray *keys=[nsarray arraywithobjects:@ "make", @ "model", @ "Modelyear", nil];

Then, a single list of properties that need to be obtained is placed in the keys array, as above

Nsdictionary *carvalues=[car Dictionarywithvaluesforkeys:keys];

Finally, use the word typical pair like store to get the dictionary value using-dictionarywithvaluesforkeys.

The following are (set values):

Nsdictionary *newvalues=[nsdictionary dictionarywithobjectsandkeys:@ "Chevy", @ "make" @ "Nova" @ "model", [NSNumber numberwithint:1964],@ "Modelyear", nil];

First, define a dictionary

[Car setvaluesforkeyswithdictionary:newvalues];

Again, set the value of the car.

c, handling undefined keys

When we use Valueforkey, we use an undefined keyword, and then we get an error at run time:

This class is not key value coding-compliant for the key xxxxxx.

When we deal with these kinds of problems, we generally use the rewriting-valueforundefinedkey and-setvalue:forundefinedkey methods

The following methods are used in the OBJECTIVE-C basic tutorial

C1, defines an empty Dictionary object stuff.

@interface garage:nsobject{

NSString *name;

Nsmutablearray *cars;

Nsmutabledictionary *stuff;

}

C2, handling, setting an assignment problem for undefined keys, placing key-value pairs in a Dictionary object

-(void) SetValue: (ID) value Forunderfinedkey: (NSString *) key{

if (Stuff=nill) {

Stuff==[[nsmutabledictionary Alloc]init];

}

[Stuff Setvaleu:value Forkey:key];

}//setvalueforundefinedkey

C3, processing, getting values for undefined keys, redirecting lookups to Dictionary objects

-(ID) Valueforundefinedkey: (NSString *) key{

ID Value=[stuff Valueforkey:key];

return (value);

}//valueforundefinedkey

Objective-c Diary-The KVC