IOS development -- Differences Between objectForKey and valueForKey in NSDictionary, objectforkey

IOS development -- Differences Between objectForKey and valueForKey in NSDictionary, objectforkey

The NSDictionary has two methods: objectForKey: And valueForKey:. What are the differences between the two methods?

First, we can see the definitions of the two methods in the NSDictionary document:

ObjectForKey: returns the value associated with aKey, or nil if no value is associated with aKey. the value of the specified key is returned. if this key is not found, nil is returned.

ValueForKey: returns the value associated with a given key. Similarly, the value of the specified key is returned.
 

Intuitively, there seems to be no difference between the two methods, but valueForKey in this document:

If key does not start with "@", invokes objectForKey :. if key does start with "@", strips the "@" and invokes [super valueForKey:] with the rest of the key. via Discussion

In general, the key can be any string combination. If the key does not start with the @ symbol, then valueForKey: is equivalent to objectForKey:. If it starts, remove the @ in the key and use the remaining part as the key to execute [super valueForKey:].
 

For example:

NSDictionary * dict = [NSDictionary dictionaryWithObject: @ "theValue"

ForKey: @ "theKey"];

NSString * value1 = [dict objectForKey: @ "theKey"];

NSString * value2 = [dict valueForKey: @ "theKey"];
 

At this time, value1 and value2 are the same results. For such a dict:

NSDictionary * dict = [NSDictionary dictionaryWithObject: @ "theValue"

ForKey: @ "@ theKey"]; // note that this key starts @

NSString * value1 = [dict objectForKey: @ "@ theKey"];

NSString * value2 = [dict valueForKey: @ "@ theKey"];
 

The value of value1 is correct, but the value of value2 is crash directly. The error message is as follows:

Terminating app due to uncaught exception 'nsunknownkeyexception', reason: '[<__ NSCFDictionary 0x892fd80> valueForUndefinedKey:]: this class is not key value coding-compliant for the key theKey .'

This is because valueForKey: Is a KVC (NSKeyValueCoding) method. In KVC, you can use a string with the same name as property to obtain the corresponding value. For example:

@ Interface Person: NSObject

@ Property (nonatomic, retain) NSString * name;

@ End

...

Person * person = [[Person alloc] init];

Person. name = @ "fannheyward ";

NSLog (@ "name: % @", [person name]);

// Name: fannheyward

NSLog (@ "name: % @", [person valueForKey: @ "name"]);

// Name: fannheyward

[Person release];
 

ValueForKey: The value is the property accessor with the same name as the specified key. If no value is available, valueForUndefinedKey: is executed. The default Implementation of valueForUndefinedKey: is to throw an NSUndefinedKeyException.

Looking back at the crash example, [dict valueForKey: @ "@ theKey"]; Removes @ from the key and changes it to [dict valueForKey: @ "theKey"];, dict does not have a property like theKey. Instead, it executes [dict valueForUndefinedKey: @ "theKey"]; and then crash out when an NSUndefinedKeyException is thrown.

ObjectForKey: And valueForKey: In most cases, the same result is returned. However, if the key starts with @, valueForKey: becomes a big pitfall. We recommend that you only use objectForKey: In NSDictionary to set the value.