TweetFollow Us on Twitter

EOKeyValueCoding Volume Number: 16 (2000)
Issue Number: 12
Column Tag: WebObjects

EOKeyValueCoding

By Sam Krishna and Patrick Taylor

Throughout this series we've asserted that programming in WebObjects is a richer, more mature experience than any competing web application development environment. A major reason for this superior programming experience can be laid at the feet of EOKeyValueCoding.

EOKeyValueCoding is simultaneously a powerful protocol (in Objective C) and interface (in Java). When the EOControl framework is imported, all Objective C classes that inherit from NSObject receive EOKeyValueCoding behavior. (The EOControl framework is imported automatically into WebObjects apps and frameworks. You do not have to explicitly import it in to your WebObjects app or framework project.) In Java, when the developer creates a subclass of EOEnterpriseObject, EOCustomObject, EOGenericRecord or WOComponent, then EOKeyValueCoding is available.

What is remarkable about EOKeyValueCoding is how universally powerful it is. With this protocol/interface, the WebObjects and EOF frameworks know how to access custom instance variables (ivars) from within your subclass. Using the methods valueForKey() and takeValueForKey() (in Objective C valueForKey: and takeValue:forKey:) WOF and EOF can access your ivars through standard API. In addition, EOKeyValueCoding provides the bridge within EOF for effective entity-relationship mapping.

In valueForKey(), this standard method allows a lookup of an ivar from both WOF and EOF classes. Consider a WOComponent subclass that needs to fill in the value of a String object in the dynamic WOString element. The WOString is bound to lastName of an EnterpriseObject (EO) which represent the employees of a corporation.

Order of Access

  1. The component subclass searches for a public accessor method based on the key name. With a key of "lastName", valueForKey() looks for a method named getLastName() or lastName(). In Objective C, it looks for a method named getLastName or -lastName.
  2. If a public accessor method isn't found, the component subclass searches for a private accessor method based on key. Note that traditionally in the Apple frameworks, a private method is distinguished by a preceding underbar. In this case, valueForKey() looks for a method named _getLastName() or _lastName(). In Objective C, it looks for a method named _getLastName or -_lastName.
  3. If an accessor method isn't found and the class method accessInstanceVariablesDirectly returns true (in Java) or YES (in Objective C), valueForKey() searches for an ivar based on the key name and returns its value directly. For the key "lastName", this would be _lastName or lastName.
  4. If neither an accessor method or an ivar is found, the default implementation invokes handleQueryWithUnboundKey() in Java or handleQueryWithUnboundKey: in Objective C. The default implementation of this method will raise an NSException.

Sometimes exceptions are raised because although ivars have changed or been deleted for some reason the component subclass or the frameworks still expect the ivar to exist unchanged. In these situations after 10-15 minutes of proverbial pounding sand, we would override handleQueryWithUnboundKey() and check to see if the disputed ivar is being looked up, and if so, return nil or null.

EOKeyValueCoding API to Access Instance Variables

For another example, suppose that you have an EO using a BigDecimal or NSDecimalNumber with "someDecimal" as one of its ivars.

Java

(BigDecimal)myEO.valueForKey("someDecimal");
note: you must always downcast your objects when using valueForKey() in Java

Objective C

[myEO valueForKey:@"someDecimal"];
Here is the operational order for takeValueForKey() (takeValue:forKey: in Objective C)
  1. The EOKeyValueCoding methods search for a public accessor method of the form setKey (setKey: in Objective C)
  2. If a public accessor method isn't found, EOKeyValueCoding searches for a private accessor method of the form _setKey (Objective C _setKey:) invoking it if such a method exists
  3. If an accessor method isn't found and the class method accessInstanceVariablesDirectly returns true or YES, takeValueForKey() searches for an ivar based on the expected key name and sets the value directly in Java. In Objective C, the old value is autoreleased and the new one is retained. For the key "lastName", this would be _lastName or lastName.
  4. If neither an accessor method or an ivar is found, the default implementation invokes handleTakeValueForUnboundKey in Java and handleTakeValue:forUnboundKey: in Objective C.

It is important that while this method has been very well debugged, sometimes phantom keys can be looked up long after they "died". It is prudent to override handleTakeValueForUnboundKey in this situation to check if the phantom key is being looked up and, if so, do nothing.

What are Protocols and Interfaces?

Objective-C protocols and Java interfaces represent essentially the same object-oriented concept: the ability to declare methods that are independent of a particular class and can be implemented by classes in any heirarchy. Any Objective-C class that declares itself to conform to a particular protocol or Java class that implements a particular interface must, by default, implement the methods declared by that protocol/interface. The approaches taken are different with the Objective C approach being arguably more flexible.

Confused? Don't worry — maybe an analogy can make things clearer. Imagine you're writing a nature simulation program which requires that various "animals" have a swimming behavior. Since entirely different kinds of animals can share behaviors even if they come from entirely different species, phyla or families, not every swimming creature inherits from the same class (whales, trout, dogs and humans are all "swimmers" even though they inherit from different classes of "animal": whales do not inherit from trout because whales are mammals while trout are fish, dogs do not inherit from humans because dogs are canines while humans are primates, etc).

In your swimming simulation, you need to have your objects implement a specific type of behavior, but not necessarily the same way. You also want to show what happens when certain types of animals are not able to swim in water (like most species of birds, for example). So, if you are writing an Objective C program, you declare a 'Swimming' protocol. Correspondingly, if you are writing a Java program, you declare a 'Swimming' interface.

The Objective-C protocol declaration may look like this:

@protocol Swimming

- (void)treadWater;
- (void)swimFreestyle;
- (void)breathe;
....

@end

The Java interface declaration may look like this:

public interface Swimming {
    public void treadWater();
    public void swimFreestyle();
    public void breathe();
    ....
}

In the swimming simulation, dogs, humans, whales, trout and ducks implement the Swimming protocol or interface method declarations and would swim in ways unique to those species. A hummingbird, on the other hand, would probably not implement the Swimming protocol or interface, and therefore drown if left in water for a prolonged period of time.

To set the ivar 'someDecimal' to a BigDecimal object of value '1', you would:

Java

myEO.takeValueForKey(new BigDecimal("1"), "someDecimal);

Objective C

[myEO takeValue:[NSDecimalNumber decimalNumberWithString:@"1"] forKey:@"someDecimal"]

What happens when your code accesses ivars that no longer exist?

This is an interesting situation. Most often, this occurs when a developer has removed an ivar from his EO without deleting all references to it from the code. EOF's default behavior is to invoke handleQueryWithUnboundKey in Java and handleQueryWithUnboundKey: in Objective C which throws an exception. Developers can override this if they desire some other behavior or have it fail gracefully (not at all recommended but if it is absolutely necessary ... ).

Corollary behavior occurs when using takeValueForKey (Objective C takeValue:forKey:)on an ivar that doesn't exist. EOF automatically invokes the method handleTakeValueForUnboundKey (Objective C handleTakeValue:forUnboundKey:). There are methods used as private API within EOF that are publicly exposed. storedValueForKey and takeStoredValueForKey in Java (storedValueForKey: and takeStoredValue:forKey: in Objective C) are used to access and/or initialize the receiver's values as they are stored in the database.

Making Life Easier

There are some methods in EOKeyValueCoding that make life considerably easier to get things done. And in the case of Objective C, it provides a very convenient way to implement the NSCoding protocol within a class.

valueForKeyPath (Objective C valueForKeyPath:) allows you to return the value at the end of a relationship path's property. For example, if you wanted to find the name of anEmployee's department, you would:

Java

(String)anEmployee.valueForKeyPath("department.name");

Objective C

[anEmployee valueForKeyPath:@"department.name"];

The relationship from anEmployee to its department was traversed and the department's name accessed. Having done this, you can use another method takeValueForKeyPath (Objective C takeValue:forKeyPath:) to set the name of a relationship's property. To change the name of anEmployee's department:

Java

anEmployee.takeValueForKeyPath("Data Processing","department.name");

ObjectiveC

[anEmployee takeValue:@"Data Processing: forKeyPath:@"department.name"];

valuesForKeys (Objective C valuesForKeys:) returns an NSDictionary of the key-value pairs of an object. It's corollary method takeValuesFromDictionary (Objective C takeValuesFromDictionary:) sets the ivars of an object to the values of the NSDictionary passed as an argument.

What are Ivars/Instance Variables?

Instance variables (also known as ivars) are the specific variables of data for an object instance. What does that mean?

Let's say you have an Employee class which generically defines what an employee looks like. An Employee has a first name, a last name, and a Social Security number. In Objective C, an Employee would look like this:

@interface Employee : NSObject
{
    NSString *firstName;
    NSString *lastName;
    NSString *ssn;
}
....
@end

And in Java, an Employee would look like this:

public class Employee extends Object {
    protected String firstName;
    protected String lastName;
    protected String ssn;
    ....
}

Employee objects, like the employees that represent the CEO and the vice-president, would have a firstName, lastName, and an ssn instance variable. For the CEO, her instance variables might look like this: firstName = "Jane", lastName = "Smith", ssn = "555-12-1234". For the vice-president, his instance variables may look like this: firstName = "John", lastName = "Brown", ssn = "555-12-9876". For the CEO Employee object, its variables contain unique data, and likewise for the vice-president Employee object as well.

EOKeyValueCoding provides a consistent API for the frameworks to access the ivars for the retrieval and modification of data. Whenever a firstName is displayed on a web page in a WebObjects app, it is retrieved through EOKeyValueCoding's APIs. The corresponding modification of ivars occurs through EOKeyValueCoding's APIs on all objects. This consistency allows developers to create applications and frameworks faster because they don't have to think about how the ivars need to be accessed.

These methods can be used to quickly implement the NSCoding protocol in Objective C. The NSCoding protocol can be used to make an object archivable to disk. It consists of two methods encodeWithCoder: and initWithCoder:. Imagining a situation where a developer wished to implement an Employee class that didn't use EOF. Some form of persistence needs to be created for the Employee objects by implementing the NSCoder protocol.

Implementing persistence

Employee.h
#import <Foundation/Foundation.h>
@interface Employee : NSObject <NSCoding>
{
    NSString *firstName;
    NSString *lastName;
    NSString *ssn;
    NSString *streetAddressOne;
    NSString *streetAddressTwo;
    NSString *city;
    NSString *state;
    NSString *zipCode;
    NSDecimalNumber *salary;
}

//  Skip the accessors....

//  EOKeyValueCoding-based methods
- (NSDictionary *)objectAsDictionary;
- (NSArray *)attributeNames;

//  NSCoding methods (which we declare for this example...)
- (id)initWithCoder:(NSCoder *)coder;
- (void)encodeWithCoder:(NSCoder *)coder;

@end

Employee.m
#import "Employee.h"
#import <EOControl/EOKeyValueCoding.h>

@implementation Employee

//  EOKeyValueCoding-based methods
- (NSArray *)attributeNames
{
    //  Return the ivar 'keys' in an NSArray
    return [NSArray arrayWithObjects:
         @"firstName",
         @"lastName",
         @"ssn",
         @"streetAddressOne",
         @"streetAddressTwo",
         @"city",
         @"state",
         @"zipCode",
         @"salary",
        nil];
}

- (NSDictionary *)objectAsDictionary
{
    //  Return the object as an NSDictionary, using the -attributeNames
    //  method to define the keys of the object
    return [self valuesForKeys:[self attributeNames]];
}

- (NSString *)description
{
    //  Inherited from NSObject
    //  Represent the object as an NSDictionary
    return [[self objectAsDictionary] description];
}

//  NSCoding methods
- (void)encodeWithCoder:(NSCoder *)coder
{
    //  NSDictionary already conforms to the NSCoding protocol,  as
    //  well as NSString and NSDecimalNumber
    [super encodeWithCoder:coder];
    [coder encodeObject:[self objectAsDictionary]];
    
    return;
}

- (id)initWithCoder:(NSCoder *)coder
{
    NSDictionary *newSelf;
    
    self = [super initWithCoder:coder];
    newSelf = [coder decodeObject];
    
    //  Since we've already archived ourselves as an NSDictionary,
    //  we can probably take our values from the decoded NSDictionary
    [self takeValuesFromDictionary:newSelf];
    
    return self;
}

@end

This is a simpler (and faster) way of implementing NSCoder, particularly as the number of ivars increasses. In our example, the traditional approach would have required 10-12 additional lines of code for dealing with all of the ivars individually. Instead of all that, the object is archived as an NSDictionary and the EOKeyValueCoding protocol takes care of the rest.

Conclusion

EOKeyValueCoding provides a consistent and convenient API to access and manipulate all of your ivars within your objects. It also provides a uniform way to represent all objects as NSDictionaries to WebObjects and EOF. This helps make object-oriented programming within WebObjects much simpler which is beneficial regardless of your level of expertise.

Unsurprisingly, EOKeyValueCoding is too useful to be kept to EOF and WebObjects alone. Apple recognized its potential by migrating it for JavaClient as NSKeyValueCoding. Even more significant, NSKeyValueCoding appears to be moving into Foundation for use with Cocoa programming. Other than some source code #import changes, all the API stays consistent which now means that persistence within classes will become much simpler.


Please feel free to contact the authors with questions or comments on the articles at webobjectseof@mac.com. We may not be able to reply personally to all emails but every one will be read.

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Latest Forum Discussions

See All

Soft Drummer (Music)
Soft Drummer 1.0 Device: iOS Universal Category: Music Price: $14.99, Version: 1.0 (iTunes) Description: Soft Drummer is the closest thing to a pro session subtle drummer in your pocket. Easy to use and fast, it's much more than a... | Read more »
Is GO Gear the Pokemon GO map app you...
Now that we've settled into something of a Pokemon GO status quo, the number one desire of most players can best be summed out by modifying a quote from Rod Tidwell of Jerry Maguire: "Show me the Pokemon!" [Read more] | Read more »
Rodeo Stampede update: Mountains, new an...
The Savannah and Jungle were just the beginning in Rodeo Stampede. Get ready to head for the Mountains. I think I heard that in a beer ad once. [Read more] | Read more »
COSMOS RINGS (Games)
COSMOS RINGS 1.0.0 Device: iOS iPhone Category: Games Price: $5.99, Version: 1.0.0 (iTunes) Description: This game cannot be played without the Apple Watch.Released anniversary sale until August 31,2016 PST! A tragic tale of time's... | Read more »
How to get started selling on Mercari
As far as ecommerce has come over the last decade or so, there's still a tremendous opportunity to make it easier for people to buy and sell goods. That's especially true when it comes to shopping apps, which should only continue to increase in... | Read more »
Human Anatomy Atlas 2017 Edition - Compl...
Human Anatomy Atlas 2017 Edition - Complete 3D Human Body 1.0.24 Device: iOS iPhone Category: Medical Price: $24.99, Version: 1.0.24 (iTunes) Description: | Read more »
Heroes of Normandie (Games)
Heroes of Normandie 1.5 Device: iOS Universal Category: Games Price: $14.99, Version: 1.5 (iTunes) Description: The game does not support iPhone 4s and below | Read more »
Why you should never power up Pokemon in...
There's no question that candy is dandy in Pokemon GO. You need big quantities of it to evolve your Pokemon, and when combined with stardust, it can be used to power up your favorite pocket monsters as well, making them more formidable for the gym... | Read more »
Webzen launches 3D MMORPG MU Origin on i...
Mu Origin is featured time and time again at the very top of App Stores in China, and within the top five worldwide top-grossing charts on Google Play.Its popularity in Korea and China, featuring more than 120 registered players in China and 6... | Read more »
Severed (Games)
Severed 1.0 Device: iOS Universal Category: Games Price: $5.99, Version: 1.0 (iTunes) Description: LAUNCH DISCOUNT ON NOW!! ENDS AUGUST 4! ==== Take control of a one-armed warrior named Sasha, wielding a living sword on her journey... | Read more »

Price Scanner via MacPrices.net

9-inch 32GB Space Gray iPad Pro on sale for $...
B&H Photo has the 9″ 32GB WiFi Space Gray Apple iPad Pro on sale for $50 off MSRP including free shipping. B&H charges sales tax in NY only: - 9″ Space Gray 32GB WiFi iPad Pro: $549 $50 off... Read more
15-inch Retina MacBook Pros on sale for up to...
B&H Photo has 15″ Retina MacBook Pros on sale for up to $200 off MSRP. Shipping is free, and B&H charges NY tax only: - 15″ 2.2GHz Retina MacBook Pro: $1849 $150 off MSRP - 15″ 2.5GHz Retina... Read more
Second-Quarter Tablet Shipments Fell 4.8% –...
The latest report from the global market research firm TrendForce finds that worldwide tablet shipments for this second quarter totaled 33.54 million units, representing a quarterly drop of 4.8% and... Read more
Global Smartphone Sales Volumes Mark Second S...
According to preliminary results from the International Data Corporation (IDC) Worldwide Quarterly Mobile Phone Tracker, vendors shipped a total of 343.3 million smartphones worldwide in the second... Read more
Apple TVs on sale for $20-$40 off MSRP
Best Buy has 32GB and 64GB Apple TVs on sale for $20-$40 off MSRP on their online store. Choose free shipping or free local store pickup (if available). Sale prices for online orders only, in-store... Read more
Mac minis on sale for $50-$100 off MSRP
B&H Photo has Mac minis on sale for $50 off MSRP including free shipping plus NY sales tax only: - 1.4GHz Mac mini: $449 $50 off MSRP - 2.6GHz Mac mini: $649 $50 off MSRP - 2.8GHz Mac mini: $949... Read more
Clearance 2015 13-inch MacBook Airs available...
B&H Photo has clearance 2015 13″ MacBook Airs available for $300 off original MSRP. Shipping is free, and B&H charges NY sales tax only: - 13″ 1.6GHz/4GB/128GB MacBook Air (MJVE2LL/A): $799... Read more
Apple certified refurbished iPad mini 4s avai...
Apple has certified refurbished iPad mini 4s now available for up to $120 off the cost of new models. An Apple one-year warranty is included with each iPad, and shipping is free. The following models... Read more
Notebook Makers In No Rush To Adopt USB-C – R...
Digitimes’ Cage Chao and Joseph Tsai note that while the USB Type-C interface is enjoying growing popularity among smartphones and tablet makers, notebook and all-in-one (AIO) PC vendors (other than... Read more
iMacs on sale for up to $250 off MSRP
B&H Photo has 21″ and 27″ Apple iMacs on sale for up to $250 off MSRP including free shipping plus NY sales tax only: - 27″ 3.3GHz iMac 5K: $2049 $250 off MSRP - 27″ 3.2GHz/1TB Fusion iMac 5K: $... Read more

Jobs Board

*Apple* Retail - Bilingual - Multiple Positi...
…speaking a plus Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the Read more
Simply Mac *Apple* Specialist- Repair Techn...
…The Technician is a master at working with our customers to diagnose and repair Apple devices in a manner that exceeds the expectations set forth by Apple Read more
*Apple* Mobile Master - Best Buy (United Sta...
What does a Best Buy Apple Mobile Master do? At Best Buy, our mission is to leverage the unique talents and passions of our employees to inspire, delight, and enrich Read more
Best Buy *Apple* Computing Master - Best Bu...
What does a Best Buy Apple Computing Master do? At Best Buy our mission is to leverage the unique talents and passions of our employees to inspire, delight, and Read more
*Apple* Valley, CA School Speech Therapy Ope...
Apple Valley, CA School Speech Therapy Openings + Job Location: Apple Valley, CA + Category: Schools - SLP - CFY + Apply Now! + Back to Results Speech Language Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.