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

Dashlane 6.2027.0 - Password manager and...
Dashlane is an award-winning service that revolutionizes the online experience by replacing the drudgery of everyday transactional processes with convenient, automated simplicity - in other words,... Read more
ffWorks 2.1.5 - Convert multimedia files...
ffWorks, focused on simplicity, brings a fresh approach to the use of FFmpeg, allowing you to create ultra-high-quality movies without the need to write a single line of code on the command-line.... Read more
Dropbox 101.4.434 - Cloud backup and syn...
Dropbox for Mac is a file hosting service that provides cloud storage, file synchronization, personal cloud, and client software. It is a modern workspace that allows you to get to all of your files... Read more
1Password 7.6 - Powerful password manage...
1Password is a password manager that uniquely brings you both security and convenience. It is the only program that provides anti-phishing protection and goes beyond password management by adding Web... Read more
EyeTV 4.0.0 - Watch and record TV on you...
EyeTV brings a rich TV experience to your Mac. Watch live TV on your Mac. Pause, rewind, and record whenever you want. EyeTV gives you powerful control over what you watch and how you watch it. Put... Read more
Tidy Up 5.3.7 - Find duplicate files and...
Tidy Up is a full-featured duplicate finder and disk-tidiness utility. Features: Supports Lightroom: it is now possible to search and collect duplicates directly in the Lightroom library. Multiple... Read more
Pinegrow 5.97 - Mockup and design web pa...
Pinegrow (was Pinegrow Web Designer) is desktop app that lets you mockup and design webpages faster with multi-page editing, CSS and LESS styling, and smart components for Bootstrap, Foundation,... Read more
BlueStacks 4.210.0 - Run Android applica...
BlueStacks App Player lets you run your Android apps fast and fullscreen on your Mac. Feature comparison chart How to install Bluestacks on your Mac Go to MacUpdate and click the green "Download"... Read more
WhatsApp 2.2027.10 - Desktop client for...
WhatsApp is the desktop client for WhatsApp Messenger, a cross-platform mobile messaging app which allows you to exchange messages without having to pay for SMS. WhatsApp Messenger is available for... Read more
Art Text 4.0.1 - $29.99
Art Text is graphic design software specifically tuned for lettering, typography, text mockups and various artistic text effects. Supplied with a great variety of ready to use styles and materials,... Read more

Latest Forum Discussions

See All

Marvel Super War launches its third seas...
Marvel Super War, NetEase's popular MOBA, has kicked off its third season today, introducing its Zone Invasion beta and an all-new playable superhero. [Read more] | Read more »
Brave Dungeon, Unlock Games' idle-R...
Brave Dungeon, Unlock Games' idle-RPG and auto chess title, has been downloaded over 1 million times since it launched last week. To commemorate reaching this milestone the developers have decided to host a series of in-game events. [Read more] | Read more »
Clash Royale: The Road to Legendary Aren...
Supercell recently celebrated its 10th anniversary and their best title, Clash Royale, is as good as it's ever been. Even for lapsed players, returning to the game is as easy as can be. If you want to join us in picking the game back up, we've put... | Read more »
Steam Link Spotlight - Disco Elysium
Steam Link Spotlight is a feature where we look at PC games that play exceptionally well using the Steam Link app. Our last entry was Signs of the Sojourner Read about how it plays using Steam Link over here. | Read more »
Distract Yourself With These Great Mobil...
There’s a lot going on right now, and I don’t really feel like trying to write some kind of pithy intro for it. All I’ll say is lots of people have been coming together and helping each other in small ways, and I’m choosing to focus on that as I... | Read more »
Pokemon Go's July Community Day wil...
Pokemon Go developers have announced the details concerning the upcoming Gastly Community Day. This particular event was selected by the players of the game after the Gas Pokemon came in second place after a poll that decided which Pokemon would... | Read more »
Clash Royale: The Road to Legendary Aren...
Supercell recently celebrated its 10th anniversary and their best title, Clash Royale, is as good as it's ever been. Even for lapsed players, returning to the game is as easy as can be. If you want to join us in picking the game back up, we've put... | Read more »
Detective Di is a point-and-click murder...
Detective Di is a point-and-click murder mystery set in Tang Dynasty-era China. You'll take on the role of China's best-known investigator, Di Renjie, as he solves a series of grisly murders that will ultimately lead him on a collision course with... | Read more »
Dissidia Final Fantasy Opera Omnia is se...
Dissidia Final Fantasy Opera Omnia, one of Square Enix's many popular mobile RPGs, has announced a plethora of in-game events that are set to take place over the summer. This will include several rewards, Free Multi Draws and more. [Read more] | Read more »
Sphaze is a neat-looking puzzler where y...
Sphaze is a neat-looking puzzler where you'll work to guide robots through increasingly elaborate mazes. It's set in a visually distinct world that's equal parts fantasy and sci-fi, and it's finally launched today for iOS and Android devices. [... | Read more »

Price Scanner via MacPrices.net

B&H Photo offers $200-$300 discounts on A...
B&H Photo has new 16″ MacBook Pros on sale today for $200-$300 off Apple’s MSRP, starting at $2149. Expedited shipping is free to many addresses in the US: – 2019 16″ 2.6GHz 6-Core MacBook Pro... Read more
Clearance 2019 15″ MacBook Pros on sale today...
Amazon-owned Woot is blowing out Apple refurbished, clearance 2019 15″ MacBook Pros starting at only $1579 and up to $950 off Apple’s original MSRP. According to Woot, “These MacBooks are Refurbished... Read more
Apple Refurbished iMac Pros available for $35...
Amazon-owned Woot is selling Apple refurbished 27″ 3.2GHz 8-Core iMac Pros for $3599.99 shipped. That’s $1400 off Apple’s original MSRP for this model. According to Woot, these iMac Pros are “Factory... Read more
Clearance 2019 13″ 2.4GHz/256GB MacBook Pro o...
B&H Photo has dropped their price on the clearance 2019 13″ 2.4GHz/256GB Quad-Core Silver MacBook Pro by $500 off Apple’s original MSRP to a new low of only $1299. Expedited shipping is free to... Read more
$219 Apple AirPods Pro are back at Verizon, s...
Verizon has Apple AirPods Pro on sale again for a limited time for $219.99 on their online store. Their price is $30 off Apple’s MSRP, and it’s the lowest price we’ve seen for AirPods Pro. Available... Read more
Apple’s $779 13″ MacBook Air deal returns to...
Apple has clearance, Certified Refurbished, 2019 13″ MacBook Airs available again starting at $779. Each MacBook features a new outer case, comes with a standard Apple one-year warranty, and is... Read more
$200 13″ MacBook Pro discounts are back at Am...
Amazon has 2020 13″ 2.0GHz MacBook Pros on sale again today for $150-$200 off Apple’s MSRP. Shipping is free. Be sure to purchase the MacBook Pro from Amazon, rather than a third-party seller, and... Read more
Deal Alert! Apple AirPods with Wireless Charg...
Sams Club has Apple AirPods with Wireless Charging Case on sale on their online store for only $149.98 from July 6, 2020 to July 9, 2020. Their price is $50 off Apple’s MSRP, and it’s the lowest... Read more
Xfinity Mobile promo: Apple iPhone XS models...
Take $300 off the purchase of any Apple iPhone XS model at Xfinity Mobile while supplies last. Service plan required: – 64GB iPhone XS: $599.99 save $300 – 256GB iPhone XS: $749.99 save $300 – 512GB... Read more
New July 2020 promo at US Cellular: Switch an...
US Cellular has introduced a new July 2020 deal offering free 64GB Apple iPhone 11 smartphones to customers opening a new line of service. No trade-in required, and discounts are applied via monthly... Read more

Jobs Board

Product Manager, *Apple* Commercial Sales -...
Product Manager, Apple Commercial Sales Austin, TX, US Requisition Number:77652 As an Apple Product Manager for the Commercial Sales team at Insight, you Read more
Physical Therapist Assistant - *Apple* Hill...
Physical Therapist Assistant - Apple Hill Rehab - Full Time Tracking Code 62519 Job Description General Summary: Under the direct supervision of a licensed Physical Read more
Operating Room Assistant, *Apple* Hill Surg...
Operating Room Assistant, Apple Hill Surgical Center - Full Time, Day Shift, Monday - Saturday availability required Tracking Code 62363 Job Description Operating Read more
Perioperative RN - ( *Apple* Hill Surgical C...
Perioperative RN - ( Apple Hill Surgical Center) Tracking Code 60593 Job Description Monday - Friday - Full Time Days Possible Saturdays General Summary: Under the Read more
Product Manager, *Apple* Commercial Sales -...
Product Manager, Apple Commercial Sales Austin, TX, US Requisition Number:77652 As an Apple Product Manager for the Commercial Sales team at Insight, you Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.