TweetFollow Us on Twitter

Reusable WebObjects Components

Volume Number: 18 (2002)
Issue Number: 8
Column Tag: WebObrjects Development

Reusable WebObjects Components

by Tom Woteki

Introduction

One of the more interesting and powerful features of WebObjects is the ability to construct reusable components. However neither the documentation nor any of the available books on WebObjects cover this topic very well. The best discussions are in Professional WebObjects 5.0 with Java (DeMan, et. al. 2001), and the Windows and version 4.5-oriented WebObjects Web Application Construction Kit (Ruzek 2001). Both of these treatments, though instructive, leave a lot to the reader. The common example that both books cover is how to construct a standard template for an application.

This article illustrates how to build reusable components using two examples drawn from a WebObjects application I wrote to maintain records of a wine collection. It also illustrates creating custom component bindings and communications between components using key-value coding. The example components have wide applicability to the maintenance of reference tables and are easily generalized to maintaining any single attribute of a database table.

This article assumes you are familiar with at least the basics of building a WebObjects database application including using interface widgets, forms, actions, data models and display groups.

The Example Problem: Maintaining a Reference Table

A common type of table in a database application is a reference table, that is, a table consisting of two columns: a primary key and the attribute of interest. An example is a table of country names. In my application there are reference tables for wine producing countries, wine regions, wine names and so on. Given such reference tables, an obvious need is to maintain the tables, either to add new records or to update existing records. Because my application uses multiple reference tables, and noting the obvious generality of the situation, I decided to develop some reusable components for maintaining any reference table. We'll see how to use these components in an example application to maintain a reference table of country names.

High-Level Design

The approach I took was to develop two components, each designed to be embedded in a form within a parent component. One component, the "selector", is used either to select an existing record to edit or to initiate insertion of a new record. The other, the "editor", is used to actually edit the selected record or create the new one. The two components communicate with each other through key-value coding whereby the selector passes the user's intent and other information to the editor. We use bindings to designate the specific table and attribute the components should edit.

Figures 1 and 2 show the respective finished products, each embedded in parent components. The selector component in Figure 1 consists only of the WOPopUpButton and the two form buttons "Edit" and "New". The former button initiates an update of a selected record, the latter initiates insertion of a new record. The remaining aspects of the interface, such as the prompt string, pertain to parent components such as the page itself. Similarly, the editor component in Figure 2 consists only of a WOTextField and two form buttons. The separate implementation of the components, apart from communications via key-value coding, and their separation from aspects of the surrounding interface, including the form they are embedded in, maximizes their reusability and adaptability to different interface designs.


Figure 1: The finished selector component in action


Figure 2: The finished editor component in action

Detailed Design

Now let's consider details of the preceding design. First of all, let's name the components we're going to build WOObjectSelector and WOObjectEditor. Viewing Figure 1 it should be clear that we must at least provide WOObjectSelector the list it should display in its WOPopUpButton menu and the page(s) to return when either its "Edit" or "New" buttons are clicked. In this design both buttons will return the same page. We might also want to provide the component a "no selection" string to display when there is no selection in the pop-up. Since we are editing a reference table, you can anticipate that we will bind a WODisplayGroup to the pop-up's display list. And you might anticipate that the return page for the buttons should be some page that encloses WOObjectEditor as a child component. Before we discuss these details, including how to provide WOObjectSelector the information it needs, let's consider what WOObjectEditor needs to do its job.

Viewing Figure 2, we can see that we need to provide the editor at least the data for the attribute of the record we are editing (in our example, the name of a country) plus the return page for the form buttons. Even more is needed, however. First of all it would be helpful to know if we are updating an existing record or inserting a new one. Second, if updating we'll need the record itself, not just the data for the attribute of interest. In order to achieve reusability, we'll provide not only the record, but the key for the attribute as well, so that we can use the powerful generality of key-value coding to update the value of any attribute. Finally, in case of inserting a new record, we'll need to create the record and insert it into the default editing context, so we'll need to provide WOObjectEditor the name of the entity to create as well as the key for the pertinent attribute.

Which component will provide WOObjectEditor the information it needs and how? WOObjectSelector knows whether the user is updating or inserting by means of the button the user clicks. If updating, it also knows the record the user is updating, namely the one corresponding to the item selected in the display list. So, WOObjectSelector is the logical provider of this information. It will do so by invoking the key-value coding method takeValueForKey (which is inherited by any class that extends WOComponent) on the parent page that encloses WOObjectEditor. It will invoke the method for each of a series of keys corresponding to the data needed by WOObjectEditor. The parent page for WOObjectEditor is also the return page for WOObjectSelector's buttons. This page, having received the required values from WOObjectSelector, will set the values needed by WOObjectEditor using API bindings. (Note that assignments specified by API bindings are performed behind the scenes by WebObjects using key-value coding.) Finally, the additional information needed by WOObjectEditor, such as the name of the entity we are working with, will be passed through from the parent page of WOObjectSelector using the same techniques.

Figure 3 neatly summarizes the flow of information and the methods for communicating between components. The parent page of WOObjectSelector passes to it the global context, namely entityName and attributeKey, plus the information the selector specifically needs using API bindings. WOObjectSelector then passes the global context along with other specifics that WOObjectEditor needs, such as the user's selection, to the latter's parent using key-value coding. Finally, the editor component's parent passes the information to it using API bindings.


Figure 3.

Now let's consider the details of each component.

WOObjectSelector

Figure 4 shows the layout and keys for WOObjectSelector. The component consists of a WOPopUpButton and two submit buttons within a table. The keys attributeKey, displayList, entityName, noSelectionString and returnPageName were mentioned previously. Their values are passed into the component by its parent.

The other keys, displayAttribute, displayObject and selectedObject are used locally. displayAttribute is a method that returns the string for each item displayed in the pop up menu; it uses key-value coding to retrieve the string using attributeKey as the key (Listing 1). The key displayObject is the local EOGenericRecord on which the display list iterates and selectedObject stores the user's selection as an EOGenericRecord. Implicit in this is that a WODisplayGroup's array of EOGenericRecords, has been bound to displayList by the parent. The actions editObjectAttribute and insertNewObject are bound to the "Edit" and "New" buttons, respectively.


Figure 4: Details of WOObjectSelector

Figure 5 shows the API editor view of WOObjectSelector with the five keys that must be bound by a parent component. Figure 6 shows how WOObjectSelector is embedded in a form element within its parent page, "Main", in the demo application, and the bindings implemented therein. (Be sure to set up the form for multiple submit buttons.) There is only one key declared in Main, namely a WODisplayGroup associated with the example application's reference table, called "Country", whose attribute of interest is "country".

Listings 2 and 3 show the implementation of the actions invoked by WOObjectSelector's buttons. Each simply creates an instance of the return page using the value bound to returnPageName and then invokes takeValueForKey on the page to set the values that WOObjectEditor will eventually need. As mentioned earlier, every child class of WOComponent inherits this key-value coding method.

There is no custom code for the Main class other than the WODisplayGroup variable countryDisplayGroup, which is bound to an entity called Country in the data model for this example application.


Figure 5: API Editor view of WOObjectSelector


Figure 6: WOObjectSelector laid out in its parent page

WOObjectEditor

Figure 7 shows the layout and keys for WOObjectEditor. The component consists of two WOConditionals, one for the case when the user is updating a record the other for creating a new record. Each conditional consists of a WOTextField and two submit buttons. Four actions are declared in Figure 7, one for each of the buttons. Recall Figure 2. Although the component has four buttons, the user only sees two depending on the choice they made from the selector component.

The keys attributeKey, entityName, insertingNewObject, updatingObject and objectToEdit were mentioned earlier. WOObjectSelector provides their values via the editor component's parent using the aforementioned key-value coding and API binding techniques. The editor's parent component provides the value for returnPageName. In our example we simply return to the Main page. As in WOObjectSelector, the key displayAttribute is a method that returns the display string corresponding to the attribute of objectToEdit that we are updating.


Figure 7: Details of WOObjectEditor

Figure 8 displays the API Editor view of WOObjectEditor with the six keys that must be bound by a parent component. Figure 9 shows how WOObjectEditor is embedded in its parent page, "CountryEditor", in the demo application and the bindings implemented therein. Notice how the Boolean values insertingNewObject and updatingObject are used not only by WOObjectEditor but also by the enclosing page itself to vary the prompt depending on the action the user selected.


Figure 8: API Editor view of WOObjectEditor

Listings 4 and 5 show the implementations for the actions insertNewObject and updateObject. The former creates a new instance of the entity specified by entityName then sets the value of the specified attribute using key-value coding. Upon inserting the new record into the default editing context, it saves the changes. The updateObject method simply sets the new value of objectToEdit using key-value coding, then saves the changes.


Figure 9: WOObjectEditor laid out in a its parent page, CountryEditor

Enhancements and Improvements

Our pair of components is already very useful and highly reusable. However, there are possibilities for improvements. One concerns validation and associated exception handling as follows:

The two action methods insertNewObject and updateObject have built-in validation rules; as written they each enforce a non-null value for every attribute of every entity. This may not be unreasonable in the case of reference tables, but it isn't as flexible as it could be. Moreover, there is no exception handling for the possibility that either insertObject or saveChanges fails. These operations could fail for a variety of reasons including constraints built in to the data model(s) used in a real application.

There are two obvious alternatives to implementing improved validation and exception handling. One is to override the method validationFailedWithException in the page that encloses WOObjectEditor. Every component inherits this method. One would probably implement it in the editor component's parent because the parent knows the context in which validation occurs. Another route would be to subclass and provide the subclass with the required context. I probably would choose the former path since the context is already known there.

Summary

This article has illustrated the design and construction of reusable WebObjects components and inter-component communication using key-value coding. The fully functional components discussed herein are widely applicable to the maintenance of reference tables, a common situation.

Bibliography and References

The interested reader may find the following books useful. Of the three, Ruzek's book is the best, in my opinion. Unfortunately, it is based on WebObjects version 4.5 and emphasizes development under Windows. Nevertheless, much is applicable and the examples are pretty good. DeMan et al's book is based on version 5 and incorporates references to OSX. Their discussion of validation, key-value coding and other important topics is very good. However, the book is a bit rough around the edges in some places perhaps reflecting its multiple authorship and the need for a bit more editing. Feiler's book is the least helpful. After a very long (60 pages) and general introduction, the book finally gets around to discussing OpenBase. For many topics the author merely recapitulates Apple's documentation, for others he provides only the most cursory treatment and no concrete examples.

  • DeMan, Michael, Frederico, Gustavo, et. al. Professional WebObjects 5.0 with Java, Wrox Press Ltd., Birmingham, UK, 2001.

  • Ruzek, George. WebObjects Web Application Development Kit, Sams Publishing, Indianapolis, 2001

  • Feiler, Jesse. Building WebObjects 5 Applications, McGraw-Hill/Osborne, Berkeley, 2002.

Listing 1: WOObjectSelector.java

displayAttribute
public String displayAttribute(){
   return (String) displayObject.valueForKey(attributeKey);
}

Listing 2: WOObjectSelector.java

editObjectAttribute
public WOComponent editObjectAttribute(){
   /* 
      The user needs to select some value to edit;
      if not, do nothing.
   */
   if (selectedObject == null) return null;
   WOComponent nextPage =
               (WOComponent)pageWithName(returnPageName);
   nextPage.takeValueForKey(entityName,"entityName");
   nextPage.takeValueForKey(selectedObject,"objectToEdit");
   nextPage.takeValueForKey(attributeKey,"attributeKey");
   nextPage.takeValueForKey(Boolean.TRUE,"updatingObject");\   
   nextPage.takeValueForKey(Boolean.FALSE,
                                             "insertingNewObject");
   return nextPage;
}

Listing 3: WOObjectSelector.java

insertNewObject
public WOComponent insertNewObject(){
   WOComponent nextPage =
               (WOComponent)pageWithName(returnPageName);
   nextPage.takeValueForKey(entityName,"entityName");
   nextPage.takeValueForKey(null,"objectToEdit");
   nextPage.takeValueForKey(attributeKey,"attributeKey");
   nextPage.takeValueForKey(Boolean.FALSE,"updatingObject");
   nextPage.takeValueForKey(Boolean.TRUE,
                                             "insertingNewObject");
   return nextPage;
}

Listing 4: WOObjectEditor.java

insertNewObject
public WOComponent insertNewObject() {
   if (displayAttribute!=null &&
                  displayAttribute.length()>0){
      /*
         the user has entered a non-blank string;
         create a new object
      */
      EOClassDescription description =
         EOClassDescription.classDescriptionForEntityName(
                                                            entityName);
      EOEnterpriseObject newObject =
         description.createInstanceWithEditingContext(null,
                                                            null);
      newObject.takeValueForKey(displayAttribute,
                                                         attributeKey);
      EOEditingConext dec =
                              this.session().defaultEditingContext();
      dec.insertObject(newObject);
      dec.saveChanges();
   }
   WOComponent nextPage =
                     (WOComponent)pageWithName(returnPageName);
   return nextPage;
}

Listing 5: WOObjectEditor.java

updateObject
public WOComponent updateObject(){
   if (displayAttribute!=null   &&
                  displayAttribute.length()>0){
      // the user has entered a non-blank string
      objectToEdit.takeValueForKey(displayAttribute,
                                                         attributeKey);
      this.session().defaultEditingContext().saveChanges();
   }
   WOComponent nextPage =
                     (WOComponent)pageWithName(returnPageName);
   return nextPage;
}

In addition to being a Macintosh and WebObjects hobbyist developer, Tom Woteki, aka Dr. Wo, is a vice president at TRW Systems. He can be reached at drwo@woteki.com

 
AAPL
$99.02
Apple Inc.
+1.35
MSFT
$43.97
Microsoft Corpora
-0.53
GOOG
$590.60
Google Inc.
+1.58

MacTech Search:
Community Search:

Software Updates via MacUpdate

OS X Yosemite Wallpaper 1.0 - Desktop im...
OS X Yosemite Wallpaper is the gorgeous new background image for Apple's upcoming OS X 10.10 Yosemite. This wallpaper is available for all screen resolutions with a source file that measures 5,418... Read more
Acorn 4.4 - Bitmap image editor. (Demo)
Acorn is a new image editor built with one goal in mind - simplicity. Fast, easy, and fluid, Acorn provides the options you'll need without any overhead. Acorn feels right, and won't drain your bank... Read more
Bartender 1.2.20 - Organize your menu ba...
Bartender lets you organize your menu bar apps. Features: Lets you tidy your menu bar apps how you want. See your menu bar apps when you want. Hide the apps you need to run, but do not need to... Read more
TotalFinder 1.6.2 - Adds tabs, hotkeys,...
TotalFinder is a universally acclaimed navigational companion for your Mac. Enhance your Mac's Finder with features so smart and convenient, you won't believe you ever lived without them. Tab-based... Read more
Vienna 3.0.0 RC 2 :be5265e: - RSS and At...
Vienna is a freeware and Open-Source RSS/Atom newsreader with article storage and management via a SQLite database, written in Objective-C and Cocoa, for the OS X operating system. It provides... Read more
VLC Media Player 2.1.5 - Popular multime...
VLC Media Player is a highly portable multimedia player for various audio and video formats (MPEG-1, MPEG-2, MPEG-4, DivX, MP3, OGG, ...) as well as DVDs, VCDs, and various streaming protocols. It... Read more
Default Folder X 4.6.7 - Enhances Open a...
Default Folder X attaches a toolbar to the right side of the Open and Save dialogs in any OS X-native application. The toolbar gives you fast access to various folders and commands. You just click... Read more
TinkerTool 5.3 - Expanded preference set...
TinkerTool is an application that gives you access to additional preference settings Apple has built into Mac OS X. This allows to activate hidden features in the operating system and in some of the... Read more
Audio Hijack Pro 2.11.0 - Record and enh...
Audio Hijack Pro drastically changes the way you use audio on your computer, giving you the freedom to listen to audio when you want and how you want. Record and enhance any audio with Audio Hijack... Read more
Intermission 1.1.1 - Pause and rewind li...
Intermission allows you to pause and rewind live audio from any application on your Mac. Intermission will buffer up to 3 hours of audio, allowing users to skip through any assortment of audio... Read more

Latest Forum Discussions

See All

Traps n’ Gemstones Review
Traps n’ Gemstones Review By Campbell Bird on July 28th, 2014 Our Rating: :: CASTLEVANIA JONESUniversal App - Designed for iPhone and iPad Fight mummies, dig tunnels, and ride a runaway minecart to discover ancient secrets in this... | Read more »
The Phantom PI Mission Apparition Review
The Phantom PI Mission Apparition Review By Jordan Minor on July 28th, 2014 Our Rating: :: GHOSTS BUSTEDUniversal App - Designed for iPhone and iPad The Phantom PI is an exceedingly clever and well-crafted adventure game.   | Read more »
More Stubies Are Coming Your Way in a Ne...
More Stubies Are Coming Your Way in a New Update Posted by Jessica Fisher on July 28th, 2014 [ permalink ] Universal App - Designed for iPhone and iPad | Read more »
The Great Prank War Review
The Great Prank War Review By Nadia Oxford on July 28th, 2014 Our Rating: :: PRANKING IS SERIOUS BUSINESSUniversal App - Designed for iPhone and iPad Though short, The Great Prank War offers an interesting and fun mix of action and... | Read more »
Marvel Contest of Champions Announced at...
Marvel Contest of Champions Announced at Comic-Con Posted by Jennifer Allen on July 28th, 2014 [ permalink ] Announced over the weekend at San Diego Comic-Con was the fairly exciting looking Marvel Contest of Champions. | Read more »
Teenage Mutant Ninja Turtles Review
Teenage Mutant Ninja Turtles Review By Jennifer Allen on July 28th, 2014 Our Rating: :: DULL SWIPINGUniversal App - Designed for iPhone and iPad The pizza power is weak when it comes to this Teenage Mutant Ninja Turtles game.   | Read more »
Exploration Focused Puzzle Game Beatbudd...
Exploration Focused Puzzle Game Beatbuddy Set to Make Transition from PC to iOS this September Posted by Jennifer Allen on July 28th, 2014 [ permalink ] | Read more »
PlanetHD
PlanetHD By Nadia Oxford on July 28th, 2014 Our Rating: :: SPACE MADNESSUniversal App - Designed for iPhone and iPad PlanetHD will keep players busy for a while, though its unpredictable physics are a handful to deal with.   | Read more »
This Week at 148Apps: July 21-25, 2014
Another Week of Expert App Reviews   At 148Apps, we help you sort through the great ocean of apps to find the ones we think you’ll like and the ones you’ll need. Our top picks become Editor’s Choice, our stamp of approval for apps with that little... | Read more »
Reddme for iPhone - The Reddit Client (...
Reddme for iPhone - The Reddit Client 1.0 Device: iOS iPhone Category: News Price: $.99, Version: 1.0 (iTunes) Description: Reddme for iPhone is an iOS 7-optimized Reddit client that offers a refreshing new way to experience Reddit... | Read more »

Price Scanner via MacPrices.net

iOS 8 and OS X 10.10 To Support DuckDuckGo As...
Writing for Quartz, Dan Frommer reports that Apple’s forthcoming iOS 8 and OS X 10.10 operating systems version updates will allow users to select DuckDuckGo as their default search engine. He notes... Read more
U.K. Hospital Using iPods and iPads To Record...
British news journal GazetteLive’s. Ian McNeal notes that the old “an apple a day keeps the doctor away” proverb is being turned on its head at http://southtees.nhs.uk/hospitals/james-cook/ James... Read more
13-inch 2.5GHz MacBook Pro on sale for $1099,...
Best Buy has the 13″ 2.5GHz MacBook Pro available for $1099.99 on their online store. Choose free shipping or free instant local store pickup (if available). Their price is $100 off MSRP. Price is... Read more
Roundup of Apple refurbished MacBook Pros, th...
The Apple Store has Apple Certified Refurbished 13″ and 15″ MacBook Pros available for up to $400 off the cost of new models. Apple’s one-year warranty is standard, and shipping is free. Their prices... Read more
Record Mac Shipments In Q2/14 Confound Analys...
A Seeking Alpha Trefis commentary notes that Apple’s fiscal Q3 2014 results released July 22, beat market predictions on earnings, although revenues were slightly lower than anticipated. Apple’s Mac’... Read more
Intel To Launch Core M Silicon For Use In Not...
Digitimes’ Monica Chen and Joseph Tsai, report that Intel will launch 14nm-based Core M series processors specifically for use in fanless notebook/tablet 2-in-1 models in Q4 2014, with many models to... Read more
Apple’s 2014 Back to School promotion: $100 g...
 Apple’s 2014 Back to School promotion includes a free $100 App Store Gift Card with the purchase of any new Mac (Mac mini excluded), or a $50 Gift Card with the purchase of an iPad or iPhone,... Read more
iMacs on sale for $150 off MSRP, $250 off for...
Best Buy has iMacs on sale for up to $160 off MSRP for a limited time. Choose free home shipping or free instant local store pickup (if available). Prices are valid for online orders only, in-store... Read more
Mac minis on sale for $100 off MSRP, starting...
Best Buy has Mac minis on sale for $100 off MSRP. Choose free shipping or free instant local store pickup. Prices are for online orders only, in-store prices may vary: 2.5GHz Mac mini: $499.99 2.3GHz... Read more
Global Tablet Market Grows 11% in Q2/14 Notwi...
Worldwide tablet sales grew 11.0 percent year over year in the second quarter of 2014, with shipments reaching 49.3 million units according to preliminary data from the International Data Corporation... Read more

Jobs Board

Sr Software Lead Engineer, *Apple* Online S...
Sr Software Lead Engineer, Apple Online Store Publishing Systems Keywords: Company: Apple Job Code: E3PCAK8MgYYkw Location (City or ZIP): Santa Clara Status: Full Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
Sr. Product Leader, *Apple* Store Apps - Ap...
**Job Summary** Imagine what you could do here. At Apple , great ideas have a way of becoming great products, services, and customer experiences very quickly. Bring Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
*Apple* Solutions Consultant (ASC) - Apple (...
**Job Summary** The ASC is an Apple employee who serves as an Apple brand ambassador and influencer in a Reseller's store. The ASC's role is to grow Apple Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.