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

 

Community Search:
MacTech Search:

Software Updates via MacUpdate

Backblaze 4.2.0.990 - Online backup serv...
Backblaze is an online backup service designed from the ground-up for the Mac. With unlimited storage available for $5 per month, as well as a free 15-day trial, peace of mind is within reach with... Read more
OmniPlan Pro 3.6 - Professional-grade pr...
With OmniPlan Pro, you can create logical, manageable project plans with Gantt charts, schedules, summaries, milestones, and critical paths. Break down the tasks needed to make your project a success... Read more
OmniPlan 3.6 - Robust project management...
With OmniPlan, you can create logical, manageable project plans with Gantt charts, schedules, summaries, milestones, and critical paths. Break down the tasks needed to make your project a success,... Read more
Little Snitch 3.7.1 - Alerts you about o...
Little Snitch gives you control over your private outgoing data. Track background activity As soon as your computer connects to the Internet, applications often have permission to send any... Read more
iMazing 2.1.3 - Complete iOS device mana...
iMazing (was DiskAid) is the ultimate iOS device manager with capabilities far beyond what iTunes offers. With iMazing and your iOS device (iPhone, iPad, or iPod), you can: Copy music to and from... Read more
AppDelete 4.3.1 - $7.99
AppDelete is an uninstaller that will remove not only applications but also widgets, preference panes, plugins, and screensavers along with their associated files. Without AppDelete these associated... Read more
FileZilla 3.23.0.2 - Fast and reliable F...
FileZilla (ported from Windows) is a fast and reliable FTP client and server with lots of useful features and an intuitive interface. Version 3.23.0.2: Bug Fixes and Minor Changes Speed up icon... Read more
PDFpen 8.3 - $74.95
PDFpen allows users to easily edit PDF's. Add text, images and signatures. Fill out PDF forms. Merge or split PDF documents. Reorder and delete pages. Even correct text and edit graphics! Features... Read more
TunnelBear 3.0.8 - Subscription-based pr...
TunnelBear is a subscription-based virtual private network (VPN) service and companion app, enabling you to browse the internet privately and securely. Features Browse privately - Secure your data... Read more
Safari Technology Preview 10.1 - The new...
Safari Technology Preview contains the most recent additions and improvements to WebKit and the latest advances in Safari web technologies. And once installed, you will receive notifications of... Read more

Latest Forum Discussions

See All

Winter comes to Darkwood as Seekers Note...
MyTona, based in the chilly Siberian city of Yakutsk, has brought a little festive fun to its hidden object game Seekers Notes: Hidden Mystery. The Christmas update introduces some new inhabitants to players, and with them a chance to win plenty of... | Read more »
Bully: Anniversary Edition (Games)
Bully: Anniversary Edition 1.03.1 Device: iOS Universal Category: Games Price: $6.99, Version: 1.03.1 (iTunes) Description: *** PLEASE NOTE: This game is officially supported on the following devices: iPhone 5 and newer, iPod Touch... | Read more »
PINE GROVE (Games)
PINE GROVE 1.0 Device: iOS Universal Category: Games Price: $1.99, Version: 1.0 (iTunes) Description: A pine grove where there are no footsteps of people due to continuous missing cases. The case is still unsolved and nothing has... | Read more »
Niantic teases new Pokémon announcement...
After rumors started swirling yesterday, it turns out there is an official Pokémon GO update on its way. We’ll find out what’s in store for us and our growing Pokémon collections tomorrow during the Starbucks event, but Niantic will be revealing... | Read more »
3 reasons why Nicki Minaj: The Empire is...
Nicki Minaj is as business-savvy as she is musically talented and she’s proved that by launching her own game. Designed by Glu, purveyors of other fine celebrity games like cult favorite Kim Kardashian: Hollywood, Nicki Minaj: The Empire launched... | Read more »
Clash of Clans is getting its own animat...
Riding on its unending wave of fame and success, Clash of Clans is getting an animated web series based on its Clash-A-Rama animated shorts.As opposed to the current shorts' 60 second run time, the new and improved Clash-A-Rama will be comprised of... | Read more »
Leaks hint at Pokémon GO and Starbucks C...
Leaked images from a hub for Starbucks employees suggests that a big Pokémon GO event with the coffee giant could begin this very week. The images appeared on Reddit and hint at some exciting new things to come for Niantic's smash hit game. | Read more »
Silent Depth Submarine Simulation (Game...
Silent Depth Submarine Simulation 1.0 Device: iOS Universal Category: Games Price: $7.99, Version: 1.0 (iTunes) Description: | Read more »
Enneas Saga lets you lead your own demon...
Defend the land of Enneas Continent from the forces of evil in the new fantasy MMORPG from Lyto Mobi: Enneas Saga. Can’t wait? No problem. It’s available to download now on Android devices. | Read more »
Great zombie games in the spirit of Dead...
Dead Rising 4 arrives tomorrow, giving enthusiasts a fresh chance to take selfies with zombies and get up to other ridiculous end-of-the-world shenanigans. To really get into the spirit of things, we've gone and gathered the best zombie games that... | Read more »

Price Scanner via MacPrices.net

Holiday sale: 12-inch Retina MacBook for $100...
B&H has 12″ Retina MacBooks on sale for $100 off MSRP as part of their Holiday sale. Shipping is free, and B&H charges NY sales tax only: - 12″ 1.1GHz Space Gray Retina MacBook: $1199 $100... Read more
Apple refurbished 13-inch MacBook Airs availa...
Apple has Certified Refurbished 13″ MacBook Airs available starting at $849. An Apple one-year warranty is included with each MacBook, and shipping is free: - 13″ 1.6GHz/8GB/128GB MacBook Air: $849 $... Read more
Apple refurbished iMacs available for up to $...
Apple has Certified Refurbished 2015 21″ & 27″ iMacs available for up to $350 off MSRP. Apple’s one-year warranty is standard, and shipping is free. The following models are available: - 21″ 3.... Read more
Apple’s Education discount saves up to $300 o...
Purchase a new Mac or iPad using Apple’s Education Store and take up to $300 off MSRP. All teachers, students, and staff of any educational institution qualify for the discount. Shipping is free: -... Read more
Back in stock: Apple refurbished Mac minis fr...
Apple has Certified Refurbished Mac minis available starting at $419. Apple’s one-year warranty is included with each mini, and shipping is free: - 1.4GHz Mac mini: $419 $80 off MSRP - 2.6GHz Mac... Read more
Twenty-Five Years Of Apple Laptops – A person...
Among many other things, the often tumultuous 16th year of the new century marked the 25th anniversary of Apple laptop computers, not counting the optimistically named 16-pound Mac Portable of 1989.... Read more
Landlordy iOS App Adds Support For Appliances...
Riga, Latvia based E-protect SIA is releasing major update (version 1.8) to its Landlordy app for managing rental business financials on the go. Landlordy is iPhone and iPad app designed for self-... Read more
Holiday sale, Apple iMacs for up to $200 off...
B&H Photo has 21″ and 27″ Apple iMacs on sale for up to $200 off MSRP including free shipping plus NY sales tax only: - 27″ 3.3GHz iMac 5K: $2099 $200 off MSRP - 27″ 3.2GHz/1TB Fusion iMac 5K: $... Read more
Holiday sale: Mac minis for $50 to $100 off M...
B&H Photo has Mac minis on sale for up to $100 off MSRP free shipping plus NY sales tax only: - 1.4GHz Mac mini: $449 $50 off MSRP - 2.6GHz Mac mini: $629 $70 off MSRP - 2.8GHz Mac mini: $899 $... Read more
Mac Pros on sale for up to $300 off MSRP, ref...
B&H Photo has Mac Pros on sale for up to $300 off MSRP. Shipping is free, and B&H charges sales tax in NY only: - 3.7GHz 4-core Mac Pro: $2799, $200 off MSRP - 3.5GHz 6-core Mac Pro: $3699, $... Read more

Jobs Board

*Apple* Retail - Multiple Positions - Apple,...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions- Trumbul...
Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, you're also the Read more
*Apple* Retail - Multiple Positions- Philade...
Job Description: Sales Specialist - Retail Customer Service and Sales Transform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Retail - Multiple Positions- San Ant...
Job Description:SalesSpecialist - Retail Customer Service and SalesTransform Apple Store visitors into loyal Apple customers. When customers enter the store, Read more
*Apple* Products Tester Needed - Apple (Unit...
…we therefore look forward to put out products to quality test for durability. Apple leads the digital music revolution with its iPods and iTunes online store, Read more
All contents are Copyright 1984-2011 by Xplain Corporation. All rights reserved. Theme designed by Icreon.