### TheElements ### 
TheElements is a sample application that provides access to the data contained in the
Periodic Table of the Elements. The Periodic Table of the Elements catalogs all the known
atomic elements in the universe.
TheElements provides this data in multiple formats, allowing you to sort the data by name,
atomic number, symbol name, and an elements physical state at room temperature.
TheElements is structured as a Model-View-Controller application. There is distinct
separation of the model data, the views used to present that data, and the controllers which
act as a liaison between the model and controller.
The application illustrates the following techniques:
 Configuring and responding to selections in a tab bar
 Displaying information in a tableview using both plain and grouped style table views
 Using navigation controllers to navigate deeper into a data structure
 Subclassing UIView
 Providing a custom UITableViewCell consisting of multiple subviews
 Implementing the UITableViewDelegate protocol
 Implementing the UITableViewDataSource protocol
 Reacting to taps in views
 Open a URL to an external web site using Safari
 Flipping view content from front to back
 Creating a reflection of a view in the interface
TheElements is a fairly large application with many classes. This document attempts to explain
those classes and their roles in the application.
To understand the flow of screens from one to the next when using the application see the
ApplicationFlow.pdf included with the project.
iOS 6.0 SDK
iOS 5.0 or later
The AtomicElement class encapsulates the data for a single atomic element. It contains the
name, atomic number, symbol, and state information, along with layout information
(horizontal and vertical positions). It returns the image that represents the state of the
object, which displayed in each visual representation of an element.
The PeriodicTable class encapsulates the collection of AtomicElement instances. It provides
access to elements in a variety of formats: sorted numerically, sorted by name, sorted by
symbol, elements for an atomic state, and elements that begin with a specific character.
This data is pre-sorted and indexed when the raw element information is read from the
Elements.plist. The PeriodicTable class is a singleton, there is one instance that is shared
by the entire application.
ElementsTableViewController is the controller class that is used for each of the four
representations of the elements data that is displayed in UITableView objects. It is
responsible for creating and configuring instances of UITableView when requested.
The ElementsTableViewController class adopts the UITableViewDelegate protocol, agreeing
to provide the cells for the tableview.
It provides instances of the custom table cell class ElementTableViewCell pre-populated
with the appropriate AtomicElement object when asked by the UITableViewDelegate for the
cell to be displayed for a row and section.
As the UITableView delegate an instance of the ElementsTableViewController class will also
receive messages when a user taps on a row in the table view. Upon receiving this event it
asks its data source object for the AtomicElement object that it represents and then instructs
the navigation controller for the view controller to push an instance of the
AtomicElementViewController class onto the navigation stack.
Each row in a UITableView displays a custom table cell of the class AtomicElementTableViewCell.
This custom class displays a graphic indicating the symbol, atomic number, and state of the
element, as well as a text label with the generally accepted name of the element.
The data source classes provide the data for the ElementsTableViewController, the UITabBarItem,
the UINavigationController and the UITableView that is displayed within those views. There
are four classes: ElementsSortedByNameDataSource, ElementsSortedByAtomicNumberDataSource,
ElementsSortedBySymbolDataSource, and ElementsSortedByStateDataSource.
These classes all adopt and implement the UITableViewDataSource protocol and are set as the
dataSource for the UITableView displayed for the data sorted in each manner. As the table
view requires data it sends messages to its dataSource (one of these class instances) and
that data is displayed.
These classes also adopt the ElementsDataSource protocol. This protocol is defined by the
application and provides a uniform means for the ElementsTableViewController to determine
the title displayed in the tab bar and navigation controller, the images displayed in the
UITabBarItem for this data representation, and the style the UITableView uses to display
the data (indexed or grouped depending on the sorted representation of the Periodic
Table data.
When a user taps on a row in a UITableView listing a collection of elements the table's
delegate (ElementsTableViewController) receives a message. Upon receiving this event the
ElementsTableViewController asks its data source object for the AtomicElement object that
it represents and then instructs the navigation controller for the view controller to push
an instance of the AtomicElementViewController class onto the navigation stack.
The AtomicElementViewController is responsible for displaying the single element view as a
large tile. The initial view displayed is the AtomicElementView. It is also responsible for
ensuring that the flipper button in the navigation bar shows that there is another view
available for the content by tapping on the tile.
The AtomicElementView is displayed in the content of the AtomicElementViewController.
It displays the atomic name, number, symbol, and state of the element. By tapping on the
element the display will 'flip' to show the AtomicElementFlippedView.
This view displays the Atomic name, number, and state of the current element, as well as a
link to the appropriate page on Wikipedia.
Version 1.12
  - Upgraded for iOS 6.0 SDK, updated to adopt current best practices for Objective-C,
  now using UITapGestureRecognizer for AtomicElementView, Storyboards, Automatic Reference Counting (ARC).
Version 1.11
  - Upgraded project to build with the iOS 4.0 SDK.
Version 1.9
  - Changed the target setting's "Code Signing Identity" to the proper value.
Version 1.8
  - Upgraded for 3.0 SDK due to deprecated APIs; in "cellForRowAtIndexPath" it now uses UITableViewCell's initWithStyle.
Version 1.7
  - Updated for and tested with iPhone OS 2.0. First public release.
  - Added additional date to the Element View
  - Fixed memory leaks in reflection implementation
Version 1.6
  - Updated with API changes for beta 6 relesae of iPhone OS SDK.
  - The UITableViewDelegate method tableView: cellForRowAtIndexPath: was
    moved to the UITableViewDataSource protocol. This resulted in a change to
    the ElementsTableViewController class and each of the datasources.
Version 1.5
  - Updated with API changes for beta 5 release of iPhone OS SDK.   
Version 1.4
  - Updated with API changes for beta 4 release of iPhone OS SDK.
  - Added code signing.
  - Subviews of cells should no longer be directly inserted into the cell
    as a subview. Instead they should be interted into the contentView of
    the cell.
  - Remove layout computations based on contentRectForBounds: in AtomicElmentTableViewCell.
    Instead the CGRect returned by self.contentRect.bounds is now used.
Version 1.3
  - Updated with API changes  for third beta release of iPhone
    - Changed designated initializer in AtomicElementTableViewCell.m to
    - Changed cell creation mechanism in ElementsTableViewController.m
      (see tableView:cellForRowAtIndexPath:)
    - Removed tabBar from Single Element View using new
      hidesBottomBarWhenPushed API (see init in
  - Added flipped indicator to navigation bar in Single Element View
    - Shows a more indicator when the front of the element is shown,
      shows a representation of the front of the element when the back
      is shown
  - Renamed various element background images to indicate pixel size
Version 1.2
  - Updated with API changes for second beta release of iPhone SDK
    - replaced toolbar references with tabBar references throughout.
    - Single Element View mode should now hide the toolbar
      automatically when it is displayed.
  - New implementation of the view reflection in the Single Element
    View mode.
    - Now creates the reflected image programmatically. This eliminates
      the cheat that included a pre-rendered vertically flipped button
    - Added a method to AtomicElementView class to return an image that
      contains the reflected image for the view, including the gradient.
    - This version is generic. It will work on any color background,
      and requires no special images for the reflection.
    - Should be reusable in most cases.
    - See AtomicElementView.m for
      reflectedImageRepresentationOfHeight:, the method that creates
      the reflection image. See AtomicElementViewController.m for the
      creation and management of the UIImageView that displays the
      reflected image.
  - Elements by Name table now displays alphabetical side index
    - Side index is now displayed rather than disclosure icon
    - Added new method to the ElementsDataSource protocol to return if
      the disclosure icon should be displayed for a sorted mode
    - See sectionIndexTitlesForTableView:and
      tableView:sectionForSectionIndexTitle:atIndex: in
Version 1.1
  - Initial release.
Copyright (C) 2008-2013 Apple Inc. All rights reserved.