A Core Data Tutorial Part 1

UPDATE 2 Aug 2011: Some of you have requested a revised version of this tutorial that covers Xcode 4, I have completed that post and it is now located here. You should also note that I will not be adding any tutorials to this site anymore as I have moved my blog to my main development website theMikeSwan.com.

This is the first in a series of tutorials that combines other tutorials I have seen and in the end will go a little past the others I have seen. My intention is to take this app from beginning to end, including the finishing touches like update checking, serial number, and icons. I am writing this to help me figure out a few things about Core Data and hopefully help others as well.

This tutorial assumes you have the Apple Developer Tools installed and are familiar with basic operation of Xcode and Interface Builder. It also assumes that you have a very basic knowledge of Core Data. If you are completely new to Core Data check out either Build a Core Data App or read chapters 11 & 30 of Aaron Hillegass’s book Cocoa Programming for Mac OS X 3rd Edition. Really if you are new to Cocoa you should have a copy of Aaron’s book no matter what, it is the one book I always go to when I’m not sure about something. We will also be doing a little with bindings, if you need a starter check out either Cocoa Bindings Intro or Chapter 11 of Aaron’s book.

This is the first tutorial in the series so we should first specify our goal. First off the purpose of the app will be to track daily expenses. Every feature we think about adding should be assessed based on that one sentence, this will hopefully help keep down the feature creep. Since the purpose of the app is to track expenses we will just call it Expenses. Now we should determine the major features of the app. It will of course need to store expenses, perhaps when, what, & how much. To make life easier it would be a good idea to be able to assign each expense a category so the user can figure out things like how much they spend on food versus movies. It would also be good to show how much was spent in a month on any one category, as well as overall (this is a feature I will need some help with). Of course we will also show yearly totals both by category and overall. We are also going to want to let the user know when we release new version of the app without having to e-mail them (they don’t want the e-mail and will ignore it). Since we are putting all this effort into writing this Expenses perhaps we want to get paid for it, in which case we will want some way to require a serial number for real use of the app. Finally in order to improve the user experience we should get crash reports so we can figure out how to reduce (if not eliminate) them. Of course along the way we will come up with a few smaller features (such as copy and paste of expenses) that we will add in, but this should be the bulk of it. We will use Core Data to build Expenses because it is extremely well suited to this kind of work and will reduce the amount of code we have to write dramatically.

Today we are going to start by just building the basic app without writing any code at all!

To start with open Xcode and start a new Document based Core Data application.

Name it Expenses (or whatever you want to call it).

Next set up the model as below.

Attributes:

Expense		Category
Name	Type	Name	Type
desc	string	name	string
date	date
amount	float

Relationships:

Expense		Category
Name	Destination	Name	Destination
category	Category	expenses	Expense (to-Many)

In the end the model should look like this:

Make sure you save.

Next let’s build the user interface.

Open MyDocument.xib

Remove the Label.

Add a tab view.

Size it to take up most of the window.

In the inspector select the size tab and set the springs and struts so that the tab view resizes with the window.

Label one of the tabs “Expenses” and the other “Categories.”

Make sure the Expenses tab is active, then drag over a Core Data Entity from the IB Library. It will bring up the “New Core Data Entity Interface” assistant:

Select Expenses -> MyDocument -> Expense as below, then click next.

In the next pane select “Detail/Master View” and select “Detail Field” & “Add/Remove.” If you want you can also select “Search Field” to get basic searching for free.

In the last pane select all of the attributes and the one relationship:

You should have something that looks similar to the image below. It needs a little work, first off lets get rid of that extra box (Layout -> Unembed Objects).

Next lets make a few other tweaks. Date should really be the first column; Description, the second; followed by Amount, and Category. Amount will need it’s number formatter adjusted to display currency in whatever way you want. You will likely want to adjust the date formatter in the date column (unless you want the really long default format). Then resize the the table and it’s columns to something resembling the image below. Once that is done collect all the detail fields and put them in a box at the bottom. you will also need to adjust the formatters for the date and amount fields to match the ones in the table view (unless you want them to be different of course). In the end it should look something like this:

Before we do the other tab we should make life easier if we have to change any bindings later. You should have two array controllers in you xib file now, one named “Expense Array Controller” and one named “Category Array Controller.” When you drop the Category entity into the other tab IB will add in another “Category Array Controller” making it very hard to tell which is which. To eliminate this problem rename “Expense Array Controller” to “ExpenseView Array Controller” and “Category Array Controller” to “CategoryPopUp Array Controller” this just makes everything way more clear for the future.

Now select the Categories tab and drop a Core Data Entity on it. follow the same procedure as before (only select the Category entity of course). Re-arrange the interface to look like below. Go ahead and rename the new “Category Array Controller” to “CategoryView Array Controller.”

If you are wondering what that huge empty space is for, it’s for the table view that you need to add now. It should have three columns; Date, Description, and Amount. Below the table view add two labels; “Total:” and a placeholder with a number formatter set to your favorite currency settings (I tend to use “$10,000,000.00” as my place holder since it should always be big enough for any numbers I will ever be entering).

Now to fill that nice new table view with some data. First add a new array controller and name it “ExpenseByCat Array Controller.” Now your xib file should look something like this:

See how easy it is to figure out which array controller does what.

In the bindings section of the inspector for “ExpenseByCat Array Controller” bind the Content Set (not the Array) to “CategoryView Array Controller.selection.expenses” this will cause the array controller to be filled by the expenses of the currently selected category.

Now that the array controller is hooked up we need to bind the table view columns. Bind each column to “ExpenseByCat Array Controller.arrangedObjects” and set the “Model Key Path:” as needed for each column.

Then select that placeholder label that you put below the table view earlier and bind it the same as the table view columns (“ExpenseByCat Array Controller.arrangedObjects”) for the “Model Key Path:” though you are going to enter a little magic: “@sum.amount” this will take care of adding up all the amounts of each expense in the category for you.

Build and Go!

You should be able to add and remove Categories and Expenses. Undo should already work. Saving and reverting should work. So should the total for each category, that’s a lot of stuff with out writing any code yet! But there are some things that would be nice to add.

In the next installment we will add:

• New expenses default to the current date.
• Alphabetize categories in the popup menu.
• Make expenses start out sorted by date and keep them that way even when the date is edited.
• Copy and paste of expenses.

Let me know what you think or if I missed something (after all I am still fairly new at this). Also if you have any ideas about how to sort out how much was spent in each category for each month please let me know.