Lab 12: Objective-C Sampler

Getting Started

  1. Open Terminal and navigate to the iosdecal-sp19 directory using cd.

  2. Use git pull to fetch this lab's starter files.

Welcome to the sampler lab for Objective-C! In this lab, you'll be using Objective-C (not Swift!) to create a simple counter that can increment/decrement and clear a label. As an added challenge, in this lab you'll be using the bracket notation for all function calls and property references! Refer to the lecture slides for a reminder on what this means.

As a reminder, sampler labs are short mini-labs designed to introduce you to a particular concept. They should be fairly simple and straightforward to implement, and shouldn't take too long to complete. You only need to complete three out of the five sampler labs we assign to recieve full credit, so we recommend you do the ones that you think might be most relevant to your project or that you find the most interesting. You have until April 30 to turn them in, so you're welcome to work on these in any order and at any time.

Part 0: Getting Started

Open the file TapCounter.xcodeproj to start the lab. We have provided the following files for you to use.

  1. Main.storyboard - The main storyboard file. We've already laid out everything with constraints and stack views for you, but you may need to come back to define outlets and actions!

  2. ViewController.h - The header file for our view controller. You'll be declaring instance variables and methods here before initializing/defining them in ViewController.m

  3. ViewController.m - The main implementation file for our view controller. You'll be using the instance variables that you defined in your header, and you'll define your methods here.

Build and run the app in your simulator, and check that the UI is laid out properly and that the buttons are interactable. Once you've confirmed everything to be working, let's get started!

Part 1: Getting the Label Working

For our counter app, we want to define an integer instance variable count to keep track of the number we've counted to so far. Define count in ViewController.h, and in ViewController.m set count to be equal to 0 when the view loads.

Note: When defining count, you should only use the nonatomic property. In this case, we're not using strong because we aren't referencing a pointer - C can handle integers natively!

Remember, use bracket notation instead of dot notation for this lab! Recall that in order to set a variable in bracket notation, you'll need to use the auto-generated setter method! Here's a quick reminder on the differences between dot and bracket notation:

self.myVariable         // dot notation
[self myVariable]       // bracket notation

self.myVariable = 20    // dot notation
[self setMyVariable:20] // bracket notation

Now that you've created a property for the count, let's hook it up to the label! In ViewController.h, create an outlet of type UILabel, and connect that outlet you defined to the label in Storyboard. Then, in ViewController.m, make the label display the current count on the view loading. You can use [[self countLabel] setText:_], replacing countLabel with whatever you called your outlet.

Note: In order to display your integer inside the label, you may need to use the [NSString stringWithFormat:_] method. You may also want to take a look at this for some guidance on format strings.

If everything was implemented correctly, once you build and run your app, the 999 in the label previously should be replaced with the 0 you initialized count to be!

Part 2: Defining Actions

Now, let's define the three actions that will correspond to our three buttons. Create IBActions in ViewController.h that correspond to the add, subtract, and clear buttons respectively. Remember, we're going to define these as minus (-) functions, since we want them to be instance rather than class functions so they'll link up to Storyboard!

Speaking of Storyboard, go there and connect the three buttons to the IBActions you just created.

Now, in ViewController.m, implement your three functions following these properties:

Remember to use the setter notation shown above to set count's value! You may also find it useful to define a helper function specifically for updating the count label if you find yourself repeating code a lot.

After this, when you build and run your app you should be able to interact with it via the buttons!

Part 3: Checkoff

Congrats on finishing this sampler lab - hopefully now you feel a little more comfortable working with Objective-C and thinking in bracket notation! Although you probably won't be using Objective-C at all for your custom app, it's still very useful to know how to read code written in the language in case you want to import some Obj-C classes into your Swift project.

As this lab is a sampler lab, you can check it off any time between when it is assigned and Tuesday, 4/30. Good luck with the rest of the sampler labs, and your final project!