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Since we need data for our app, let's create a model class. For this app our class is going to consist of an array containing separate instances of a playlist represented as a dictionary.
Teacher's Notes
Code Snippets
_library = @[@{kTitle: @"Rise and Shine",
kDescription: @"Get your morning going by singing along to these classic tracks as you hit the shower bright and early!",
kIcon: @"coffee.pdf",
kLargeIcon: @"coffee_large.pdf",
kBackgroundColor: @{@"red": @255.0, @"green": @204.0, @"blue": @51.0, @"alpha": @1.0},
kArtists: @[@"American Authors", @"Vacationer", @"Matt and Kim", @"MGMT", @"Echosmith", @"Tokyo Police Club", @"La Femme"]
},
@{kTitle: @"Runner's Rampage",
kDescription: @"Hit the track hard and get in beast mode with everything from dance tracks to classic hip hop. All the right fuel to motivate you to push your limits.",
kIcon: @"running.pdf",
kLargeIcon: @"running_large.pdf",
kBackgroundColor: @{@"red": @255.0, @"green": @102.0, @"blue": @51.0, @"alpha": @1.0},
kArtists: @[@"Avicii", @"Calvin Harris", @"Pitbull", @"Iggy Azalea", @"Rita Ora", @"Drake", @"Lil Wayne"]
},
@{kTitle: @"Joy Ride",
kDescription: @"Let this eclectic playlist take you wherever your heart desires. Cruise along in style to these energetic beats.",
kIcon: @"helmet.pdf",
kLargeIcon: @"helmet_large.pdf",
kBackgroundColor: @{@"red": @153.0, @"green": @102.0, @"blue": @204.0, @"alpha": @1.0},
kArtists: @[@"Afrojack", @"Kid Cudi", @"Daft Punk", @"kIcona Pop", @"Gesaffelstien", @"Roksnoix", @"deadmau5"]
},
@{kTitle: @"In The Zone",
kDescription: @"Keep calm and focus. Shut out the noise around you and grind away with some mind sharpening instrumental tunes.",
kIcon: @"laptop.pdf",
kLargeIcon: @"laptop_large.pdf",
kBackgroundColor: @{@"red": @51.0, @"green": @153.0, @"blue": @204.0, @"alpha": @1.0},
kArtists: @[@"Dr. Dre", @"Snoop Dogg", @"Com Truise", @"D12", @"Flying Lotus", @"Kanye West", @"Rundfunk"]
},
@{kTitle: @"Button Masher",
kDescription: @"Turn up the speakers and get out of my way! The ultimate gaming playlist to get you hyped up and ready for the crazy fun that’s about to happen.",
kIcon: @"joystick.pdf",
kLargeIcon: @"joystick_large.pdf",
kBackgroundColor: @{@"red": @51.0, @"green": @204.0, @"blue": @102.0, @"alpha": @1.0},
kArtists: @[@"Skrillex", @"The Game", @"2 Chainz", @"Jay Z", @"T.I.", @"Rihanna", @"Eminem"]
},
@{kTitle: @"Futbal Remix",
kDescription: @"There’s nothing like the world’s game. Kick around the field with this eclectic playlist from around the world. Futbal for life.",
kIcon: @"ball.pdf",
kLargeIcon: @"ball_large.pdf",
kBackgroundColor: @{@"red": @255.0, @"green": @102.0, @"blue": @153.0, @"alpha": @1.0},
kArtists: @[@"Shakira", @"Santana", @"Wyclef Jean", @"Aloe Blacc", @"Pitbull", @"Enrique Iglesias", @"Ricky Martin"]
}];
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Let's start our data model, by adding a new class to our project.
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So let's go to File > New, and then File again, and
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under iOS source, select Cocoa Touch Class.
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We're going to name this file MusicLibrary, so let's get rid of that.
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We want this file to be a subclass of NSObject, and
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make sure your language is objectiveC and then hit next.
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Now, we want to add it to the algorithm group, in the algorithm project, so
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hit Create.
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Okay, so we're going to set this class up as our data model.
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Now, there are a couple different ways we could structure our data model, but
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we're going to keep it relatively simple.
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Going to move these down.
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Our data model is going to consist of an array, that represents our music library.
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We're worked with arrays before, so this should be familiar territory to you.
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Inside our array, we're going to store dictionary incidences.
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Each dictionary will contain relevant information about each playlist,
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that we can use to display on our playlist detail page.
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If you are unfamiliar with the basics of arrays and
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dictionaries, we have some great content.
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Check out the objective C basics before proceeding.
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Now we'll start by going to the header file, and
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we'll create an array called library as a property.
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We'll first start with the property declaration, specify strong nonatomic.
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It's gonna be of type NSArray.
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And we're going to call this library.
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Now, to this array, we're going to append dictionaries.
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Each dictionary will represent a playlist, and will have key value pairs for
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things like the playlist name, description, artists included, and so on.
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Now we'll do this using an initializer method.
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Again if you're unfamiliar with the basics on init methods,
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check out the link to the relevant video in a previous course.
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So let's go over to the implementation file.
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And in between, we're gonna start typing init.
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And hit Auto complete to start the init method.
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First, we need to initialize our super class, and assign it to self.
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So, let's do self, equal super nit.
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Now, let's check for self using an if statement.
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If true, we set up our library array inside the if' block.
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And then, after this if, we return self.
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Each object in the array is going to be an instance of
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NSDictionary containing the relevant key value page.
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Now, there are a couple of different ways, we can instantiate our dictionaries.
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We could use the NS dictionary class method, dictionary with objects for keys.
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Let's give that a try.
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Now, you don't need to type out what I'm about to do.
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I'm just going to explain this to you, and then, I'll give you this block of code to
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copy paste, since there's quite a bit of code, and it's mostly repetitive code.
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First I'm going to make some space.
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So that, we only have the editor on screen.
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First, lets create an instance of a playlist dictionary using
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the NSDictionary class method, dictionary with object for keys.
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Now this method takes in two arrays.
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An array of values, and an array of keys.
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I'm going to use a keyboard shortcut to paste in some code.
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Okay.
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So we could create an instance of our playlist like this.
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The first array using the dictionaryWithObjects for keys method,
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takes in an array of objects, that is a list of values.
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And then the second array is the list of keys.
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Now we'll go over what these keys mean, in just a bit.
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But, creating it this way this code is just for a single playlist.
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We have six playlists, so we would need to have six of these.
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With just one playlist, it's already relatively unreadable.
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Then after we have all six of these,
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we'd have to add this to the library array, which is our property.
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So for the bottom here,
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we can add it to the array using the NS array initializer method with objects.
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And then we were passing each dictionary, as an object to this array.
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Now this works, but there's a couple issues with our code.
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First, it's going to be unreadable.
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But more than that, we would be repeating code quite often.
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If we replicated this six times,
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we would calling NSDictionary, dictionaryWithObjects, forKeys six times.
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And then repeating all this code over and over again.
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Especially the keys.
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We would be retyping the keys six times.
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Now an easier way to do this, is using the NSDictionary literal syntax.
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Now to create a dictionary using the literal syntax, go something like this.
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Let's do NSDictionary riseAndShine, just like our last dictionary.
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We start with an @ symbol,
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followed by curly braces to create an empty dictionary.
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We can then add key value pairs just like before.
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So we could do someKey to someValue.
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So this is a first step.
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We could use this and get rid of the dictionary with objects, method call.
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Gonna use another shortcut, to show you what that looks like.
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And that would look like this.
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Now we created the exact same dictionary object,
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as we had before this time using literal syntax.
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As you can see, it's a bit easier to read.
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If you've noticed here in our artists key we have
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a value we're passing in, an array.
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Now we're creating an array using the literal syntax just like the dictionary.
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So we could construct our library array, using this literal syntax as well.
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We'd start with an at symbol, and
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then instead of curly braces we use square brackets.
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So to add this dictionary to our library array,
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we can just wrap it in the array literal syntax.
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For example, self.library equal rise and shine.
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Now, there's only one problem though.
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With six instances of playlist dictionaries,
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we're repeating the same keys over and over again.
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Even though this syntax is usually used.
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I still have to write this out five more times, to create all my playlists.
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By doing this we're not adhering to the DRY, that is,
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don't repeat yourself principle.
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We're writing code over and over again.
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But on top of that, we're bound to make mistakes somewhere and
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introduce errors in our code.
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These keys are hardcoded strings.
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And repeating them five times,
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increases the chance that we're going to make an error somewhere.
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What we can do is create constants, or
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constant strings, to represent each dictionary key, and
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then use those variables as keys instead of retyping the string every single time.
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This is a common pattern in iOS development.
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Let's start by defining our constants at the top,
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under the import statement at the top of our file.
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Gonna make some room.
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We start with the class.
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So, NSString, cuz we're creating a string, then the asterisk.
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But instead of naming the string as we usually do,
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we're going to add the keyword const.
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This creates a constant pointer to a string.
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When creating constants in objective C, it's standard practice to start the name
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of a constant with a lower case k followed by the name of the constant.
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So in our case, let's create a constant for the first key title.
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We'll do kTitle, and then we'll assign it this string,
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gonna copy paste it, put some space.
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There we go, title.
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Now, in our dictionary code, instead of using this string title.
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We can just use the variable.
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Similarly, let's go ahead and create constants for all our keys.
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[BLANK_AUDIO]
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Okay, so now we can replace all the hard coded strings,
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using our constants that we just created.
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But, rather than having you watch me retype all these keys, and
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then add five other playlist dictionaries,
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I'm just going to get rid of this code and copy paste that in.
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So now we have self.library, which is
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assigned an array, in using the little syntax.
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Inside that array, we using the literal syntax again,
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we create dictionaries containing playlist instead, with this
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dictionaries we're using a constant string, as the key for key value pick.
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Now, the structure of our dictionary is simple enough.
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We have a key for example title along with the value as a string.
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Now all values are stored as strings, except for artists, which is an array
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containing names of the artists included in the playlist, stored as strings.
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Also the background color.
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This contains a second dictionary,
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as you can see here by the dictionary literal syntax.
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We're storing the red, green, blue, and alpha values here, so
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that we can construct a background color for our cover images.
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Even our Icon keys return a string with the name of the image that we're going to
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use as our playlist cover.
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The Icon strings will match up to the images we loaded,
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in the image assets folder earlier.
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That is how a single playlist dictionary is structured.
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Since we have six playlists,
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we had six dictionaries in our array, to represent our entire music library.
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Now as in a site, if we were building a real music library app
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we would not be hard coding data like this.
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In reality, we would be pulling that information off of
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a server on the Internet and creating our data models as we get that information.
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That's a bit more advanced, though, and to start off, we're just going to
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hard code our data, so we don't have to worry about all those details.
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Now, just to reiterate, we created these constants for a couple different reasons.
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First we're using variables here, so you see that I don't have to
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retype the hard coded string every single time for all six dictionary instances.
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But even more important, down the road let's say I wanted to change this title.
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Let's say I want to change this key in my dictionary.
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If I had hardcoded strings I would have to go in everywhere,
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wherever the key is used in my map and change that string every single time.
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Now, now that we have constants I can just come up here, and
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change this title to something else say title,
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and that's going to be reflected in all the dictionaries in my library instance.
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That makes a lot more sense, and
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we're being better coders by adhering to the dry principle.
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Let's change this back.
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Creating the library was our first step.
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Every time we want information about a playlist, we don't want to
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have to deal with the entire library array and it's dictionaries.
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Why?
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Because it would be fairly cumbersome.
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We'd have to iterate over the array grab each dictionary,
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know the correct keys to get the values we want and so on.
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To make the data easier to work with we're going to create a second class called
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playlist, that will pull the information we need from the library and
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package it up for us to use.
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