NSURL- AppleURL- Apple (mostly same as the above,URLis new to swift and can be used interchangeably withNSURL)NSBundle- AppleJSONSerialization- AppleCreating and Modifying an NSURL in Your Swift App- Coding Explorer Blog
We've already works a bit with the concepts of converting JSON into data models. We're going to extend this further in this into by showing you how to import data from a local .json file, rather than taking an Dictionary of values and converting them into your data model. This practice is very common when making web requests, so we'll just be getting our feet wet with this example.
Locating a file is done via querying the filesystem using NSURL/URL. As briefly stated in the official Apple docs for NSURL/URL:
An NSURL object represents a URL that can potentially contain the location of a resource on a remote server, the path of a local file on disk, or even an arbitrary piece of encoded data.
In the project you will find Main.storyboard already containing a UITableViewController with an embedded UINavigationController that is set to be the "Initial View Controller".
- Change the custom class of the table view controller to
InstaCatTableViewController - Give the prototype cells an identifier of
InstaCatCellIdentifier - Switch over to
InstaCatTableViewController.swiftand add in anInstaCatstruct to house the data for anInstaCat
- For this part, refer to your tests to know how you should construct the model
- Now add in the following instance variables to the
InstaCatTableViewControllerclass
internal let InstaCatTableViewCellIdentifier: String = "InstaCatCellIdentifier"
internal let instaCatJSONFileName: String = "InstaCats.json"
internal var instaCats: [InstaCat] = []Let's start getting used to creating our functions before we start typing them out completely. This will help to understand that a little bit of planning and preparation can go a long way in development. How exactly do we know what we'll need though? We should think of the task as being a series of functions that take an input and return some output. Our goal is to locate a file, get the data of that file, and try to create [InstaCat] from that data. Combining those two concepts (series of functions and goals) we can derive a good estimate of what we'll need:
- We know that we're going to need to create an array of
InstaCatfromData. That means we know that the input to a function will beDataand its output is going to be[InstaCat], but in all likelihood (and as is common practice) we should not guarantee that the data will create[InstaCat]so we return an optional. - In order to get that
Data, we're going to need the file'sURLso that we can locate it - The name of the file is available to us as a
String, but we'll need aURLto be able to do the searching
So, from the above we can deduce:
internal func getResourceURL(from fileName: String) -> URL? {
return nil
}
internal func getData(from url: URL) -> Data? {
return nil
}
internal func getInstaCats(from jsonData: Data) -> [InstaCat]? {
return nil
}Note: I have a preference for immediately adding a return value for functions that I write that expect one. I do this only because I prefer not to see pre-compiler errors.
And now, in viewDidLoad, we can add all of the following, even if we haven't filled out the functions yet:
guard let instaCatsURL: URL = self.getResourceURL(from: instaCatJSONFileName),
let instaCatData: Data = self.getData(from: instaCatsURL), // sorry, this should be Data, not NSData!
let instaCatsAll: [InstaCat] = self.getInstaCats(from: instaCatData) else {
return
}
self.instaCats = instaCatsAllEach of the projects we've created, compile into a self-encapsulated application bundle (aka. "app bundle"). The NSBundle/Bundle class helps with locating resources within your app's bundle - for example a file. For the most part, you're only ever going to be using Bundle.main (which refers to the directory within your application bundle where the contents of your project are commonly kept).
internal func getResourceURL(from fileName: String) -> URL? {
// 1. There are many ways of doing this parsing, we're going to practice String traversal
guard let dotRange = fileName.rangeOfCharacter(from: CharacterSet.init(charactersIn: ".")) else {
return nil
}
// 2. The upperbound of a range represents the position following the last position in the range, thus we can use it
// to effectively "skip" the "." for the extension range
let fileNameComponent: String = fileName.substring(to: dotRange.lowerBound)
let fileExtenstionComponent: String = fileName.substring(from: dotRange.upperBound)
// 3. Here is where Bundle.main comes into play
let fileURL: URL? = Bundle.main.url(forResource: fileNameComponent, withExtension: fileExtenstionComponent)
return fileURL
}Getting the Data contents of the file located at fileURL is fairly straightforward
internal func getData(from url: URL) -> Data? {
// 1. this is a simple handling of a function that can throw. In this case, the code makes for a very short function
// but it can be much larger if we change how we want to handle errors.
let fileData: Data? = try? Data(contentsOf: url)
return fileData
}Using the tests provided and following snippet of code (with hints), finish out the rest of this project so that you can parse out the data contained in InstaCats.json to create [InstaCat and output the info to the InstaCatTableViewController as pictured below:
internal func getInstaCats(from jsonData: Data) -> [InstaCat]? {
// 1. This time around we'll add a do-catch
do {
let instaCatJSONData: Any = try JSONSerialization.jsonObject(with: jsonData, options: [])
// 2. Cast from Any into a more suitable data structure and check for the "cats" key
// 3. Check for keys "name", "cat_id", "instagram", making sure to cast values as needed along the way
// 4. Return something
}
catch let error as NSError {
// JSONSerialization doc specficially says an NSError is returned if JSONSerialization.jsonObject(with:options:) fails
print("Error occurred while parsing data: \(error.localizedDescription)")
}
return nil
}