This is an iOS app that reads in a list of interesting photos from Flickr using its API.
This project was created to learn how to make web service requests using URLSession, parse JSON data using JSONSerialization, implement data persistence using Core Data framework and how to present content using UICollectionView. This project also allowed an opportunity to practice implementing segues.
This GIF shows the app in action.
- Web service requests can be done with URL and key-value pairs that are part of the URL's format are called query items.
- Enumerations can have raw values associated with them, which can be strings, integer or floating point numbers, and characters.
- To declare a type-level property or a method in structs, use the static keyword. Classes may also use the class keyword for type properties and methods.
- The default level of access control for properties and methods is internal. Going from least restrictive to most restrictive, the levels are: open, public, internal, fileprivate, and private.
- Computed properties calculate a value instead of storing it. They provide a getter and an optional setter.
- URLRequest contains the information needed for the web service request. An instance of URLRequest can be created with an URL object, which in turn can be properly formatted with the help of an URLComponents instance.
- URLSessionTask is an abstract class that is responsible for the actual work of communicating with a server. The three types of concrete session tasks are: URLSessionDataTask for retrieving data and returning it as Data in memory, URLSessionDownloadTask for downloading files and URLSessionUploadTask for uploading files. To start a task, .resume() needs to be called as all tasks are created in a suspended state.
- URLSession serves as a factory for URLSessionTask instances. An instance of URLSession can create multiple tasks with common properties defined by it's configuration.
- In JSON documents, curly braces ( { and } ) denote a dictionary while square brackets ( [ and ] ) denote an array.
- Enumerations can also have associated values. Associated values in enums can be a great way to handle an operation's, such as a web service request, success or failure; the success case can have data following a successful execution and the failure case can have error info tied to it. Unlike raw values, associated values can be of any type.
- It is a good idea to add a completion closure with the @escaping annotation to the method where the web service request is taking place. Since getting data from the request will be asynchronous, the closure will be called once the particular instance of URLSessionTask completes the request.
- A URLSessionDataTask will run it's completion handler on the background thread. To run any instructions inside the completion handler on the main thread, use OperationQueue.main.addOperation(_:) and place such instructions inside it.
- To refactor the UICollectionView and UITableView data source methods to a separate class, the class must conform to NSObjectProtocol on top of conforming to the respective data source protocol.
- To customize the layout of a UICollectionView, access it's collectionViewLayout property as UICollectionViewFlowLayout and adjust the properties as needed. Alternatively, the view controller with the collection view can conform to UICollectionViewDelegateFlowLayout and implement the optional methods.
- The method awakeFromNib() is called when a cell is first created and prepareForResuse() when a cell is about to get reused.
- The UIActivityIndicatorView class can be used to display a spinning wheel for loading content as the user waits.
- To compare data items in a collection, conform the type of the data to the Equatable protocol. For example, conforming to the Equatable protcol and implementing ==(::) function allows a type to see if two of its instances have equal values.
- Extensions offer more functionality to a class/type by adding computed properties, adding methods and conforming to protocols. Stores properties, however, cannot be added to extensions.
- In Core Data, a Swift type is called an entity and the properties are called attributes. If a property's type is not one of the certain data types that Core Data can store in its stores, declare that property as a transformable attribute. The attributes in Core Data are also modelled to be optionals.
- NSManagedObject is the default class of an object fetched with Core Data.
- When creating an instance of NSPersistentContainer, the name must match the name of the data file model which holds the different entities. After its instantiation, the container needs to load its persistent stores and this operation is an asynchronous one.
- A NSPersistentContainer object's viewContext is what allows interaction with the entities in Core Data. This viewContext property is of type NSManagedObjectContext. Use the performAndWait(_:) synchronous method to return after its block is executed.
- Saving in Core Data is relatively simple with calling save() on the viewContext of the NSPersistentContainer object. On the other hand, loading/fetching objects back requires executing a NSFetchRequest. The sortDescriptors property of a NSFetchRequest object needs to be supplied an array of NSSortDescriptor instances. This sortDescriptors property has a key that matches an attribute of the entities to be loaded and a boolean for ascending/descending sorting. The array helps sorting through potential collisions, e.g. people with the same last name could be sorted based on their first names.
- A NSPredicate object can contain a condition that can be true or false. Such a predicate can be added to a NSFetchRequest instance via it's predicate property to check if an entity matches a specific criteria.
- In Core Data, entities can have two kinds of relationships: to-one and to-many. In a to-many relationship, an instance of one entity has a reference to a Set containing instances of the other entity. Two entities can have a bidirectional relationship with each other, called an inverse relationship.
- Instead of using the viewContext of NSPersistentContainer, use performBackgroundTask(_:) to run expensive operations on the background and avoid blocking the main queue.
Copyright 2019 Soumik Barua
Licensed under the Apache License, Version 2.0 (the "License");
you may not use this file except in compliance with the License.
You may obtain a copy of the License at
http://www.apache.org/licenses/LICENSE-2.0
Unless required by applicable law or agreed to in writing, software
distributed under the License is distributed on an "AS IS" BASIS,
WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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