conversations.from: human
conversations.value: What could be a title for a book about with the genre: Computers & Technology, Programming?
conversations.from: assistant
conversations.value: A title for this book could be 'Pro Design Patterns in Swift'
conversations.from: human
conversations.value: Give me the first five paragraphs for this book.
conversations.from: assistant
conversations.value: Pro Design Patterns in Swift
  This code uses a for loop to create asynchronous GCD blocks that check out and return Book objects from the Library. To make the example a little more realistic, I have added a delay after the Book is obtained from the Library and before it is returned again, like this:
  Start the application to see the results. It will take a few seconds to run, so be patient. When all of the blocks generated in the main.swift file have executed, you will see output similar to the following:
  Your results may be different because the random nature of the delay I added will vary the sequence in which objects are taken from and returned to the pool.
  Care is required when implementing the object pool pattern because it is easy to create a pool that doesn't work or doesn't suit the application to which it has been applied. It is easy to get carried away trying to create the perfect pool, especially when considering the variations that I describe in Chapter 8. The result can be code that is difficult to maintain and an unstable object pool that behaves unpredictably once it has been deployed.
  Care is also required when protecting an object pool against concurrent access in order to avoid creating a pool that locks up unexpectedly, even if it works during your development tests. Be conservative and give preference to safety over performance—and, most importantly, test your code with as many different usage scenarios as you can. I recommend keeping object pools as simple as you can and focus on producing something that works and can be easily tested.
conversations.from: human
conversations.value: Give me five more paragraphs.
conversations.from: assistant
conversations.value: The Spreadsheet class has a Dictionary property that stores a collection of Cell objects, each of which is indexed by a Coordinate object. The Coordinate stores a column and row value, such as A45, where A is the column and 45 is the row, in order to create a grid. The Cell object is used to store an Int value at a given position in the grid and also has details of the coordinate that its value relates to. The initializer for the Spreadsheet class creates a grid with 26 columns and 50 rows and sets the value property of each Cell to the row index. The setValue method changes the value property of a Cell at a specified Coordinate, and the total property computes the sum of the value properties of all the Cell objects in the grid.
  The global function called == compares two Coordinate objects for equality, which allows them to be used as keys in a Dictionary collection.
  The problem addressed by the flyweight pattern is the impact of creating large numbers of identical objects, both in terms of the memory that they consume and the amount of time taken to create them. The Spreadsheet class that I defined in the previous section creates Cell and Coordinate objects for every position in the grid it maintains, which means that each Spreadsheet object generates a relatively large number of objects. Listing 17-2 shows the code I added to the main.swift file to demonstrate the problem.
  I create two Spreadsheet objects, set values for different cells, and write out the totals. I then write out the total number of Cell objects that are in the Dictionary collections maintained by the Spreadsheet objects. Running the application produces the following output:
  I have ended up with a large number of Cell objects for such a simple operation, and most of them still have the default value that was assigned during instantiation.
conversations.from: human
conversations.value: Give me the last five paragraphs.
conversations.from: assistant
conversations.value: Notice how I have been able to extend the functionality of the example application without needing to change any of the existing model, view, or controller code. I defined a new set of commands and then added new code to respond to them and display the data that was generated. Each of the components in the application is focused on a narrow range of tasks and is loosely coupled to the other components, making it easy to isolate a component for testing or to add or change features.
  There are many variations in the way that the MVC pattern is implemented and variations on the pattern itself. The only variation that matters to Swift developers is using a predefined MVC framework. The most commonly used frameworks are UIKit and AppKit. Creating your own MVC framework is an interesting experiment, and it reveals a lot about how the different sections of the pattern fit together, but for real applications, stick to the MVC frameworks provided by Apple, which provide a lot of ready-made functionality and which have been thoroughly tested.
  The most common pitfall is to define code in one section that belongs in another. As I explained at the start of the chapter, this isn't always a clear-cut decision, and the best way to avoid this pitfall is through experience. Implementing the MVC pattern successfully depends on understanding the way that your development processes work, and the more often you apply the MVC pattern, the better your understanding of what works for your projects and what doesn't. Take your time to think through what you are doing, be prepared to refactor when you change your mind about a feature, and leave yourself opportunities to experiment.
  The most obvious examples of the MVC pattern are the AppKit and UIKit frameworks, which use MVC to enforce structure in UI applications.
  In this chapter I described the MVC pattern and explained how it is used to add structure to a complete application. I explained the different sections that make up the MVC pattern and created an example application and framework as an example implementation.