Playgrounds are an interactive work environment that lets us write code and see the results immediately in the sidebars. As changes are made to the code, the results in the sidebar also change in real time. This means that Playgrounds are a great way to learn and experiment with Swift.

Playgrounds also make it incredibly easy to try out new APIs, prototype new algorithms, and demonstrate how code works. We will be using Playgrounds throughout this book to show how our sample code works. Therefore, before we really get into Swift development, let's spend some time learning and getting comfortable with Playgrounds.

If the Swift code does not make a lot of sense right now, do not worry; as we go through the book, this code will begin to make sense. We are simply trying to get a feel of Playgrounds right now.

A Playground has three sections, as follows:

The following screenshot shows how the sections are arranged in a Playground:

Let's start a new Playground. The first thing we need to do is to start Xcode. Once Xcode has started, we can select the Get started with a playground option, as shown in the following screenshot:

Alternatively, we can navigate to File | New | Playground from the top menu bar, as shown in the following screenshot:

Next, we should see a screen similar to the following screenshot that lets us name our Playground and select whether the Playground is an iOS or OS X Playground. For most of the examples in this book, it is safe to assume that you can select either iOS or OS X unless it is otherwise noted.

Finally, we are asked for the location to save our Playground too. After we select the location, the Playground will open up and look similar to the following screenshot:

From the preceding screenshot, we can see that the coding area of the Playground looks like the coding area for an Xcode project. What is different is the sidebar on the right-hand side. This sidebar is where the results of our code are shown. The code in the previous screenshot imports iOS's UIKit framework and sets a variable named str to the string Hello, playground. You can see the content of the str string in the sidebar to the right of the code.

By default, a new Playground does not open the timeline sidebar. You can open it manually by pressing the Command, Option, and Enter keys together. We will show you why the timeline sidebar is so important later in this chapter.

When you start a new iOS Playground, the Playground imports UIKit (Cocoa Touch). This gives us access to the UIKit framework that provides the core infrastructure for iOS applications. When we start a new OS X Playground, the Playground imports Cocoa. This gives us access to the OS X Cocoa framework.

What the last paragraph means is, if we want to experiment with specific features of either UIKit or Cocoa, we need to open the correct Playground. As an example, if we have an iOS Playground open and we want to create an object that represents a color, we would use a UIColor object. If we had an OS X playground open, we would use an NSColor object to represent a color.

As you will see throughout this book, Playgrounds are great at showing the results of code as text in the results sidebar. Playgrounds can also do a lot more than just text; they can also do images, graphs, and display views. Let's take a look at how we would show an image in a Playground. The first thing we need to do is to load the image into the resource directory of our Playground. The following steps show how to load an image into the resource directory:

We can also graph the value of numeric variables over time with the timeline sidebar. This feature is really useful when we are prototyping new algorithms. It allows us to see the value of the variable throughout the course of the calculations. To see how graphing works, take a look at the following Playground:

In this playground, we set the variable j to 1. We then create a for loop that assigns numbers 1 through 5 to the variable i. At each step in the for loop, we set the value of the variable j to the current value of j multiplied by i. The graph in the timeline sidebar shows the value of the variable j at each step of the for loop. We will be covering for loops in detail later in this book.

To bring up the graph, click on the symbol that is shaped like a circle with a dot in it. We can then move the timeline slider to see the values of variable j at each step of the for loop.

There is a lot more that we can do with Playgrounds, and we have only scratched the surface in our quick introduction here. As we proceed through the book, we will be using Playgrounds for almost all of the sample code and will demonstrate other features of Playgrounds as they are used.

Before we leave this brief introduction, let's take a look at what Playgrounds are not, so we can understand when not to use Playgrounds:

Writing comments in Swift code is a little different from writing comments in Objective-C code. We still use the double slash // for single line comments and the /* and */ for multiline comments.

What has changed is how we document the parameters and the return value. To document any parameter, we use the :parm: field, and for the return value, we use the :return: field. The following Playground shows examples of both single line and multiline comments to properly comment a function:

To write good comments, I would recommend using single line comments within a function to give quick one-line explanations of your code. We will then use the multiline comments outside of functions and classes to explain what the function and class does. The preceding Playground shows a good use of comments. By using proper documentation, as we did in the preceding screenshot, we can use the documentation feature within Xcode. If we hold down the option key, and then click on the function name anywhere in our code, Xcode will display a popup with the description of the function. This next screenshot shows what that popup would look like:

You probably noticed from the code samples so far, that we are not using semicolons at the end of lines. The semicolons are optional in Swift; therefore, both lines in the following Playground are valid in Swift. You can see the results of the code in the results sidebar, as shown in the following screenshot:

For style purposes, it is strongly recommended that you do not use semicolons in your Swift code. If you are really set on using semicolons in your code (I would not recommend it), then I would recommend just being consistent and using it on every line of code; however, Swift will not warn you if you forget them.

In Swift, parentheses around conditional statements are optional, for example, both if statements in the following Playground are valid. You can see the results of the code in the sidebar, as shown in the following screenshot:

For style purposes, it is recommended that you do not include the parentheses in your code unless you have multiple conditional statements on the same line. For readability purposes, it is good practice to put parentheses around the individual conditional statements that are on the same line. See the following Playground for samples:

In Swift, unlike most other languages, the curly bracket is required after statements. This is one of the safety features that are built into Swift. Arguably, there have been numerous security bugs that may have been prevented if the developer would have used curly braces. A good example of this is Apple's Goto Fail bug. These bugs could also have been prevented by other means, as well as unit testing and code reviews, but requiring developers to use curly braces, in my opinion, is a good security standard. The following Playground shows you what error you get if you forget to include the curly braces:

In most other languages, the following line of code is valid, but it probably is not what the developer meant to do:

if (x =1) {
}

In Swift, this statement is not valid. Using an assignment operator (=) in a conditional statement (if and while) will throw an error. This is another safety feature built into Swift. It prevents the developer from forgetting the second equals sign (=) in a comparison statement. This error is shown in the following Playground:

For both conditional (if and while) and assignment (=) statements, the white spaces are optional. Therefore, in the following Playground, both the i block and j block of code are valid.