You can find the code for this chapter on GitHub in the Chapter 2 folder: https://github.com/PacktPublishing/Animating-SwiftUI-Applications.

Let’s consider the book’s definition of animation. Animation is a series of static images displayed in rapid succession to create the illusion of motion. The images, or frames, are usually displayed at a rate of 24 or 30 frames per second from the beginning to the end, which is fast enough to create the illusion of continuous motion. These can be created using a variety of techniques, including hand-drawn, computer-generated, and stop-motion animations.

Looking at this definition, we can see that there is a start point and an end point to animation, and the images in between are all slightly different; when played, our eyes cannot pick out the individual images, which means we perceive movement or animation.

In SwiftUI, we have to define the start point (where the animation begins) and the end point (where the animation ends). However, when we code, we don’t actually put a bunch of still images between both end points (though we can); what we usually do is use a single image and then animate the properties of that image, such as its location on the screen, its opacity, or its color.

In addition to images, we can also animate RGB colors by changing their hue or the corner radius of shapes, and if we draw a shape, we can animate the individual lines (paths) or the strokes around it.

It works like this: if we want a rectangle to move from the bottom left to the top right of the iPhone or iPad screen, we declare it in code (that declarative syntax thing again) by using the animation modifier. Then SwiftUI does the magic for us, moving the object, or in SwiftUI speak, “transitioning” the object from a start point to an end point, filling in all the gaps along the way with whatever values we are working with (integers, colors, opacity, etc.). The process of going through all the values to create a smooth fluid motion is called interpolation.

SwiftUI fills in those gaps for us really well, but it cannot animate every property of every view. Only properties considered “animatable” can be animated; things such as a view’s color, opacity, rotation, size, position, corner radius, and strokes. Nearly all properties that have a numerical value are animatable.

SwiftUI includes basic animations with default or custom easing or timing curves (a timing curve refers to the speed at the start and end of the animation), as well as spring animations. Spring animations have a bouncy effect to them and can be adjusted from a slight bounce to a very pronounced bounce, similar to a rubber ball bouncing on the floor.

You can also change many customization options, such as the speed of an animation, add a “wait” time before the animation starts, and have an animation repeat itself.

Let’s continue by diving deeper into animation timing curves, seeing what they look like and how they affect an animation.

Animations have what are called curves. A curve, or more specifically, a timing curve, refers to the speed at which the animation starts and how it should end.

SwiftUI provides several timing curves to choose from that we can use inside the animation modifier. It’s called a timing curve because if you were to plot each point of the animation’s movement from start to finish on a graph, and draw a line by connecting those points, most of them would create a curved line, as in this illustration:

Figure 2.1: The ease timing curves

This graph shows three animation timing curves: easeIn, easeOut, and easeInOut. The beginning of the animation is at the bottom left, and the end is at the top right. In the easeInOut timing curve, the animation starts slow, speeds up, and then finally slows down before coming to a complete stop.

There is also a linear timing curve. An animation using this curve will have the same rate of speed at its beginning as it does at its end. If you were to plot it on a graph, it would be a straight line, like so:

Figure 2.2: The linear timing curve

Timing curves are not complicated – we get to choose the ones we want based on how we want the animation to look and perform. If you don’t specify a timing curve, you will get the default curve, the easeInOut one. We will use some of these SwiftUI-provided timing curves in our projects.

In the next section, I want to explain the two distinct types of animation in SWiftUI: implicit and explicit.

There are two types of animations in SwiftUI: implicit and explicit. Let’s look at what these animation types do and the difference between the two.

An implicit animation is an animation that is automatically applied to a property when its value is changed. In SwiftUI, implicit animations are created using the .animation modifier. Here is an example:

struct ContentView: View {
    @State private var grow: CGFloat = 1
    var body: some View {
        Circle()
            .frame(width: 100, height: 100)
            .foregroundColor(.blue)
            .onTapGesture {
                self.grow += 0.5
            }
            .scaleEffect(grow)
            .animation(.default, value: grow)
    }
}

In this example, we use a tap gesture to scale up the size of a circle; when tapped, the circle will grow by half its size. The .animation modifier uses the default animation style, which will animate the circle using an easeInOut timing curve by default.

You can also use other animation styles. For example, here, I added a spring style instead of the default style:

var body: some View {
        Circle()
••••••••
            .scaleEffect(grow)
            .animation(.spring(dampingFraction: 0.3,blendDuration: 0.5),value: grow)
    }

This style will animate the circle and add a springy effect to it.

So implicit animations are a convenient way to add basic animations to your SwiftUI app without having to write any explicit animation code. The animations are applied using the animation modifier.

Sometimes though, you may want more from your animation, and implicit animations may not give you the degree of control that you are looking for. In this case, you can use explicit animations instead.

An explicit animation is an animation you create and add inside the code block using the withAnimation function. Here’s an example:

struct ContentView: View {
    @State private var scaleUp: CGFloat = 1
    var body: some View {
        Button(action: {
            //Animate the scale of the view when the button is               tapped
            withAnimation(.interpolatingSpring(stiffness: 60,               damping: 2)) {
                scaleUp *= 1.4
            }
        }) {
            Text("Scale up")
                .scaleEffect(scaleUp)  // explicit animation
        }
    }
}

In this example, tapping the button will animate the scale of the text using a spring animation. The duration of the animation will be determined by the system’s default animation settings, but the curve of the animation will be customized using the interpolatingSpring function.

Additionally, you can customize the duration of the animation by specifying a duration parameter in the interpolatingSpring function. The following is an example of this:

withAnimation(.interpolatingSpring(stiffness: 60, damping: 2, duration: 2.5)) { scaleUp *= 1.5 }

This will cause the animation to last for 2.5 seconds.

So, the difference when using explicit animations versus implicit animations is that they can give you more control over the details of the animation, or when you want to animate multiple properties simultaneously; you can put as much code into the withAnimation block as needed. However, they can be more work to set up than implicit animations.

You can also have the animation repeat itself a pre-set number of times or indefinitely. Here’s an example of repeating the previous animation forever and auto-reversing it:

withAnimation(.interpolatingSpring(stiffness: 60, damping: 2).repeatForever(autoreverses: true)) {
                    scaleUp *= 1.4
                }

In the previous example, I altered the code in the withAnimation function to include the repeatForever option and set the autoreverses parameter to true. When you run the code, the text will scale up with a springy effect, and when it’s done bouncing (about 3 seconds or so), the animation will start over, repeating endlessly or until the app is stopped.

Those are the two types of animations; next is a list of ways to trigger animations.

So, how do we trigger animations? There are several ways to do so in SwiftUI, including using the following SwiftUI modifiers, methods, and functions:

• The .animation() modifier: This modifier allows you to specify the type of animation used when a view appears or disappears or when its state changes.
• The withAnimation() function: This function allows you to wrap a block of code that changes the state of a view, and it will automatically animate the changes.
• A gesture: This is a way to interact with a view by performing actions such as tapping, dragging, or pinching. You can use a gesture to trigger an animation when a certain action is performed on a view.
• A timer: This allows you to specify an animation to be performed over a certain period of time. You can use a timer to animate the changes to a view’s state over a specific duration.
• The onAppear() and onDisappear() modifiers: These modifiers allow you to specify code to be executed when a view appears or disappears. These modifiers can trigger an animation when a view appears or disappears.
• Button and other control views: A button, slider, picker, or other control type view in SwiftUI can be a trigger for an animation.

There are other ways to trigger animations, but these are the main ones that we will cover here. Whatever you choose will depend on the specific needs of your app and the behavior you want to produce. We will explore these different triggers when we start building our projects in the coming chapters.

Let’s continue by looking at various properties that can be animated in SwiftUI.

In this section, we will explore some of the animatable properties. Here is a list of the ones that we will look at:

• Offset
• Hue rotation
• Opacity
• Scale
• Stroke
• Trim
• Corner radius

Let’s take a look at them in more detail.

The offset property

The first property we will look at that is animatable is the offset property. This property is responsible for offsetting a view on the x- and y-axes, basically giving you control to place that view anywhere on the screen. The following is an example of animating the offset modifier that has been placed on a rectangle shape:

struct Offset_Example: View {
    @State private var moveRight = false
    var body: some View {
       //MARK: - ANIMATE OFFSET
        VStack {
            RoundedRectangle(cornerRadius: 20)
                .foregroundColor(.blue)
                .frame(width: 75, height: 75)
                .offset(x: moveRight ? 150 : 0, y: 350 )
                .animation(Animation.default, value: moveRight)
            Spacer()
            Button("Animate") {
                moveRight.toggle()
            }.font(.title2)
        }
    }
}

After you put that code into your ContentView file, your preview will look like Figure 2.3. When you press the Animate button, the blue rectangle will move to the right, and when you press it again, it will return to its original starting position.

Figure 2.3: Animating the offset

This is how the code works. When the Animate button is pressed, the moveRight variable has its value toggled or changed to true, and the offset modifier has a ternary operator in there for its x parameter.

A ternary operator is an operator that accepts a Boolean variable and checks to see whether it’s true or false. If the variable is true, the value to the left of the colon is used, but if the variable is false, the value to the right of the colon is used. This makes it similar to an if statement but different because an if statement can check for multiple conditions.

So, if moveRight is true, then the rounded rectangle is placed 150 points to the right; otherwise, if false, it is left where it is (the 0 value means do nothing). The animation modifier also picks up on any change because it has the moveRight variable in there for the value parameter. This value parameter takes the variable that you are using for the animation. The animation modifier will then interpolate over the values from the start to the finish point and move the object smoothly, creating a nice, fluid animation.

Here’s a way to really see how the animation modifier is working. If you comment out the animation statement in the code and press the button, you will see that the object still moves 150 points to the right, but it does so instantly; there is no gliding across the screen now; the object just appears at its new location 150 points to the right. To create smooth, fluid animations, we need that animation modifier and its behind-the-scenes interpolating magic. This is part of the reason why we use less code in SwiftUI versus coding animation in UIKit; much of the heavy lifting is already done for us in the background with SwiftUI.

This was an example of animating an object from one point to another by changing the numerical value for the x parameter in the offset modifier. Let’s look at another property that’s animatable: HueRotation.

Hue rotation

Hue rotation is a type of color effect that can be applied to views and other components. It’s a modifier that allows you to adjust the hue of a color by adding or subtracting a fixed angle from its hue value. You can use hue rotation to create a range of related colors.

The modifier has an angle parameter that takes a value in radians or degrees. That value is based on a circle, which is 360 degrees and represents a wheel of all the colors that we can think of.

Let’s look at an Xcode example:

struct Hue_Rotation_Example: View {
    @State private var hueRotate = false
        var body: some View {
        //MARK: - ANIMATE HUE ROTATION
        VStack(spacing: 20) {
            Text("ANIMATE HUE ").font(.title2).bold()
            // rotate the colors and stop halfway around the               color wheel
            RoundedRectangle(cornerRadius: 25)
                .frame(width: 200, height: 200)
                .foregroundColor(.red)
                .hueRotation(Angle.degrees(hueRotate ? 180 :                   0))
                .animation(.easeInOut(duration: 2), value:                   hueRotate)
            // rotate the colors around the color wheel one               full revolution (360 degrees)
            Divider().background(Color.black)
            Text("ANIMATE HUE WITH GRADIENT").font(.title2).              bold()
            AngularGradient(gradient: Gradient(colors: [Color.              red, Color.blue]), center: .center)
                .hueRotation(Angle.degrees(hueRotate ? 360 :                   0))
                .animation(.easeInOut(duration: 2), value:                   hueRotate)
                .mask(Circle())
                .frame(width: 200, height: 200)
            Button("Animate") {
                hueRotate.toggle()
            }
            .font(.title)
        }
    }
}

When you add the code into Xcode, your previews will look like Figure 2.4:

Figure 2.4: Animating the hue rotation

I created two objects in this example: a rounded rectangle and an angular gradient circle.

In the rounded rectangle, I used a ternary operator to check whether the hueRotate variable is true. When it becomes true via the Animate button being pressed, the value to the left of the colon inside the ternary operator is used, 180 degrees. Then the animation starts going through the color spectrum and stops halfway through to display that color.

Notice the use of the duration function just after the timing curve. This function allows us to set the duration of the animation; do we want it to happen fast, or do we want the animation to happen over a longer period? It has one parameter, and that is the amount of time we want the animation to take to complete; for that, we use an integer value. I set the value to 2, so it can slow things down a little, taking the animation 2 seconds to complete.

Looking at the angular gradient example, I’m using a value of 360 degrees. When we press the Animate button, the code animates through the entire color wheel and stops where it started (360 degrees is one revolution of a circle), thus displaying the original color.

Taking a look at the hueRotate variable inside the body of the button, we have two ways that we can start the animation. The first is by explicitly setting hueRotate to true, like this:

hueRotate = true

Or by using the toggle method like we’re doing in the code:

hueRotate.toggle()

The difference between these two ways of starting the animation is that firstly, the animation starts and then finishes, but it never reverses itself with subsequent button presses. If you’d like the animation to start and finish, and on the next button press reverse itself, then use the toggle method.

What’s also very interesting is that we can animate the colors of shapes and other objects, but also animate the colors that make up images, as we will see in the upcoming projects.

You can also use the hueRotation() modifier in combination with other modifiers, such as brightness() or saturation(), to create some complex and interesting color adjustments.

Let’s continue looking at the different properties to animate and at a very common property, opacity.

Opacity

Opacity refers to the transparency of a view, whether you can see it clearly, whether it is partially visible, or maybe you can’t see it at all. We can use the opacity modifier to make views appear and disappear. When we add animation to the opacity, the transition from shown to hidden is interpolated over, so it smoothly fades in and out.

Here's an example of adding opacity to an animation:

struct Opacity__Example: View {
    @State private var appear = true
    var body: some View {
//MARK: - ANIMATE OPACITY
        VStack{
            Text("Appear/Disappear")
                .font(.title).bold()
            Circle()
                .foregroundColor(.purple)
                .opacity(appear ? 1 : 0)
                .animation(.easeIn, value: appear)
                .frame(height: 175)
            Button("Animate") {
                appear.toggle()
            }.font(.title2)
            //MARK: - OVERLAPPING OPACITY
            VStack{
                Text("Overlapping Opacity").bold()
                    .font(.title)
                Circle()
                    .foregroundColor(.yellow)
                    .frame(height: 100)
                    .opacity(0.5)
                Circle()
                    .foregroundColor(.red)
                    .frame(height: 100)
                    .opacity(0.5)
                    .padding(-60)
            }.padding(60)
        }
    }
}

The preceding code will produce the following results, as shown in Figure 2.5:

Figure 2.5: Animating the opacity

In our first example, the animation variable is called appear, and it’s set to true as its default value, which shows the circle. When the Animate button is pressed, the variable gets toggled to false, and the circle animates itself until it completely disappears. And when the button is pressed again, the animation gets set to true, and the circle becomes visible again. Again, using the animation modifier initiates the interpolation over the start and end values, so the circle doesn’t just appear or disappear instantly; there is a gradual change of state until the end of the animation is reached.

The second example of the two circles overlapping at the bottom of the screen demonstrates a unique component of opacity in SwiftUI. When we apply the opacity modifier to a view that already had its opacity transformed, the modifier will multiply the overall effect. For example, the yellow and red circles have their opacity set at 50%, overlapping each other. The top red circle allows some of the bottom yellow circle to show through, thus multiplying the opacity effect so that area is a little darker, and at the same time, mingling the two colors, creating orange.

Next, let’s take a look at animating the size or scale of a view, which we can do with the scaleEffect modifier.

Scale

Every view has a specific size, and we can change that size by scaling it up or down with animation. We can do this with the scaleEffect modifier. Here’s an example of how we can animate the scale of a view:

struct Scale_Example_One: View {
    @State private var scaleCircle = false
    var body: some View {
            //MARK: - ANIMATE THE SCALE OF A CIRCLE SHAPE
            VStack {
                Text("SCALE SHAPE").font(.title).bold()
                Circle()
                    .frame(width: 150)
                    .foregroundColor(.green)
                    .scaleEffect(scaleCircle ? 0.1 : 1)
                    .animation(.default, value: scaleCircle)
                Button("Scale Shape") {
                    scaleCircle.toggle()
                }
            }.font(.title2)
        }
    }

The preceding code will produce the following results, as shown in Figure 2.6:

Figure 2.6: Scaling a shape

You should be starting to recognize much of the code that we’re using; for example, we use a VStack to hold our views, so they get stacked vertically, and we can use the button control as a way to start the animation.

In the example, I’m creating a simple green circle and using the scaleEffect modifier, passing in our animating variable. When the state changes to true, the circle scales down to one-tenth of its size, and when false, it goes back to its original size.

We’re using the animation modifier again with the default timing curve. The default curve is an easeInOut curve, which we discussed earlier in the chapter. An easeInOut curve will cause the animation to start slowly, then ramp up to its top speed, and then finish by easing out slowly again.

Let’s look at another example of scaling up and down, but instead of scaling a shape that we created using the circle initializer, we’re using a system image to show you that you can also scale images:

struct Scale_Example_Two: View {
    @State private var scaleBug = false
    var body: some View {
        //MARK: - ANIMATE THE SCALE OF A SYSTEM IMAGE
        VStack{
            Text("SCALE IMAGE").font(.title).bold()
            Image(systemName: "ladybug.fill")
                .renderingMode(.original) //allows multicolor                   for SF Symbols
                .resizable()
                .frame(width: 150, height: 150, alignment:                   .center)
                .scaleEffect(scaleBug ? 0.1 : 1)
                .animation(.default, value: scaleBug)
                .padding(10)
            Button("Scale Image") {
                scaleBug.toggle()
            }
        }.font(.title2)
    }
}

The preceding code will produce the following results, as shown in Figure 2.7:

Figure 2.7: Scaling an image

This particular image is a system image from the SF Symbols app. If you don’t have this app yet, I highly recommend it. You can download it for free at the Apple Developer portal. In it, Apple has given us thousands of images we can use in our code. And what’s new in the latest release is that now, many of the images can be rendered in multicolor: we have to set the rendering mode to .original so the image gets shown with colors, instead of just black or white.

Note

Not all images can be colored. Look in the SF Symbols app to see which ones can be colored.

Finally, in this third example of a scaling animation, we use the anchor method, which scales the view by the given amount in both the horizontal and vertical directions relative to an anchor point:

struct Scale_Example_Three: View {
    @State private var scaleFromAnchor = true
    var body: some View {
                VStack{
            Text("SCALE FROM ANCHOR ").font(.title).bold()
            Image(systemName: "heart.fill")
                .renderingMode(.original) //allows the use of                   multicolor for SF Symbols
                .resizable()
                .frame(width: 150, height: 125, alignment:                   .center)
                .scaleEffect(scaleFromAnchor ? 1 : 0.2, anchor:                   .bottomTrailing)
                .animation(.default, value: scaleFromAnchor)
                .padding(10)
            Button("Scale from Anchor") {
                scaleFromAnchor.toggle()
            }
        }.font(.title2)
    }
}

The preceding code will produce the following results, as shown in Figure 2.8:

Figure 2.8: Scaling from an anchor point

All views have an anchor point, which is usually in the middle of the view. But we can change that anchor point, and have the animation scale the object based on where that anchor point is. In the code, I used the .bottomTrailing option as the anchor point, so when we press the button, the heart image scales down and toward the trailing edge (the right side of the screen), rather than scaling from the center of the object. However, SwiftUI also gives us the following anchor points to choose from:

• bottomTrailing
• trailing
• bottom
• center
• top
• bottomLeading
• topLeading
• topTrailing
• leading

In this final section, we will look at three more properties that can be animated: stroke, trim, and cornerRadius.

Stroke, trim, and corner radius

Let’s now look at three more properties we can animate: the stroke of a line, the trim of a circle, and the corner radius of a rectangle.

The stroke of a shape is the outline or border of the shape. It has a particular color and width and can have various attributes such as line cap style or line join style. Let’s animate the stroke of a rectangle, so it gets thicker or thinner with each button press:

struct Stroke_Example: View {
    @State private var animateStroke = false
    var body: some View {
        //MARK: - ANIMATE THE STROKE OF THE ROUNDED RECT
        VStack{
            Text("ANIMATE STROKE").font(.title).bold()
            RoundedRectangle(cornerRadius: 30)
                .stroke(Color.purple, style:                   StrokeStyle(inewidth: animateStroke ? 25 :                   1))
                .frame(width: 100, height: 100)
                .animation(.default, value: animateStroke)
            Button("Animate Stroke") {
                animateStroke.toggle()
            }
        }.font(.title2)
    }
}

Either a thick or thin stroke line is created around the rectangle, as shown in Figure 2.9:

Figure 2.9: Animating the stroke

The first thing we do is define our animation variable, setting its initial value to false. Looking inside the stroke modifier, I pass the animateStroke variable as an argument to the line width parameter, so when it does become true, it changes stroke to 25 points (otherwise, it will be 1 point). Again, we also use the default timing curve inside the animation modifier, and when we run this, stroke is smoothly modified from a thickness of 25 points, then back to 1 point when the button is pressed again.

Here is another example where we are using the trim modifier this time:

struct Trim_Example: View {
    @State private var animateTrim = false
    @State private var circleTrim: CGFloat = 1.0
    
    var body: some View {
        //MARK: - ANIMATE THE TRIM MODIFIER OF A CIRCLE
        VStack {
            Text("ANIMATE TRIM").font(.title).bold()
                .padding(.top, 10)
            Circle()
                .trim(from: 0, to: circleTrim)
                .stroke(Color.red, style: StrokeStyle(inewidth:                   30, lineCap: CGLineCap.round))
                .frame(height: 150)
                .rotationEffect(.degrees(180))
                .animation(.default, value: animateTrim)
                .padding(.bottom, 20)
            Button("Animate Trim") {
                animateTrim.toggle()
                circleTrim = animateTrim ? 0.25 : 1
            }
        }.font(.title2)
    }
}

The trim modifier takes two parameters: from (meaning what part of the circle we want to start trimming from) and to (meaning where we want to end the trimming). The from parameter is set to 0, which means there will be a complete circle on the screen as we are not trimming yet. The code produces the following results, a circle that has its line trimmed off and restored when the button is pressed, as shown in Figure 2.10:

Figure 2.10: Animating the trim

Also, notice we use two @State variables to work with the trim modifier, one called animateTrim, to trigger the animation, and one called circleTrim, which is a numerical value of the CGFloat type. This variable will hold the amount of circle we want to trim off. Initially, it gets set to 1, so the whole circle is visible.

Note

A CGFloat type is a floating-point number. CG stands for Core Graphics, which is an older coding paradigm was used in Apple’s graphic framework, but is still used in SwiftUI.

Looking inside the button code, then within the circleTrim variable, we’re storing one of two values using the ternary operator: either .25, or 1. This means that when animateTrim toggles to true, the code trims off 75% of the circle and leaves 25%; when animateTrim toggles to false, the value of 1 is used, which represents 100% of the circle. So, the values in the ternary operator represent how much of the circle to keep.

If we run the code, we see we have a nice trimming animation of this circle. The line of code called CGLineCap.round refers to the shape of the line that’s drawn at the end points, and it can be round, square, or butt line cap.

And just to have a little fun here, if we go into the trim modifier and change the from parameter to 0.5 instead of 0, we now start the drawing halfway through the circle. Run the code, and it looks like we’re animating or painting a smile and then removing the smile when we press the button again.

Note

If this code seems a bit confusing, where you see the trim modifier being set for the circle, and the circleTrim variable being set in the button body, then think of the trim modifier as the “where” part of the trimming. This means where do we want to start and end the trimming? Then, think of the ternary operator inside the button as the “how much,” meaning how much do we want to trim off the circle and how much of it do we want to keep?

Let’s now move on to the final example. In this example, we will take a look at how you can animate the corner radius of a rectangle. The corner radius refers to how sharp you want to make the corners of a rectangle; you can go all the way from a 90° angle up to a much higher value to create a smooth, rounded corner.

All the code is similar to what we’ve used so far except for the use of the cornerRadius modifier. The following is an example of the code:

struct Corner_Radius_Example: View {
    @State private var animateCornerRadius = false
    
    var body: some View {
        //MARK: - ANIMATE THE CORNER RADIUS
        VStack{
            Text("ANIMATE CORNER RADIUS").font(.title).bold()
                .padding(.top, 30)
            Rectangle()
                .foregroundColor(.green)
                .frame(width: 150, height: 150)
            .cornerRadius(animateCornerRadius ? 0 : 75)
            .animation(.default, value: animateCornerRadius)
            .padding(.bottom, 20)
            Button("Animate Corner Radius") {
                animateCornerRadius.toggle()
            }
        }.font(.title2)
    }
}

This code produces the following results: a rectangle with its corner radius changed from a 90° angle all the way up to create a circle. So, we’re changing a rectangle into a circle and back again when the button is pressed, as shown in Figure 2.11:

Figure 2.11: Animating the corner radius

In the code, the line that’s doing most of the work is this one:

.cornerRadius(animateCornerRadius ? 0 : 75) 

The animateCornerRadius variable gets passed into the cornerRadius modifier, which then gets checked for the true or false values; if it is false, it gets a value of 75 placed into it, which will make the size of this rectangle animate into a perfectly round circle. And when toggled back to true, the circle animates into a rectangle with 90-degree corners by having its corner radius changed to 0.

Note that the reason why the code creates a perfect circle is that we set the frame of the rectangle’s width and height to 150 points, thus creating a square, and anytime you set a corner radius to half the width or height of a square, you will always get a perfect circle.

SwiftUI gives us more ways to animate objects in addition to these, and we will explore them in the coming chapters when we start building projects.

In this chapter, we looked at how animations work, the two types of animation in SwiftUI, implicit and explicit, and many of the properties that can be animated. These include hue rotation, opacity, a view’s position on the screen and size, stroke, trim, corner radius, and timing curves.

This was an important step needed to guide you along on your SwiftUI animations adventure. Remember, if a property is a numerical value, it almost always can have an animation applied to it.

In the next chapter, we will start working on some projects. For our first project, we will create an app similar to Apple’s breathing app (very popular on Apple watches) and learn how to combine more than one animation in a view.