Ella Walter
In iOS app development, developers often need to implement gesture recognition to allow users to interact with the app through touch-based actions. One such gesture is panning, which involves the user dragging their finger across the screen to move or manipulate a view. To enable this functionality in Swift, developers can utilise the UIPanGestureRecognizer class, which simplifies the process of detecting and interpreting panning gestures. By attaching a UIPanGestureRecognizer to a view, developers can access the translation property, which indicates how far the user's finger has moved from its initial touchpoint. This enables the creation of interactive and dynamic user interfaces, such as slide-out trays or hamburger menus, where views can be dragged and dropped within the app layout.
| Characteristics | Values |
|---|---|
| Frameworks | UIKit, UIPresentationController, UIPanGestureRecognizer, UIGestureRecognizer |
| Methods | didPanTray, viewDidLoad, didPan(_:) |
| Variables | trayDownOffset, trayUp, trayDown, velocity, translation, initialCenter |
| Gesture States | Began, Changed, Ended |
| Tools | Xcode, Storyboard |
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What You'll Learn
- Declare a constant property with the name 'pannableView' of type UIView
- Add pannableView to the view hierarchy and centre it in its superview
- Define instance variables to store the tray's position when it's up and down
- Define a variable to hold the initial position of the view
- Create a conditional statement to check the current gesture state during the pan
Declare a constant property with the name 'pannableView' of type UIView
When working with pan gesture recognizers in Swift, you can declare a constant property with the name `pannableView` of type `UIView`. This allows you to create and configure a `UIView` instance that can be used for panning gestures.
Here's an example code snippet demonstrating how to declare the `pannableView` constant property:
Swift
Import UIKit
Class ViewController: UIViewController {
// Declare a private constant property named pannableView of type UIView
Private let pannableView: UIView = {
Let view = UIView()
View.backgroundColor = .blue
View.translatesAutoresizingMaskIntoConstraints = false
View.widthAnchor.constraint(equalToConstant: 200).isActive = true
View.heightAnchor.constraint(equalToConstant: 200).isActive = true
Return view
}()
}
In the code above, we use a self-executing closure to initialize and configure the `pannableView` instance. The view has a blue background color and is set to a size of 200 points in width and height. We also set `translatesAutoresizingMaskIntoConstraints` to `false` to disable auto-layout for the view.
Once you have declared the `pannableView` property, you can add it to the view hierarchy and center it within its superview in the `viewDidLoad()` method:
Swift
Override func viewDidLoad() {
Super.viewDidLoad()
View.addSubview(pannableView)
PannableView.center = view.center
}
By declaring the `pannableView` constant property, you can now utilize it throughout your code to handle panning gestures and update its position accordingly.
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Add pannableView to the view hierarchy and centre it in its superview
To add pannableView to the view hierarchy and centre it in its superview, you need to perform the following steps:
Firstly, open ViewController.swift and declare a private, constant property with the name pannableView of type UIView. You can use a self-executing closure to create and configure the UIView instance. Set the view's background colour, width and height, and set translatesAutoresizingMaskIntoConstraints to false if you are not using Auto Layout to size and position the view.
Next, in the view controller's viewDidLoad() method, add pannableView to the view hierarchy and centre it in its superview, which is the view controller's view. You can do this by setting its centre property.
In the didPan(_:) method, the view controller moves pannableView by updating its centre property. The view controller asks the pan gesture recognizer for the location of the user's finger in the superview of pannableView. This is done by invoking location(in:) on the pan gesture recognizer and passing in the view controller's view. The location(in:) method returns a CGPoint object, which is then assigned to the centre property of pannableView.
Note that setting the centre of pannableView to the location of the user's finger can cause the view to snap to a location below the user's finger. This occurs because the user will not touch the view exactly at its centre. To address this, you can declare a private variable property, initialCentre, of type CGPoint, and set its value to zero. Update the value of initialCentre when the pan gesture recognizer detects a pan gesture.
Additionally, ensure that you have not changed the centre/location of your parent view, as this can impact the centring of the pannableView.
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Define instance variables to store the tray's position when it's up and down
When working with pan gesture recognizers in Swift, it is important to define instance variables to store the tray's position when it is up and down. This allows for a seamless user experience when interacting with the tray view. Here is an explanation of how to achieve this:
To begin, you need to define the instance variables that will store the tray's position. In your ViewController Swift file, above the viewDidLoad method, add the following instance variables:
Swift
Var trayDownOffset: CGFloat!
Var trayUp: CGPoint!
Var trayDown: CGPoint!
These variables will store the position of the tray when it is in the up and down states. `trayDownOffset` represents the amount the tray moves down from its original position. `trayUp` and `trayDown` will store the CGPoint values for the center position of the tray when it is in the up and down states, respectively.
Now, within the `viewDidLoad` method, you need to assign values to these variables. Here is an example of how you can do this:
Swift
Override func viewDidLoad() {
Super.viewDidLoad()
// Set the initial position of the tray
TrayDownOffset = 160 // Adjust this value as needed
TrayUp = trayView.center
TrayDown = CGPoint(x: trayView.center.x, y: trayView.center.y + trayDownOffset)
}
In the code above, `trayDownOffset` is set to 160, but you can adjust this value based on the specific size of your tray. The `trayUp` variable is assigned the initial center position of the `trayView`. The `trayDown` variable calculates the center position for the down state by adding the `trayDownOffset` value to the y-coordinate of the `trayView.center`.
By defining these instance variables and assigning them appropriate values, you can now easily access and use them throughout your code to manage the tray's position when it is in the up and down states. This ensures that the tray view responds smoothly to the user's pan gestures.
Additionally, it is worth noting that while Swift's extensions allow you to add methods and computed properties, they do not support the addition of stored properties. Therefore, when working with instance variables, ensure that they are defined within the main class declaration.
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Define a variable to hold the initial position of the view
When creating a draggable view in Swift, it is important to define a variable to hold the initial position of the view. This allows the program to keep track of the view's position as it is being dragged by the user.
To do this, you can declare a private variable property, such as "initialCenter", of type CGPoint. This variable will store the initial position of the view. You can set the initialCenter property to the center of the view, which can be accessed through the pan gesture recognizer.
Swift
Import UIKit
Class ViewController: UIViewController {
// Define a variable to hold the initial position of the view
Private var initialCenter: CGPoint = .zero
// Other properties and methods...
@objc private func didPan(_ sender: UIPanGestureRecognizer) {
// Update the view's position based on the pan gesture
PannableView.center = sender.location(in: view)
// Reset the view's position to the initial position when the gesture ends
If sender.state == .ended {
PannableView.center = initialCenter
}
}
}
In this example, the `initialCenter` variable is used to store the initial position of the view. When the user begins dragging the view, the `didPan(_:)` method is called, and the view's position is updated based on the user's finger location. If the user releases the view, indicated by the `sender.state == .ended`, the view's position is reset to the initial position stored in the `initialCenter` variable.
By defining a variable to hold the initial position of the view, you can easily reset the view's position or perform other actions based on the view's movement relative to its initial position. This provides a more flexible and interactive user experience.
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Create a conditional statement to check the current gesture state during the pan
To create a conditional statement to check the current gesture state during the pan in Swift, you can use the following code snippet:
Swift
If sender.state == .began {
// Code for when the gesture begins
} else if sender.state == .changed {
// Code for when the user is panning
} else if sender.state == .ended {
// Code for when the user stops panning
}
In this code snippet, we are checking the `sender.state` property of the gesture recognizer to determine the current gesture state. The possible values for `sender.state` are:
- `.began`: This indicates that the gesture has just started. You can use this case to initialize any necessary variables or perform setup tasks.
- `.changed`: This indicates that the user is actively panning or dragging their finger across the screen. This is where you would typically update the position of the panned view.
- `.ended`: This indicates that the user has lifted their finger off the screen, ending the gesture. You can use this case to perform any necessary cleanup tasks or animations after the gesture has ended.
For example, let's say you want to move a tray view up and down based on the user's pan gesture. Here's how you can use the conditional statement within your `didPanTray` method:
Swift
@IBAction func didPanTray(sender: UIPanGestureRecognizer) {
Let translation = sender.translation(in: view)
Print("translation \(translation)")
If sender.state == .began {
// Store the initial center position of the tray
TrayOriginalCenter = trayView.center
} else if sender.state == .changed {
// Update the tray's center position based on the translation
TrayView.center = CGPoint(x: trayOriginalCenter.x, y: trayOriginalCenter.y + translation.y)
} else if sender.state == .ended {
// Animate the tray to its final position based on the velocity
Let velocity = sender.velocity(in: view)
If velocity.y > 0 {
// Animate the tray moving down
UIView.animate(withDuration: 0.3) {
TrayView.center = trayDownCenter
}
} else {
// Animate the tray moving up
UIView.animate(withDuration: 0.3) {
TrayView.center = trayUpCenter
}
}
}
In this example, when the gesture begins, we store the initial center position of the `trayView`. As the user pans, we update the `trayView`\'s center position based on the translation value, ignoring the x-component to allow for only vertical movement. When the gesture ends, we check the velocity's y-component to determine whether the tray should animate to the up or down position.
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Frequently asked questions
You can use the UIPanGestureRecognizer class to detect and interpret pan gestures.
It is a distinct gesture recognizer that interprets panning gestures. It is a subclass of the UIGestureRecognizer class.
You can add a View in Storyboard or add a view programmatically. If you are adding a view in Storyboard, create an outlet for your view. Attach a UIPanGestureRecognizer to your Tray view and create an Action/Method.
You can define this within your ViewController Swift file, right below where you define your variables. The code to make the Tray draggable will go inside our didPanTray method.
You can define Instance variables to store the tray's position when it's "up" and "down" as well as the offset amount that the tray will move down when in its down position.
