Swift Table View Animations Tutorial: Drop-In Cards

Swift Table View Animations Tutorial: Drop-In Cards
Rotate table cell

Add some flair to your tables.

Update note: This tutorial was updated for iOS 8 and Swift by Ray Fix. Original post by Tutorial Team member Brian Broom and Code Team member Orta Therox.

The standard UITableView is a powerful and flexible way to present data in your apps; chances are that most apps you write will use table views in some form. However, one downside is that without some level of customization, your apps will look bland and blend in with the thousands of apps just like it.

To prevent boring table views, you can add some subtle animation to liven up the actions of your app. You may have seen this in the Google+ app where the cards fly in through the side with a cool animation. If you haven’t seen it yet, download it here (it’s free)! You might also want to check out the design guidelines that Google released at the 2014 I/O conference. It contains many tips and examples on how to use animation well.

In this table view animations tutorial, you’ll be using Swift to enhance an existing app to rotate the cells of a table as you scroll. Along the way, you’ll learn about how transforms are achieved in UIKit, and how to use them in a subtle manner so as not to overwhelm your user with too much happening on-screen at once. You will also get some advice on how to organize your code to keep responsibilities clear and your view controllers slim.

Before beginning, you should know how to work with UITableView and the basics of Swift. If you need an introduction to these topics, you might want to start with the Swift Tutorial series that will teach you the basics of Swift in a table view app.

At publication time, our understanding is we cannot post screenshots of iOS 8 while it’s in beta. Any screenshots shown are from iOS 7, which will be close to what you see in iOS 8.

Getting Started

Download the starter project and open it up in Xcode 6. You’ll find a simple storyboard project with a UITableViewController subclass (MainViewController) and a custom UITableViewCell (CardTableViewCell) for displaying team members. You will also find a model class called Member that encapsulates all of the information about a team member and knows how to fetch that information from a JSON file stored in the local bundle.

Build and run the project in the simulator; you’ll see the following:

Starter project

A perfectly good design, ready to be spiced up.

The app is off to a good start, but it could use a little more flair. That’s your job; you’ll use some Core Animation tricks to animate your cell.

Define the Simplest Possible Animation

To get the basic structure of the app going you’ll start by creating a super simple fade-in animation helper class. Go to File\New\File… and select type iOS\Source\Swift File for an empty Swift file. Click Next, name the file TipInCellAnimator.swift and then click Create.

Replace the file contents with the following:

import UIKit
 
class TipInCellAnimator {
  // placeholder for things to come -- only fades in for now
  class func animate(cell:UITableViewCell) {
    if let view = cell.contentView {
      view.layer.opacity = 0.1
      UIView.animateWithDuration(1.4) {
        view.layer.opacity = 1
      }
    }
  }
}

This simple class provides a method that takes a cell, gets its contentView and sets the layer’s initial opacity to 0.1. Then, over the space of 1.4 seconds, the code in the closure expression animates the layer’s opacity back to 1.0.

Note: A Swift closure is just a block of code that can also capture external variables. For example, { } is a simple closure. Functions declared with the func keyword are just examples of named closures. It is even perfectly legal to declare functions inside of other functions in Swift.

If you pass a closure as the last argument of a function, you can use the special trailing closure syntax and move the closure outside of the function call. You can see this in the UIView.animateWithDuration() call.

You can read more about closures in the Swift Programming Language book or read about the rich history of closures on Wikipedia.

Now that you have the animation code ready, you need the table view controller to trigger this new animation as the cells appear.

Trigger the Animation

To trigger your animation, open MainViewController.swift and add the following method to the class:

override func tableView(tableView: UITableView!, willDisplayCell cell: UITableViewCell!,
    forRowAtIndexPath indexPath: NSIndexPath!) {
  TipInCellAnimator.animate(cell)
}

This method is declared in the UITableViewDelegate protocol and gets called just before the cell is shown on the screen. It calls the TipInCellAnimator‘s class method animate() on each cell as it appears to trigger the animation.

Build and run your app. Scroll through the cards and watch the cells slowly fade in:

SwiftDrop-InFadeAnimation

Getting Fancy with Rotation

Now it’s time to make the app a little more fancy with some animated rotation. This works in the same way as the fade-in animation, except you are specifying both start and end transformations.

Open TipInCellAnimator.swift, and replace its contents with:

import UIKit
import QuartzCore // 1
 
class TipInCellAnimator {
  class func animate(cell:UITableViewCell) {
    if let view = cell.contentView {
      let rotationDegrees: CGFloat = -15.0
      let rotationRadians: CGFloat = rotationDegrees * (CGFloat(M_PI)/180.0)
      let offset = CGPointMake(-20, -20)
      var startTransform = CATransform3DIdentity // 2
      startTransform = CATransform3DRotate(CATransform3DIdentity,
        rotationRadians, 0.0, 0.0, 1.0) // 3
      startTransform = CATransform3DTranslate(startTransform, offset.x, offset.y, 0.0) // 4
 
      // 5
      view.layer.transform = startTransform
      view.layer.opacity = 0.8
 
      // 6
      UIView.animateWithDuration(0.4) {
        view.layer.transform = CATransform3DIdentity
        view.layer.opacity = 1
      }
    }
  }
}

This time the animation is quicker (0.4 seconds), the fading in is more subtle, and you get a nice rotation effect. The key to the above animation is defining the startTransform matrix and animating the cell back to its natural identity transformation. Let’s dig into that and see how it was done:

  1. This class now requires QuartzCore to be imported because it uses core animation transforms.
  2. Start with an identity transform, which is a fancy math term for “do nothing.” This is a view’s default transform.
  3. Call CATransform3DRotate to apply a rotation of -15 degrees (converted to radians), where the negative value indicates a counter-clockwise rotation. This rotation is around the axis 0.0, 0.0, 1.0; this represents the z-axis, where x=0, y=0, and z=1.
  4. Applying just the rotation to the card isn’t enough, as this simply rotates the card about its center. To make it look like it’s tipped over on a corner, add a translation or shift where the negative values indicate a shift up and to the left.
  5. Set this rotated and translated transform as the view’s initial transform.
  6. Animate the view back to it’s original values.

Notice how you were able to build up the final transformation one step at a time as shown in the image below:

Building up transformations

Building up transformations to produce the desired effect.

Note: An arbitrary chain of transformations can ultimately be represented by one matrix. If you studied matrix math in school, you may recognize this as multiplication of matrices. Each step multiplies a new transformation until you end up with the final matrix.

You’ll also notice that you’re transforming a child view of the cell, and not the cell itself. Rotating the actual cell would cause part of it to cover the cell above and below it, which would cause some odd visual effects, such as flickering and clipping of the cell. A cell’s contentView contains all of its constituent parts.

Not all properties support animation; the Core Animation Programming Guide provides a list of animatable properties for your reference.

Build and run your application. Watch how the cells tilt into view as they appear!

Rotation of card cell.

A Swift Refactor

The original Objective-C version of this tutorial made sure to compute the starting transform once. In the above version of the code it is computed each time animate() gets called. How might you do this in Swift?

One way is to use an immutable stored property that is computed by calling a closure. Replace the contents of TipInCellAnimator.swift with:

import UIKit
import QuartzCore
 
let TipInCellAnimatorStartTransform:CATransform3D = {
  let rotationDegrees: CGFloat = -15.0
  let rotationRadians: CGFloat = rotationDegrees * (CGFloat(M_PI)/180.0)
  let offset = CGPointMake(-20, -20)
  var startTransform = CATransform3DIdentity
  startTransform = CATransform3DRotate(CATransform3DIdentity,
    rotationRadians, 0.0, 0.0, 1.0)
  startTransform = CATransform3DTranslate(startTransform, offset.x, offset.y, 0.0)
 
  return startTransform
}()
 
class TipInCellAnimator {
  class func animate(cell:UITableViewCell) {
    if let view = cell.contentView {
 
      view.layer.transform = TipInCellAnimatorStartTransform
      view.layer.opacity = 0.8
 
      UIView.animateWithDuration(0.4) {
        view.layer.transform = CATransform3DIdentity
        view.layer.opacity = 1
      }
    }
  }
}

Notice the code that generates the startTransform is now in its own stored property TipInCellAnimatorStartTransform. Rather than defining this property with a getter to create the transform each time its called, you set its default property by assigning it a closure and following the assignment with the empty pair of parenthesis. The parenthesis force the closure to be called immediately and assign the return value to the property. This initialization idiom is discussed in Apple’s Swift book in the chapter on initialization. See “Setting a Default Property Value with a Closure or Function” for more information.

Note: It would have been nice to make TipInCellAnimatorStartTransform a class property of TipInCellAnimator but as of this writing, class properties are not yet implemented in Swift.

Adding Some Limits to Your Transformation

Although the animation effect is neat, you’ll want to use it sparingly. If you’ve ever suffered through a presentation that overused sound effects or animation effects, then you know what effect overload feels like!

In your project, you only want the animation to run the first time the cell appears — as it scrolls in from the bottom. When you scroll back toward the top, the cells should scroll without animating.

You need a way to keep track of which cards have already been displayed so they won’t be animated again. To do this, you’ll use a Swift Dictionary collection that provides fast key lookup.

Note: A set is an unordered collection of unique entries with no duplicates, while an array is an ordered collection that does allow duplicates. The Swift Standard Library does not currently include a Set type, but it can easily be simulated with a Dictionary of Bools. The abstraction penalty for using a Dictionary in this way is very small, which is probably why the Swift team left it out of the initial release.

The general disadvantage of a set or dictionary keys is that they don’t guarantee an order, but the ordering of your cells is already handled by the data source, so it isn’t an issue in this case.

Open MainViewController.swift and add the following property to the class:

var didAnimateCell:[NSIndexPath: Bool] = [:]

This declares an empty dictionary that takes NSIndexPaths as keys and Bools as values. Next, replace the implementation of tableView(tableView:, willDisplayCell:, forRowAtIndexPath:) with the following:

override func tableView(tableView: UITableView!, willDisplayCell cell: UITableViewCell!, 
                        forRowAtIndexPath indexPath: NSIndexPath!) {        
    if didAnimateCell[indexPath] == nil || didAnimateCell[indexPath]! == false {
        didAnimateCell[indexPath] = true
        TipInCellAnimator.animate(cell)
    }
}

Instead of animating every cell each time it appears as you scroll up and down the table, you check to see if the cell’s index path is in the dictionary. If it isn’t, then this is the first time the cell has been displayed; therefore you run the animation and add the indexPath to your set. If it was already in the set, then you don’t need to do anything at all.

Build and run your project; scroll up and down the tableview and you’ll only see the cards animate the first time they appear on-screen.

Drop-In-UpDownScroll

Where To Go From Here?

In this tutorial you added animation to a standard view controller. The implementation details of the animation were kept out of the MainViewController class and instead put into a small, focused, animation helper class. Keeping class responsibilities focused, particularly for view controllers, is one of the main challenges of iOS development.

You can download the final project for this tutorial here.

Now that you’ve covered the basics of adding animation to cells, try changing the values of your transform to see what other effects you can achieve. Some suggestions are:

  1. Faster or slower animation
  2. Larger rotation angle
  3. Different offsets; if you change the rotation angle, you will likely need to change the offset to make the animation look right. What does the animation look like if you drop the offset entirely and use (0, 0) for the parameters?
  4. Go nuts and create some whacked-out transforms.
  5. Advanced: Can you get the card to rotate along the horizontal or vertical axis? Can you make it look like it flips over completely?
  6. Advanced: Add an else clause to tableView(tableView:, willDisplayCell:, forRowAtIndexPath:) and perform a different animation when cells are displayed a second time.
Crazy Rotations

Crazy rotations that you can (but maybe shouldn’t) apply to your cells. See if you can duplicate these!

A great exercise is to try and identify animations in your favorite apps. Even with the simple animation from this tutorial, there are countless variations you can produce on this basic theme. Animations can be a great addition to user actions, such as flipping a cell around when selected, or fading in or out when presenting or deleting a cell.)

If you have any questions about this table view animations tutorial or this technique in general, please join the forum discussion below!

Swift Table View Animations Tutorial: Drop-In Cards is a post from: Ray Wenderlich

The post Swift Table View Animations Tutorial: Drop-In Cards appeared first on Ray Wenderlich.

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