Summary of Chapter 22. Animation
You've seen that you can create your own animations using the Xamarin.Forms timer or
Task.Delay, but it is generally easier using the animation facilities provided by Xamarin.Forms. Three classes implement these animations:
ViewExtensions, the high-level approach
Animation, more versatile but harder
AnimationExtension, the most versatile, lowest-level approach
Generally, animations target properties that are backed by bindable properties. This is not a requirement but these are the only properties that dynamically react to changes.
There is no XAML interface for these animations, but you can integrate animations into XAML using techniques discussed in Chapter 23. Triggers and Behaviors.
Exploring basic animations
The basic animation functions are extension methods found in the
ViewExtensions class. These methods apply to any object that derives from
VisualElement. The simplest animations target the transforms properties discussed in
Chapter 21. Transforms.
RotateTo method changes the
Rotation property of the
Button from 0 to 360 over the course of one-quarter second (by default). If the
Button is tapped again, however, it does nothing because the
Rotation property is already 360 degrees.
Setting the animation duration
The second argument to
RotateTo is a duration in milliseconds. If set to a large value, tapping the
Button during an animation starts a new animation beginning at the current angle.
RelRotateTo method performs a relative rotation by adding a specified value to the existing value. This method allows the
Button to be tapped multiple times, and each time increases the
Rotation property by 360 degrees.
All the animation methods in
Task<bool> objects. This means that you can define a series of sequential animations using
bool completion return value is
false if the animation finished without interruption or
true if it was cancelled by the
CancelAnimation method, which cancels all animations initiated by the other method in
ViewExtensions that are set on the same element.
You can mix awaited and non-awaited animations to create composite animations. These are the animations in
ViewExtensions that target the
Scale transform properties:
TranslateTo potentially affects both the
Task.WhenAll and Task.WhenAny
It is also possible to manage simultaneous animations using
Task.WhenAll, which signals when multiple tasks have all concluded, and
Task.WhenAny, which signals when the first of several tasks has concluded.
Rotation and anchors
The CircleButton demonstrates this technique by revolving a
Button around the center of the page.
Generally animations are linear from a start value to an end value. Easing functions can cause animations to speed up or slow down over their course. The last optional argument to the animation methods is of type
Easing, a class that defines 11 static read-only fields of type
Linear, the default
In suffix indicates that the effect is at the beginning of the animation,
Out means at the end, and
InOut means that it's at the beginning and end of the animation.
The BounceButton sample demonstrates the use of easing functions.
Your own easing functions
You can also define you own easing functions by passing a
Func<double, double> to the
Easing also defines an implicit conversion from
Func<double, double> to
Easing. The argument to the easing function is always in the range of 0 to 1 as the animation proceeds linearly from beginning to end. The function usually returns a value in the range of 0 to 1, but could be briefly negative or greater than 1 (as is the case with the
SpringOut functions) or could break the rules if you know what you're doing.
The SwingButton sample also demonstrates a custom easing function, and also a technique of changing the
AnchorY properties within a sequence of rotation animations.
One popular type of animation occurs when a page first appears. Such an animation can be started in the
OnAppearing override of the page. For these animations, its best to set up the XAML for how you want the page to appear after the animation, and then initialize and animate the layout from code.
At the other extreme, "forever" animations run until the program is terminated. These are generally intended for demonstration purposes.
PalindromeAnimation displays a palindrome, and then sequentially rotates the individual letters by 180 degrees so they're all upside-down. Then the entire string is flipped 180 degrees to read the same as the original string.
The CopterAnimation sample rotates a simple
BoxView helicopter while revolving it around the center of the screen.
BoxView spokes around the center of the screen, and then rotates each spoke itself to create interesting patterns:
However, progressively increasing the
Rotation property of an element might not work in the long term, as the RotationBreakdown sample demonstrates.
Animating the bounds property
The only extension method in
ViewExtensions not yet demonstrated is
LayoutTo, which effectively animates the read-only
Bounds property by calling the
Layout method. This method is normally called by
Layout derivatives as will be discussed in Chapter 26. CustomLayouts.
LayoutTo method should be restricted to special purposes. The BouncingBox program uses it to compress and expand a
BoxView as it bounces off the sides of a page.
The XamagonXuzzle sample uses
LayoutTo to move tiles in an implementation of the classic 15-16 puzzle that displays a scrambled image rather than numbered tiles:
Your own awaitable animations
Deeper into animations
The Xamarin.Forms animation system can be a little confusing. In addition to the
Easing class, the animation system comprises the
You've already seen
ViewExtensions. It defines nine methods that return
CancelAnimations. Seven of the nine methods target transform properties. The other two are
FadeTo, which targets the
Opacity property, and
LayoutTo, which calls the
The animation object is then started with a call to the
AnimationExtensions class contains mostly extension methods. There are several versions of an
Animate method and the generic
Animate method is so versatile that it's really the only animation function you need.
Working with the Animation class
Beyond the high-level animation methods
The ConcurrentAnimations sample also demonstrates how to perform animations that go beyond the properties targetted by the
ViewExtensions methods. In one example, a series of periods get longer; in another example, a
BackgroundColor property is animated.
More of your own awaitable methods
The Xamarin.FormsBook.Toolkit library contains a
MoreViewExtensions class with
TranslateYTo extension methods that don't have this problem, as well as
CancelTranslateYTo methods for cancelling those animations.
The SpringSlidingEntrance demonstrates the
Implementing a Bezier animation
It is also possible to develop an animation that moves an element along the path of a Bezier spline. The Xamarin.FormsBook.Toolkit library contains a
BezierSpline structure that encapsulates a Bezier spline and a
BezierTangent enumeration to control orientation.
The BezierLoop sample demonstrates animating an element along a Beizer path.
Working with AnimationExtensions
One type of animation missing from the standard collection is a color animation. The problem is that there is no right way to interpolate between two
Color values. It's possible to interpolate the individual RGB values, but just as valid is interpolating the HSL values.
For this reason, the
MoreViewExtensions class in the Xamarin.FormsBook.Toolkit library contains two
Color animation methods:
HslColorAnimation. (There are also two cancellation methods:
The ColorAnimations sample demonstrates using these two types of color animations.
Structuring your animations
It's sometimes useful to express animations in XAML and use them in conjunction with MVVM. This is covered in the next chapter, Chapter 23. Triggers and Behaviors.