Storyboards, Visual States and SketchFlow Animations
What are these things – are they different ways of doing the same task? When would I use one in preference to another? Do they all work in all project types?
This post will try to answer those questions by describing the animation and control customization tools that are available to you in Expression Blend 3 + SketchFlow, and discussing what jobs each tool is meant to do. I’ll be classifying project types along two independent axes: WPF or Silverlight, and Blend or SketchFlow.
In the first release of Blend, if you wanted to change the value of a property over time, then the Storyboard was your one option. Using a Storyboard is also known as ‘keyframing’. You create a new Storyboard (or create a BeginStoryboardAction and let that workflow create a Storyboard for you), move the playhead to various times and then use the artboard or the property inspector to change values. Each time you change a value, a keyframe is added to Blend’s Timeline meaning that, at that time, the property has that value. During the interval between keyframes, the property value smoothly takes on intermediate values in a process known as interpolation. By default, the interpolation between two values is linear, meaning the value changes steadily as time passes to form a straight gradient on a graph. And you can control interpolation between keyframes by describing an easing curve. Whether you were changing the Y coordinate of a bouncing ball, or changing the color of a rectangle in a Button template in response to a mouse click, in the first release of Blend you would have used a Storyboard to do it. Something you would have known about, too, is a handoff animation. This is an animation that has no keyframe at time 0 so that, when the Storyboard begins, property values are snapshotted and gradually changed to the value of the earliest keyframe. Handoff animations are important when defining the transition into a control state because you need the animation to be continuous even when it is interrupting an already in-flight transition (say you move the mouse over and then away from a control before the transition into the mouseover state completes).
Storyboards are available in all project types. They’re just as useful today as ever, and they are worth learning how to use, because at some point you’ll probably need to use them. They give you the most control over animation, but control can come at the cost of some effort.
For the task of customizing the look and transitions of a control’s visual states, there’s an alternative and arguably simpler mental model than using a Storyboard to define the transition into a state. The simpler mental model is that you draw the control in each of its states then, if it’s important to you, specify how long any of the transitions take. I say ‘draw’ because that’s morally what you’re doing; strictly speaking you select a state in Blend’s States panel, set properties, select another state, and so on, but you’re drafting a static image of how the control looks in each state. You needn’t be concerned with animation, although it’s interesting to note that the runtime that supports this mental model (that runtime is known as the Visual State Manager, or VSM for short) does generate a handoff animation for each state transition. For practical purposes, drawing states and setting transition durations like this gets the job done much of the time without needing to see Blend’s Timeline at all. But, if you need a steady state animation (say you want the blue focus rectangle to pulse all the time a control is focused), then yes you’ll need to open the Timeline and drop a couple of keyframes and set the animation to repeat indefinitely. Or if you want a shiny reflection to flash across a glass button during the transition from Normal to MouseOver, then again you’ll need to know what a Storyboard is.
Of course you can leverage the Visual State Manager in your own UserControls too. This is because states can apply at the level of the individual control (e.g. in the MouseOver state a Brush is a different color) as well as at the level of a page or scene (e.g. in the ShoppingCartOpen state an otherwise hidden panel is visible). So, you can add states to one of your UserControls that represents a page or scene, set different properties in different states, then use GoToStateActions to drive state changes in response to events.
The Visual State Manager is fully integrated into Blend Silverlight projects and SketchFlow Silverlight projects. You can also use VSM in WPF projects although, while the UserControl experience is the same as for Siverlight, not all WPF custom controls support VSM. I’ve written previously about the States panel and WPF controls.
The last tool I’ll mention is the SketchFlow Animation, and this tool is available in SketchFlow projects only, both WPF and Silverlight. A SketchFlow Animation is logically a ‘storyboard’ in the true sense of the word: a sequence of frames that tells a story. When you’re building a prototype, you don’t want to implement a feature fully in order to demonstrate it. Playing back a scripted example of the interaction you have in mind gets the job done at the prototyping stage. So if you want to show off how you imagine your application will reorganize and animate in response to the user dragging a product into the shopping cart, you could create a new SketchFlow animation and then draw a few frames showing how the product gets dragged between containers and how the layout of those containers responds, and even specify the easing between frames.
For those who like to know how things work under the hood, a SketchFlow Animation is represented internally as a VSM state group. But you don’t need to be familiar with VSM to use a SketchFlow Animation. Nor do you need to be aware of what a Storyboard is, nor be able to use Blend’s Timeline. In a sense, each frame (or state) in a SketchFlow Animation is a keyframe, but at a macro level so that each keyframe defines the entire scene at a point in time rather than the micro keyframes in a Storyboard that define a single property’s value at a point in time.
Now that you have an idea of what these different pieces do, and when they’re available, you’ll be able to pick the most efficient tool for each animation job you want to do.