Finish up Animator docs first pass

docs/guide/animator.md

@@ -1,11 +1,11 @@
 # Animator
 
 Textual ships with an easy-to-use system which lets you add animation to your application.
-To get a feel for what animation looks like in Textual and try out different easing functions, run `textual easing` in your terminal.
+To get a feel for what animation looks like in Textual, run `textual easing` from the command line.
 
 !!! note
 
-    The easing preview requires the `dev` extras (using `pip install textual[dev]`).
+    The `textual easing` preview requires the `dev` extras to be installed (using `pip install textual[dev]`).
 
 ## Animating styles
 
@@ -23,9 +23,8 @@ The app below contains a single `Static` widget which is immediately animated to
 --8<-- "docs/examples/guide/animator/animation01.py"
 ```
 
-Internally, the animator deals with updating the value of the `opacity` attribute on the `styles` object.
-In a single line, we've achieved a fading animation:
-
+Internally, the animator repeatedly updates the value of the `opacity` attribute on the `styles` object.
+With a single line of code, we've achieved a fading animation:
 
 === "After 0s"
 
@@ -43,28 +42,56 @@ In a single line, we've achieved a fading animation:
     ```
 
 Remember, when the value of a property on the `styles` object gets updated, Textual automatically updates the display.
-This means there's no additional code required to trigger a display update.
+This means there's no additional code required to trigger a display update - the animation just works.
 
 In the example above we specified a `duration` of two seconds, but you can alternatively pass in a `speed` value.
 
-## Animating other attributes
+## The `Animatable` protocol
 
-You can animate non-style attributes on widgets too.
-This could be used to drive more complex animations involving styles, or to keep animations in sync with each other.
+You can animate `float` values and any type which implements the `Animatable` protocol.
+
+To implement the `Animatable` protocol, add a `def blend(self: T, destination: T, factor: float) -> T` method to the class.
+The `blend` method should return a new object which represents `self` blended with `destination` by a factor of `factor`.
+The animator will repeatedly call this method to retrieve the current value to display for the current.
+
+An example of an object which implements this protocol is [Color][textual.color.Color].
+It follows that you can use `animate` to animate from one `Color` to another.
+
+## Animating widget attributes
+
+You can animate non-`style` attributes on widgets too, assuming they implement `Animatable`.
 Again, the animation system will take care of updating the attribute on the widget as time progresses.
 
-If the attribute being animated is [reactive](./reactivity.md), Textual can handle the refreshing of the display each time the animator updates the value.
+If the attribute being animated is [reactive](./reactivity.md), Textual can refresh the display each time the animator updates the value.
 
-## Animating arbitrary values
+The example below shows a simple incrementing timer that counts from 0 to 100 over 100 seconds.
 
-Sometimes, you'll want to animate a value that isn't directly accessible as an attribute on a widget.
-For example, perhaps the value to be animated is nested inside some object structure, and you don't want to restructure your code to make it a top-level attribute.
+=== "animation04.py"
 
-In these cases, you can make use of an "unbound" animator.
-These are animators which aren't pre-emptively associated with an object.
-They let you pass in an object, _and_ the name of the attribute you wish to animate on it.
+    ```python
+    --8<-- "docs/examples/guide/animator/animation04.py"
+    ```
+
+=== "Output"
+
+    ```{.textual path="docs/examples/guide/animator/animation04.py"}
+    ```
+
+Since `value` is reactive, the display is automatically updated each time the animator modifies it.
+
+## Animating Python object attributes
+
+Sometimes you'll want to animate a value that exists inside a plain old Python object.
+
+In these cases, you can make use of the "unbound" animator.
+An unbound animator is an animator which isn't pre-emptively associated with (bound to) an object.
+
+Unbound animators let you pass the name of the attribute you wish to animate, _and_ the object that attribute exists on.
 This is unlike the animators discussed above, which are already _bound_ to the object they were retrieved from.
 
+You can retrieve the unbound animator from the `App` instance via `App.animator`, and call the `animate` method on it.
+This method is the same as the one described earlier, except the first argument is the object containing the attribute.
+
 ## Easing functions
 
 Easing functions control the "look and feel" of an animation.
@@ -72,6 +99,15 @@ The easing function determines the journey a value takes on its way to the targe
 Perhaps the value will be transformed linearly, moving towards the target at a constant rate.
 Or maybe it'll start off slow, then accelerate towards the final value as the animation progresses.
 
+Easing functions take a single input representing the time, and output a "factor".
+This factor is what gets passed to the `blend` method in the `Animatable` protocol.
+
+!!! warning
+
+    The factor output by the easing function will usually remain between 0 and 1.
+    However, some easing functions (such as `in_out_elastic`) will produce values slightly below 0 and slightly above 1.
+    Because of this, any implementation of `blend` should support values outwith the range 0 to 1.
+
 Textual supports the easing functions listed on this [very helpful page](https://easings.net/).
 In order to use them, you'll need to write them as `snake_case` and remove the `ease` at the start.
 To use `easeInOutSine`, for example, you'll write `in_out_sine`.