Retrieves the details of the key frame at index_ inside this.
The this must already have key frames set, and index_ must be smaller than the number of
key frames.
| this | |
| index_ |
the index of the key frame |
| key |
return location for the key, or |
| mode |
return location for the easing mode, or |
| value |
a Value initialized with the type of the values |