Pro Android with Kotlin

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CHAPTER 9: User Interface
 
While the dragging is in progress, the dragged target gets visually 
represented by a moving 
shadow
object. You are free to define it. It 
could be a star, a box, or any kind of graphics resembling the drag 
source. You only have to tell the system how to paint it. The positioning 
during the drag operation gets automatically handled by the Android 
OS. Because the drag source stays in place during the drag operation, 
the layout remains static all the time, and the dragging does not thwart 
layout operations by the layout manager.
 
When the drag shadow enters the area of a possible drop target, the 
listener gets informed about that, and you can react by, for example, 
changing the visual appearance of the drop candidate.
 
Because all drag sources and possible drop targets get informed about 
the various dragging states, you are free to visually express that by 
using different view appearances.
 
Once a drop happens, the listener gets informed and you can freely 
react on such drop events.
It is also possible to not use a dedicated drag-and-drop listener but instead overwrite some 
specific methods of the views taking part in drag-and-drop operations. The downside of 
that is you have to use the customized views inside the layout description, which makes 
it somewhat less readable. Also, from an architectural point of view, the views then know 
too much of what is happening 
to
them, which is an external concern and as such is better 
handled from outside objects. We therefore follow the listener approach and in the following 
sections describe what exactly to do for this methodology.
Defining Drag Data
If your drag-and-drop operation defines some kind of data transfer from one object 
represented by a view to another, you define a 
ClipData
object, for example, as follows:
val item = ClipData.Item(myView.tag.toString())
val dragData = ClipData(myView.tag.toString(),
arrayOf(MIMETYPE_TEXT_PLAIN), item)
The first argument to the constructor is a user-readable label for the clip datum, the second 
describes the type of the contents by assigning appropriate MIME types, and the 
item
argument 
represents the datum to be transported, which here is the string given by the 
tag
attribute of the 
view. Other item types are intents and URIs, to be specified in the constructor of 
ClipData.Item
.
Defining a Drag Shadow
The drag shadow is the visible element painted underneath your finger while the dragging is 
in progress. You define the shadow in an object as follows:
class DragShadow(val resources: Resources, val resId:Int,
view: ImageView) : View.DragShadowBuilder(view) {
val rect = Rect()

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