What is delta, globalPosition, localPosition and primaryDelta in Flutter? - flutter

Question

I am struggling to understand the properties delta, globalPosition, localPosition and primaryDelta of DragUpdateDetails. I read the documentation for DragUpdateDetails but it didn't really help much.
I found some question asked: What is primaryDelta and delta in DragUpdateDetails and What is the difference between globalposition and localposition in flutter?. First one has no answer whereas second one only has explanation for globalPosition and localPosition.
This is a sample code for dragging a container:
import 'package:flutter/material.dart';
void main() => runApp(MyApp());
class MyApp extends StatefulWidget {
#override
_MyAppState createState() => _MyAppState();
}
class _MyAppState extends State<MyApp> {
double _xPosition = 0;
double _yPosition = 0;
#override
Widget build(BuildContext context) {
return MaterialApp(
debugShowCheckedModeBanner: false,
home: Scaffold(
body: SafeArea(
child: Container(
child: Stack(
children: [
Positioned(
top: _yPosition,
left: _xPosition,
child: GestureDetector(
onPanUpdate: (DragUpdateDetails e) {
setState(() {
_xPosition += e.delta.dx;
_yPosition += e.delta.dy;
});
},
child: Container(
height: 200,
width: 200,
color: Colors.black,
),
),
)
],
),
),
),
),
);
}
}
In this example why are we using delta but not globalPosition or localPosition. I mean of course dragging will not work as expected but how to know when to use delta or globalPosition or localPosition?
Can someone provide me a quick explanation for these properties? It would really mean a lot. Thanks!

Answer 1

According to the Source code of DragUpdateDetailsclass:
delta → Offset
The amount the pointer has moved in the coordinate space of the event receiver since the previous update
Meaning, The distance covered by dragging since the last pointer contact. Delta gives dx for horizontal distance and dy for vertical distance.
primaryDelta → double
The amount the pointer has moved along the primary axis in the coordinate space of the event receiver since the previous update
primaryDelta gives the absolute distance in only one primary direction of dragging, meaning if the drag is primarily in horizontal axis(GestureDragUpdateCallback + Horizontal only) then this value represents the drag distance in the horizontal axis. If the drag in is vertical axis (GestureDragUpdateCallback + Vertical only) then this value represents the drag amount in the vertical axis.
Note: if the GestureDragUpdateCallback is for a two-dimensional drag (e.g., a pan), then this value is null.
globalPosition → Offset
The pointer's global position when it triggered this update.
The position of the pointer on the screen with reference to the whole screen area and origin point at the top-left corner of the screen. Global Position gives x for horizontal co-ordinate and y for vertical co-ordinate
localPosition → Offset
The local position in the coordinate system of the event receiver at which the pointer contacted the screen.
The position of the pointer just like the global position, except the referential frame being the widget/render object instead of the whole screen. Here, The widget is the one that has received the pointer contact.
Explanation with respect to the example code
According to the example code that is provided, I can safely say that when you are dealing with the positioning of any draggable widget you must have an initial position.
For the initial position, globalPosition or localPosition can be used. Which one to use is specific to the widget tree and use of the app.
Once the initial position is set, you can use delta or primaryDelta to find the new position for the draggable widget to move to when dragged by following formula:
newXPosition = initialXPosition + (dx or primaryDelta in horizontal axis)
newYPosition = initialYPosition + (dy or primaryDelta in vertical axis)
One thing to keep in mind while using delta and primaryDelta is that, if the widget can/should recognize the drag events in both axis, only delta will be provided, primaryDelat will be null.
Otherwise, if only one direction drag is expected then using only primaryDelta will work as expected, at this point delta will have only one value as a non-zero value and other as 0 based on which direction the drag is to be recognized.
Let me know if you have any other questions about this in the comments.