add arbitrary position support

Author: fbchen

example/arbitrary_position.dart

@@ -0,0 +1,67 @@
+import 'dart:math';
+
+import 'package:flutter/material.dart';
+import 'package:flutter_constraintlayout/src/constraint_layout.dart';
+
+import 'custom_app_bar.dart';
+
+class ArbitraryPositionExample extends StatefulWidget {
+  const ArbitraryPositionExample({Key? key}) : super(key: key);
+
+  @override
+  State createState() => ArbitraryPositionExampleState();
+}
+
+class ArbitraryPositionExampleState extends State<ArbitraryPositionExample> {
+  @override
+  Widget build(BuildContext context) {
+    return Scaffold(
+      appBar: const CustomAppBar(
+        title: 'Arbitrary Position',
+        codePath: 'example/arbitrary_position.dart',
+      ),
+      body: ConstraintLayout(
+        debugPrintLayoutTime: true,
+        children: [
+          Container(
+            color: Colors.black,
+            child: const Text(
+              'Arbitrary position gives you more freedom\nadjust the window size to see the effect',
+              style: TextStyle(
+                color: Colors.white,
+                fontSize: 16,
+              ),
+              textAlign: TextAlign.center,
+            ),
+          ).applyConstraint(
+            width: matchParent,
+            height: 50,
+            top: parent.top,
+          ),
+          Container(
+            color: Colors.orange,
+          ).applyConstraint(
+            size: matchConstraint,
+            anchors: [sId(-1)],
+            calcSizeCallback: (parent, anchors) {
+              return const BoxConstraints.tightFor(
+                width: 100,
+                height: 100,
+              );
+            },
+            calcOffsetCallback: (parent, self, anchors) {
+              return Offset(
+                  min(
+                      max(
+                          300,
+                          anchors[0].getRight() -
+                              anchors[0].getMeasuredWidth() / 2),
+                      700),
+                  max(400, parent.size.height - self.getMeasuredHeight()));
+            },
+          )
+        ],
+      ),
+    );
+  }
+}

example/home.dart

@@ -1,6 +1,7 @@
 import 'package:flutter/material.dart';
 import 'package:flutter_constraintlayout/flutter_constraintlayout.dart';
 
+import 'arbitrary_position.dart';
 import 'badge.dart';
 import 'barrier.dart';
 import 'charts.dart';
@@ -40,6 +41,7 @@ class ExampleHome extends StatelessWidget {
     'Staggered Grid': const StaggeredGridExample(),
     'Circle Position': const CirclePositionExample(),
     'Pinned Position': const PinnedPositionExample(),
+    'Arbitrary Position': const ArbitraryPositionExample(),
     'Self wrapContent': const SelfWrapContentExample(),
     'Margin': const MarginExample(),
     'Charts': const ChartsExample(),