hyzp_ybqx-Commit162：优化my_flutter_drag_scale插件，实现双击图片放大后，点击位置位于容器中央

lib/my_flutter_drag_scale/lib/core/touchable_container.dart

@@ -1,3 +1,5 @@
+import 'dart:math';
+
 import 'package:flutter/material.dart';
 import './custom_gesture_detector.dart' as gd;
 
@@ -51,7 +53,7 @@ class _TouchableContainerState extends State<TouchableContainer>
   @override
   void initState() {
     super.initState();
-    _controller = new AnimationController(vsync: this)..addListener(_handleFlingAnimation);
+    _controller = AnimationController(vsync: this)..addListener(_handleFlingAnimation);
   }
 
   @override
@@ -65,8 +67,30 @@ class _TouchableContainerState extends State<TouchableContainer>
   //也就是最小值是原点0，0，点从最大值到0的区间，也就是这个图可以从最大值移动到原点
   Offset _clampOffset(Offset offset) {
     final Size size = context.size; //容器的大小
-    final Offset minOffset = new Offset(size.width, size.height) * (1.0 - _scale);
-    return new Offset(offset.dx.clamp(minOffset.dx, 0.0), offset.dy.clamp(minOffset.dy, 0.0));
+    // print("context.size = ${context.size}"); // context.size = Size(298.0, 298.0)
+    final Offset minOffset = Offset(size.width, size.height) * (1.0 - _scale);
+    return Offset(offset.dx.clamp(minOffset.dx, 0.0), offset.dy.clamp(minOffset.dy, 0.0));
+    // return Offset(minOffset.dx / 2, minOffset.dy / 2);
+  }
+
+  // 实现双击图片放大后，点击位置位于容器中央
+  Offset _clampOffset2(Offset position) {
+    final Size size = context.size; //容器的大小
+    // print("context.size = ${context.size}"); // context.size = Size(298.0, 298.0)
+    RenderBox renderBox = context.findRenderObject();
+    Offset offsetTopLeft = renderBox.localToGlobal(Offset.zero);
+    print("offsetTopLeft = $offsetTopLeft"); // offsetTopLeft = Offset(31.0, 111.0)
+
+    double x = position.dx - offsetTopLeft.dx;
+    x = max(x * _scale - size.width * _scale / 4, 0);
+    x = min(x, size.width * _scale / 2);
+
+    double y = position.dy - offsetTopLeft.dy;
+    y = max(y * _scale - size.height * _scale / 4, 0);
+    y = min(y, size.height * _scale / 2);
+    print("Offset(x, y) = ${Offset(-x, -y)}");
+
+    return Offset(-x, -y);
   }
 
   void _handleFlingAnimation() {
@@ -92,7 +116,7 @@ class _TouchableContainerState extends State<TouchableContainer>
       // Ensure that image location under the focal point stays in the same place despite scaling.
       _offset = _clampOffset(details.focalPoint - _normalizedOffset * _scale);
     });
-    ScaleChangedModel model = new ScaleChangedModel(scale: _scale, offset: _offset);
+    ScaleChangedModel model = ScaleChangedModel(scale: _scale, offset: _offset);
     if (widget.scaleChangedCallback != null) widget.scaleChangedCallback(model);
   }
 
@@ -102,7 +126,7 @@ class _TouchableContainerState extends State<TouchableContainer>
     final Offset direction = details.velocity.pixelsPerSecond / magnitude;
     final double distance = (Offset.zero & context.size).shortestSide;
     _flingAnimation =
-        new Tween<Offset>(begin: _offset, end: _clampOffset(_offset + direction * distance))
+        Tween<Offset>(begin: _offset, end: _clampOffset(_offset + direction * distance))
             .animate(_controller);
     _controller
       ..value = 0.0
@@ -111,33 +135,41 @@ class _TouchableContainerState extends State<TouchableContainer>
 
   void _onDoubleTap(gd.DoubleDetails details) {
     _normalizedOffset = (details.pointerEvent.position - _offset) / _scale;
+    print("_scale = ${_scale}, _offset = ${_offset}");
+    print("position = ${details.pointerEvent.position}, _normalizedOffset = ${_normalizedOffset}");
+    // _scale = 1.0, _offset = Offset(0.0, 0.0)
+    // position = Offset(178.0, 260.7), _normalizedOffset = Offset(178.0, 260.7)
     if (!widget.doubleTapStillScale && _scale != 1.0) {
       setState(() {
         _scale = 1.0;
         _offset = Offset.zero;
       });
-      ScaleChangedModel model = new ScaleChangedModel(scale: _scale, offset: _offset);
+      ScaleChangedModel model = ScaleChangedModel(scale: _scale, offset: _offset);
       if (widget.scaleChangedCallback != null) widget.scaleChangedCallback(model);
       return;
     }
     setState(() {
       if (widget.doubleTapStillScale) {
         _scale *= (1 + 0.5);
+        // Ensure that image location under the focal point stays in the same place despite scaling.
+        // _offset = doubleDownPositon;
+        _offset = _clampOffset(details.pointerEvent.position - _normalizedOffset * _scale);
       } else {
         _scale *= (2);
+        _offset = _clampOffset2(details.pointerEvent.position);
+        // _offset = Offset.zero; // 对齐左上角
+        print("_scale = ${_scale}, _offset = ${_offset}");
+        // _scale = 2.0, _offset = Offset(-175.3, -270.0)
       }
-      // Ensure that image location under the focal point stays in the same place despite scaling.
-      // _offset = doubleDownPositon;
-      _offset = _clampOffset(details.pointerEvent.position - _normalizedOffset * _scale);
     });
 
-    ScaleChangedModel model = new ScaleChangedModel(scale: _scale, offset: _offset);
+    ScaleChangedModel model = ScaleChangedModel(scale: _scale, offset: _offset);
     if (widget.scaleChangedCallback != null) widget.scaleChangedCallback(model);
   }
 
   @override
   Widget build(BuildContext context) {
-    return new gd.GestureDetector(
+    return gd.GestureDetector(
       // onPanDown: _onPanDown,
       onDoubleTap: _onDoubleTap,
       onScaleStart: _handleOnScaleStart,
@@ -151,8 +183,8 @@ class _TouchableContainerState extends State<TouchableContainer>
           minWidth: double.maxFinite,
           minHeight: double.infinity,
         ),
-        child: new Transform(
-            transform: new Matrix4.identity()
+        child: Transform(
+            transform: Matrix4.identity()
               ..translate(_offset.dx, _offset.dy)
               ..scale(_scale, _scale, 1.0),
             child: widget.child),