Preserve selection when folding and unfolding blocks (#95)

* Preserve selection when folding and unfolding blocks (#81)

* Minor fixes (#81)

* Fix after review (#81)

lib/src/code_field/code_controller.dart

@@ -505,18 +505,26 @@ class CodeController extends TextEditingController {
   }
 
   void foldAt(int line) {
+    final oldCode = _code;
     _code = _code.foldedAt(line);
+
     super.value = TextEditingValue(
       text: _code.visibleText,
-      // TODO(alexeyinkin): Preserve selection, https://github.com/akvelon/flutter-code-editor/issues/81
+      selection: _code.hiddenRanges.cutSelection(
+        oldCode.hiddenRanges.recoverSelection(value.selection),
+      ),
     );
   }
 
   void unfoldAt(int line) {
+    final oldCode = _code;
     _code = _code.unfoldedAt(line);
+
     super.value = TextEditingValue(
       text: _code.visibleText,
-      // TODO(alexeyinkin): Preserve selection, https://github.com/akvelon/flutter-code-editor/issues/81
+      selection: _code.hiddenRanges.cutSelection(
+        oldCode.hiddenRanges.recoverSelection(value.selection),
+      ),
     );
   }
 

lib/src/hidden_ranges/hidden_ranges.dart

@@ -160,6 +160,67 @@ class HiddenRanges {
     );
   }
 
+  /// Translates the [position] in the full text to the position in
+  /// the visible text.
+  ///
+  /// If [position] is hidden, returns the last visible position before it.
+  int cutPosition(int position) {
+    if (ranges.isEmpty || position < ranges.first.start) {
+      return position;
+    }
+
+    if (position >= ranges.last.end) {
+      return position - hiddenCharactersBeforeRanges.last;
+    }
+
+    int lowerRange = 0;
+    int upperRange = ranges.length - 1;
+
+    // Linear interpolation search.
+    while (upperRange > lowerRange) {
+      // Full characters at the lower range start and at the upper range end.
+      final lowerChar = ranges[lowerRange].start;
+      final upperChar = ranges[upperRange].end;
+
+      final rangeIndex = lowerRange +
+          ((position - lowerChar) /
+                  (upperChar - lowerChar) *
+                  (upperRange - lowerRange))
+              .floor();
+
+      if (rangeIndex < lowerRange) {
+        return position - hiddenCharactersBeforeRanges[lowerRange];
+      }
+      if (rangeIndex > upperRange) {
+        return position - hiddenCharactersBeforeRanges[upperRange + 1];
+      }
+
+      final range = ranges[rangeIndex];
+
+      switch (range.compareToPosition(position)) {
+        case 0:
+          return range.start - hiddenCharactersBeforeRanges[rangeIndex];
+        case -1:
+          lowerRange = rangeIndex + 1;
+          break;
+        case 1:
+          upperRange = rangeIndex - 1;
+          break;
+      }
+    }
+
+    // upper == lower - 1, or lower, or lower + 1
+    final range = ranges[upperRange];
+    switch (range.compareToPosition(position)) {
+      case -1:
+        return position - hiddenCharactersBeforeRanges[upperRange + 1];
+      case 1:
+        return position - hiddenCharactersBeforeRanges[upperRange];
+    }
+
+    return range.start - hiddenCharactersBeforeRanges[upperRange];
+  }
+
   /// Translates the [position] in the visible text to the position in
   /// the full text.
   ///
@@ -178,8 +239,9 @@ class HiddenRanges {
     int lowerRange = 0;
     int upperRange = ranges.length - 1;
 
+    // Linear interpolation search.
     while (upperRange > lowerRange) {
-      // Visible characters if the range collapse positions.
+      // Visible characters at the ranges' collapse positions.
       final lowerChar =
           ranges[lowerRange].end - hiddenCharactersBeforeRanges[lowerRange + 1];
       final upperChar =
@@ -245,6 +307,46 @@ class HiddenRanges {
     }
   }
 
+  TextSelection cutSelection(TextSelection selection) {
+    if (selection.isCollapsed) {
+      final position = cutPosition(selection.start);
+      return selection.copyWith(
+        baseOffset: position,
+        extentOffset: position,
+      );
+    }
+
+    return selection.copyWith(
+      baseOffset: cutPosition(selection.baseOffset),
+      extentOffset: cutPosition(selection.extentOffset),
+    );
+  }
+
+  TextSelection recoverSelection(TextSelection selection) {
+    if (selection.isCollapsed) {
+      final position = recoverPosition(
+        selection.start,
+        placeHiddenRanges: TextAffinity.downstream,
+      );
+
+      return selection.copyWith(
+        baseOffset: position,
+        extentOffset: position,
+      );
+    }
+
+    return selection.copyWith(
+      baseOffset: recoverPosition(
+        selection.baseOffset,
+        placeHiddenRanges: TextAffinity.downstream,
+      ),
+      extentOffset: recoverPosition(
+        selection.extentOffset,
+        placeHiddenRanges: TextAffinity.downstream,
+      ),
+    );
+  }
+
   @override
   int get hashCode => Object.hash(
         Object.hashAll(ranges),

test/src/code_field/code_controller_folding_test.dart

@@ -152,8 +152,7 @@ int c;
           controller.value,
           const TextEditingValue(
             text: _codeFolded1,
-            // TODO(alexeyinkin): Selection.
-            //selection: TextSelection(baseOffset: 1, extentOffset: 2),
+            selection: TextSelection(baseOffset: 1, extentOffset: 2),
           ),
         );
 
@@ -163,8 +162,7 @@ int c;
           controller.value,
           TextEditingValue(
             text: textBefore,
-            // TODO(alexeyinkin): Selection.
-            //selection: TextSelection(baseOffset: 1, extentOffset: 2),
+            selection: const TextSelection(baseOffset: 1, extentOffset: 2),
           ),
         );
       });
@@ -212,8 +210,7 @@ public class MyClass {
   }
 }
 ''',
-            // TODO(alexeyinkin): Selection.
-            //selection: TextSelection(baseOffset: 1, extentOffset: 2),
+            selection: TextSelection(baseOffset: 0, extentOffset: 6),
           ),
         );
       });
@@ -253,8 +250,7 @@ public class MyClass {
   }
 }
 ''',
-            // TODO(alexeyinkin): Selection.
-            //selection: TextSelection(baseOffset: 1, extentOffset: 2),
+            selection: TextSelection(baseOffset: 0, extentOffset: 6),
           ),
         );
       });
@@ -302,8 +298,7 @@ int n;
   }
 }
 ''',
-            // TODO(alexeyinkin): Selection.
-            //selection: TextSelection(baseOffset: 1, extentOffset: 2),
+            selection: TextSelection.collapsed(offset: 91),
           ),
         );
       });

test/src/hidden_ranges/common.dart

@@ -0,0 +1,20 @@
+import 'package:flutter_code_editor/src/hidden_ranges/hidden_range.dart';
+import 'package:flutter_code_editor/src/hidden_ranges/hidden_ranges.dart';
+
+final hiddenRanges = HiddenRanges(
+  ranges: const [
+    //            How many characters are hidden by the beginning of this range:
+    HiddenRange(20, 23, firstLine: 0, lastLine: 0, wholeFirstLine: true), //   0
+    HiddenRange(31, 42, firstLine: 0, lastLine: 0, wholeFirstLine: true), //   3
+    HiddenRange(67, 91, firstLine: 0, lastLine: 0, wholeFirstLine: true), //  14
+    HiddenRange(100, 101, firstLine: 0, lastLine: 0, wholeFirstLine: true), //38
+    HiddenRange(102, 103, firstLine: 0, lastLine: 0, wholeFirstLine: true), //39
+    HiddenRange(104, 105, firstLine: 0, lastLine: 0, wholeFirstLine: true), //40
+    HiddenRange(106, 107, firstLine: 0, lastLine: 0, wholeFirstLine: true), //41
+    HiddenRange(108, 109, firstLine: 0, lastLine: 0, wholeFirstLine: true), //42
+    HiddenRange(110, 111, firstLine: 0, lastLine: 0, wholeFirstLine: true), //43
+    HiddenRange(113, 123, firstLine: 0, lastLine: 0, wholeFirstLine: true), //44
+    //                                                                        54
+  ],
+  textLength: 140,
+);