Fix emoji selection issue.

Author: Yuncong Zhang

Runtime/rendering/editable.cs

@@ -327,16 +327,34 @@ int _handleControl(bool rightArrow, bool leftArrow, bool ctrl, int newOffset) {
 
         int _handleHorizontalArrows(bool rightArrow, bool leftArrow, bool shift, int newOffset) {
             if (rightArrow && this._extentOffset < this.text.text.Length) {
-                newOffset += 1;
-                if (shift) {
-                    this._previousCursorLocation += 1;
+                if (newOffset < this.text.text.Length - 1 && char.IsHighSurrogate(this.text.text[newOffset])) {
+                    // handle emoji, which takes 2 bytes
+                    newOffset += 2;
+                    if (shift) {
+                        this._previousCursorLocation += 2;
+                    }
+                }
+                else {
+                    newOffset += 1;
+                    if (shift) {
+                        this._previousCursorLocation += 1;
+                    }
                 }
             }
 
             if (leftArrow && this._extentOffset > 0) {
-                newOffset -= 1;
-                if (shift) {
-                    this._previousCursorLocation -= 1;
+                if (newOffset > 1 && char.IsLowSurrogate(this.text.text[newOffset - 1])) {
+                    // handle emoji, which takes 2 bytes
+                    newOffset -= 2;
+                    if (shift) {
+                        this._previousCursorLocation -= 2;
+                    }
+                }
+                else {
+                    newOffset -= 1;
+                    if (shift) {
+                        this._previousCursorLocation -= 1;
+                    }
                 }
             }
 
@@ -637,12 +655,10 @@ public float textScaleFactor {
         }
 
         public TextSelection selection {
-            get { return this._selection; }
+            get {
+                return this._selection;
+            }
             set {
-                if (this._selection == value) {
-                    return;
-                }
-
                 this._selection = value;
                 this._selectionRects = null;
                 this.markNeedsPaint();

Runtime/service/text_input.cs

@@ -203,8 +203,23 @@ public class TextEditingValue : IEquatable<TextEditingValue> {
 
         public TextEditingValue(string text = "", TextSelection selection = null, TextRange composing = null) {
             this.text = text;
-            this.selection = selection ?? TextSelection.collapsed(-1);
             this.composing = composing ?? TextRange.empty;
+
+            if (selection != null) {
+                // handle emoji, which takes 2 bytes
+                // if selection cuts in the middle of the emoji, move it to the end
+                int start = selection.start, end = selection.end;
+                if (start < text.Length && char.IsLowSurrogate(text[start])) {
+                    start++;
+                }
+                if (end < text.Length && char.IsLowSurrogate(text[end])) {
+                    end++;
+                }
+                this.selection = selection.copyWith(start, end);
+            }
+            else {
+                this.selection = TextSelection.collapsed(-1);
+            }
         }
 
         public static TextEditingValue fromJson(JSONObject json) {