Support single-char emoji.

Author: Yuncong Zhang

Runtime/painting/text_span.cs

@@ -21,7 +21,7 @@ public class TextSpan : DiagnosticableTree, IEquatable<TextSpan> {
         public TextSpan(string text = "", TextStyle style = null, List<TextSpan> children = null,
             GestureRecognizer recognizer = null, HoverRecognizer hoverRecognizer = null) {
             this.text = text;
-            this.splitedText = !string.IsNullOrEmpty(text) ? EmojiUtils.splitBySurrogatePair(text) : null;
+            this.splitedText = !string.IsNullOrEmpty(text) ? EmojiUtils.splitByEmoji(text) : null;
             this.style = style;
             this.children = children;
             this.recognizer = recognizer;

Runtime/ui/painting/canvas_impl.cs

@@ -752,7 +752,7 @@ void _drawTextBlob(TextBlob textBlob, Offset offset, Paint paint) {
             var subText = textBlob.text.Substring(textBlob.textOffset, textBlob.textSize);
 
             Texture tex = null;
-            bool alpha = !char.IsHighSurrogate(subText[0]);
+            bool alpha = !char.IsHighSurrogate(subText[0]) && !EmojiUtils.isSingleCharEmoji(subText[0]);
             if (alpha) {
                 font.RequestCharactersInTextureSafe(subText, fontSizeToLoad, style.UnityFontStyle);
                 tex = font.material.mainTexture;

Runtime/ui/painting/txt/mesh_generator.cs

@@ -139,8 +139,9 @@ public MeshMesh resolveMesh() {
             var fontInfo = FontManager.instance.getOrCreate(style.fontFamily, style.fontWeight, style.fontStyle);
             var font = fontInfo.font;
 
-            if (char.IsHighSurrogate(text[this.textBlob.textOffset])) {
-                D.assert(this.textBlob.textSize % 2 == 0);
+            // Handling Emoji
+            char startingChar = text[this.textBlob.textOffset];
+            if (char.IsHighSurrogate(startingChar) || EmojiUtils.isSingleCharEmoji(startingChar)) {
                 var vert = new List<Vector3>();
                 var tri = new List<int>();
                 var uvCoord = new List<Vector2>();
@@ -151,29 +152,40 @@ public MeshMesh resolveMesh() {
                 var minY = minMaxRect.top;
                 var maxY = minMaxRect.bottom;
 
-                for (int i = 0; i < this.textBlob.textSize; i += 2) {
-                    char a = text[this.textBlob.textOffset+i], b = text[this.textBlob.textOffset+i+1];
-                    D.assert(char.IsLowSurrogate(b));
+                for (int i = 0; i < this.textBlob.textSize; i++) {
+                    char a = text[this.textBlob.textOffset + i];
+                    int code = a;
+                    if (char.IsHighSurrogate(a)) {
+                        D.assert(i+1 < this.textBlob.textSize);
+                        D.assert(this.textBlob.textOffset+i+1 < this.textBlob.text.Length);
+                        char b = text[this.textBlob.textOffset+i+1];
+                        D.assert(char.IsLowSurrogate(b));
+                        code = char.ConvertToUtf32(a, b);
+                    } else if (char.IsLowSurrogate(a) || EmojiUtils.isEmptyEmoji(a)) {
+                        continue;
+                    }
+                    var uvRect = EmojiUtils.getUVRect(code);
 
                     var pos = this.textBlob.positions[i];
-                    
+
+                    int baseIndex = vert.Count;
                     vert.Add(new Vector3(pos.x + minX, pos.y + minY, 0));
                     vert.Add(new Vector3(pos.x + maxX, pos.y + minY, 0));
                     vert.Add(new Vector3(pos.x + maxX, pos.y + maxY, 0));
                     vert.Add(new Vector3(pos.x + minX, pos.y + maxY, 0));
 
-                    tri.Add(i * 2);
-                    tri.Add(i * 2 + 1);
-                    tri.Add(i * 2 + 2);
-                    tri.Add(i * 2);
-                    tri.Add(i * 2 + 2);
-                    tri.Add(i * 2 + 3);
-                    var code = char.ConvertToUtf32(a, b);
-                    var uvRect = EmojiUtils.getUVRect(code);
+                    tri.Add(baseIndex);
+                    tri.Add(baseIndex + 1);
+                    tri.Add(baseIndex + 2);
+                    tri.Add(baseIndex);
+                    tri.Add(baseIndex + 2);
+                    tri.Add(baseIndex + 3);
                     uvCoord.Add(uvRect.bottomLeft.toVector());
                     uvCoord.Add(uvRect.bottomRight.toVector());
                     uvCoord.Add(uvRect.topRight.toVector());
                     uvCoord.Add(uvRect.topLeft.toVector());
+                    
+                    if(char.IsHighSurrogate(a)) i++;
                 }
                 MeshMesh meshMesh = new MeshMesh(null, vert, tri, uvCoord);
                 _meshes[key] = new MeshInfo(key, meshMesh, 0);