refactor: Tag class support in registerTag()

docs/source/tutorials/register-filters-tags.md

@@ -12,8 +12,8 @@ engine.registerTag('upper', {
     parse: function(tagToken: TagToken, remainTokens: TopLevelToken[]) {
         this.str = tagToken.args; // name
     },
-    render: async function(ctx: Context) {
-        var str = await this.liquid.evalValue(this.str, ctx); // 'alice'
+    render: function*(ctx: Context) {
+        const str = yield this.liquid.evalValue(this.str, ctx); // 'alice'
         return str.toUpperCase() // 'ALICE'
     }
 });
@@ -22,6 +22,25 @@ engine.registerTag('upper', {
 * `parse`: Read tokens from `remainTokens` until your end token.
 * `render`: Combine scope data with your parsed tokens into HTML string.
 
+For complex tag implementation, you can also provide a tag class:
+
+```typescript
+// Usage: {% upper name:"alice" %}
+import { Hash, Tag, TagToken, Context, Emitter, TopLevelToken, Liquid } from 'liquidjs'
+
+engine.registerTag('upper', class UpperTag extends Tag {
+    private hash: Hash
+    constructor(tagToken: TagToken, remainTokens: TopLevelToken[], liquid: Liquid) {
+        super(tagToken, remainTokens, liquid)
+        this.hash = new Hash(tagToken.args)
+    }
+    * render(ctx: Context) {
+        const hash = yield this.hash.render();
+        return hash.name.toUpperCase() // 'ALICE'
+    }
+});
+```
+
 See existing tag implementations here: <https://github.com/harttle/liquidjs/tree/master/src/builtin/tags>
 See demo example here: https://github.com/harttle/liquidjs/blob/master/demo/typescript/index.ts