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engine/gulp/util/drop-pure-export.js

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+
+//  Drop unused `export` syntax to indicate that the object will only be used inside the module at runtime.
+//  So the expression can be evaluated by uglify.
+//
+//  Replace:
+//      /*@__DROP_PURE_EXPORT__*/
+//      export xxx
+//  As:
+//      /*@__DROP_PURE_EXPORT__*/
+//      xxx
+
+const es = require('event-stream');
+const stream = require('stream');
+
+const COMMENT_ANNOTATION = `/*@__DROP_PURE_EXPORT__*/`;
+const REG = /(\/\*@__DROP_PURE_EXPORT__\*\/\s*)(export\s+)?/g;
+
+module.exports = function (filename) {
+    if (filename.endsWith('.js') || filename.endsWith('.ts')) {
+        return es.map(function (data, callback) {
+            var content = data.toString();
+            if (content.includes(COMMENT_ANNOTATION)) { // fast test
+                let error = false;
+                content = content.replace(REG, function (match, g1, g2) {
+                    if (g2) {
+                        return g1;
+                    }
+                    else {
+                        // console.log(content);
+                        error = true;
+                        return match;
+                    }
+                });
+                if (error) {
+                    return callback(new Error(`Uncaptured comment annotation "${COMMENT_ANNOTATION}" in "${filename}", please use it in the correct way.`));
+                }
+                data = new Buffer(content);
+            }
+            callback(null, data);
+        });
+    }
+    else {
+        return stream.PassThrough();
+    }
+};