diff --git a/manifest.json b/manifest.json
index 9536651..bab72c9 100755
--- a/manifest.json
+++ b/manifest.json
@@ -16,7 +16,8 @@
   },
   "background": {
     "scripts": [
-      "scripts/shared.js",
+      "scripts/shared/tools.js",
+      "scripts/shared/common.js",
       "scripts/background/messaging.js",
       "scripts/background/result.js",
       "scripts/background/signiture.js",
@@ -29,7 +30,8 @@
   "content_scripts": [{
     "matches": ["*://*/*"],
     "js": [
-      "scripts/shared.js",
+      "scripts/shared/tools.js",
+      "scripts/shared/common.js",
       "scripts/content/content.js"
     ],
     "run_at": "document_idle"
diff --git a/scripts/background/actions.js b/scripts/background/actions.js
index 526baf0..7c25b0e 100644
--- a/scripts/background/actions.js
+++ b/scripts/background/actions.js
@@ -105,7 +105,7 @@ function extractTabData(tab, itemsSelector, fieldSelectors) {
         itemsSelector: itemsSelector,
         fieldSelectors: fieldSelectors
     }
-    let cond = r => r !== undefined;
+    let cond = r => !MSG_ELEMENT_NOT_FOUND.isEqual(r);
     return sendMessage(tab, req, 'Extract data from the tab...', cond);
 }
 
diff --git a/scripts/content/content.js b/scripts/content/content.js
index f6099f3..f8f7d8c 100644
--- a/scripts/content/content.js
+++ b/scripts/content/content.js
@@ -24,26 +24,30 @@
 );
 
 function extract(itemsSelector, fieldSelectors) {
-    // let fieldNotFound = false;
+    // since some elements may be loaded asynchronously.
+    // if one field is never found, we should return undefined,
+    // so that senders can detect to retry until elements loaded.
+    // If user writes wrong selectors, the task retries infinitely.
+    let fieldFound = {};
     let items = Array.from(document.querySelectorAll(itemsSelector));
-    if (!items.length) return [];
+    // items may not loaded yet, tell the sender to retry.
+    if (!items.length) return MSG_ELEMENT_NOT_FOUND;
     let results = items.map(
         item => {
             return fieldSelectors.map(
                 selector => {
                     let [cls, attr] = selector.split('@').slice(0, 2);
-                    // TODO: close tab to cancel task tip
-                    // if (fieldNotFound) return;
                     let fieldVals = Array.from(item.querySelectorAll(cls));
-                    // if (!fieldVals.length) {
-                    //     fieldNotFound = true;
-                    //     return;
-                    // }
+                    if (!fieldVals.length) {
+                        return;
+                    }
+                    fieldFound[selector] = true;
                     return fieldVals.map(find => attr ? find[attr] : find.textContent.trim()).join('\n')
                 }
             )
         }
     );
-    // return fieldNotFound ? [] : results
-    return results;
+    // if it exists a field, which is not found in any row, the sender should retry.
+    let shouldWait = fieldSelectors.reduce((p, c) => p || !fieldFound[c], false);
+    return shouldWait ? MSG_ELEMENT_NOT_FOUND : results
 }
\ No newline at end of file
diff --git a/scripts/shared/common.js b/scripts/shared/common.js
new file mode 100644
index 0000000..cd0b5d8
--- /dev/null
+++ b/scripts/shared/common.js
@@ -0,0 +1,8 @@
+const EXT_NAME = "DataExtracter";
+
+const ACTION_EXTRACT = `${EXT_NAME}:Extract`;
+const ACTION_GOTO_URL = `${EXT_NAME}:GoToTUL`;
+const ACTION_REPORT_IN = `${EXT_NAME}:ReportIn`;
+const ACTION_QUERY_URL = `${EXT_NAME}:QueryURL`;
+
+const MSG_ELEMENT_NOT_FOUND = new ConstMessage(1, "No element found for at least one selector, maybe it's not loaded yet");
diff --git a/scripts/shared.js b/scripts/shared/tools.js
similarity index 82%
rename from scripts/shared.js
rename to scripts/shared/tools.js
index 79dcaa5..30cb6eb 100644
--- a/scripts/shared.js
+++ b/scripts/shared/tools.js
@@ -1,8 +1,13 @@
-const EXT_NAME = "DataExtracter";
-const ACTION_EXTRACT = `${EXT_NAME}:Extract`;
-const ACTION_GOTO_URL = `${EXT_NAME}:GoToTUL`;
-const ACTION_REPORT_IN = `${EXT_NAME}:ReportIn`;
-const ACTION_QUERY_URL = `${EXT_NAME}:QueryURL`;
+class ConstMessage {
+    constructor(id, message) {
+        this.id = id;
+        this.message = message;
+    }
+    isEqual(err) {
+        if (!err || !err.id) return false;
+        return this.id == err.id;
+    }
+}
 
 function saveFile(data, mimeType, fileName) {
     fileName = fileName || document.title || "result";
@@ -34,4 +39,4 @@ function saveFile(data, mimeType, fileName) {
     } else {
         location.href = url
     }
-}
\ No newline at end of file
+}