wait for elements

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
 }