Add property to tell if the list is currently loading or loading more

We'll need to display a spinner of some kind in the front-end, I think.

Contributes to issue CURA-8556.

plugins/Marketplace/PackageList.py

@@ -1,7 +1,7 @@
 # Copyright (c) 2021 Ultimaker B.V.
 # Cura is released under the terms of the LGPLv3 or higher.
 
-from PyQt5.QtCore import Qt
+from PyQt5.QtCore import pyqtProperty, pyqtSignal, pyqtSlot, Qt
 from typing import List, TYPE_CHECKING
 from UM.Qt.ListModel import ListModel
 
@@ -20,10 +20,39 @@ class PackageList(ListModel):
 
     PackageRole = Qt.UserRole + 1
 
+    ITEMS_PER_PAGE = 20  # Pagination of number of elements to download at once.
+
     def __init__(self, parent: "QObject" = None):
         super().__init__(parent)
 
         self._packages: List[PackageModel] = []
+        self.setIsLoading(True)
+
+        self.requestFirst()
+
+    def requestFirst(self) -> None:
+        """
+        Make a request for the first paginated page of packages.
+
+        When the request is done, the list will get updated with the new package models.
+        """
+        self.setIsLoading(True)
+
+    isLoadingChanged = pyqtSignal()
+
+    @pyqtSlot(bool)
+    def setIsLoading(self, is_loading: bool) -> None:
+        if(is_loading != self._is_loading):
+            self._is_loading = is_loading
+            self.isLoadingChanged.emit()
+
+    @pyqtProperty(bool, notify = isLoadingChanged, fset = setIsLoading)
+    def isLoading(self) -> bool:
+        """
+        Gives whether the list is currently loading the first page or loading more pages.
+        :return: ``True`` if the list is downloading, or ``False`` if not downloading.
+        """
+        return self._is_loading
 
     def _update(self) -> None:
         # TODO: Get list of packages from Marketplace class.