In answer to your questions:
So is there somehow a way to reload data into a DataSource that already contains data from a remote service?
The recommended way to reload the DataSource's data from remote service is by using the read method
Can the add() method if the DataSource be used for something like this?
No, add method is intended to be used for inserting additional records in the DataSource on the client side. If you want to refresh the data from the remote service you should use the read method.
On a side note, for the scenario you described you can use hierarchical dataSource - mobile ListView supports hierarchical data binding:
Last but not least the following part of your description is not clear:
There is a DataSource in the cars view which is created everytime the view is loaded (init event). This is basically ok, but the data is loaded everytime the view is accessed although we already have loaded the data from the server.
The init event fires only once when the view is accessed for first time. Any subsequent navigation to the View should not execute the init event handler unless some additional configuration is set.
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