In a Windows Form window, multiple events can trigger an asynchronous method. This method downloads a file and caches it. My problem is that I want that method to be executed once. In other words, I want to prevent the file to be downloaded multiple times.
If the method downloading the file is triggered twice, I want the second call to wait for the file (or wait for the first method to be done).
Does someone have an idea on how to achieve that?
UPDATE: I am simply trying to prevent unnecessary downloads. In my case, when a client put its mouse over an item in a ListBox for more than a couple milliseconds, we start to download. We make the assumption that the user will click and request the file. What can potentially happen is that the user keeps his mouse over the item for one second and then click. In this case two downloads start. I am looking for the best way to handle such scenario.
UPDATE 2:: There is a possibility that the user will move its mouse over multiple items. In consequences, multiple downloads will occur. I’ve not really tough of this scenario, but right now if we face such scenario we don’t abandon the download. The file will be downloaded (files are usually around 50-100kb) and then are going to be cached.
Maintain the state of what’s happening in a form variable and have your async method check that state before it does anything. Make sure you synchronize access to it, though! Mutexes and semaphores are good for this kind of thing.
If you can download different files simultaneously, you’ll need to keep track of what’s being downloaded in a list for reference.
If only one file can be downloaded at a time, and you don’t want to queue things up, you could just unhook the event while something is being downloaded, too, and rehook it when the download is complete.