Image has a cache case, the picture name is unchanged, the link is unchanged, only the picture changes, how to know the picture changes need to update the picture?

Since the picture is written to the cache, then you request data, you must first from the cache to see, the cache has no this picture, if there is to read from the cache image information, not networking. This is the normal way of thinking, but now the picture is updated. But you cache is still in, this is you request data, go cache read, cache or have picture, so you can't go to Network load. So your picture is still cached and cannot be updated.

So since there is a local cache, the cache does not time out, even if the background updates the picture, we can not get the latest, unless the cache time-out or the user cleared the cache, so that the network, this time to get a new picture. Otherwise, I think it is impossible to get a new picture, which is contradictory.

Image has a cache case, the picture name is unchanged, the link is unchanged, only the picture changes, how to know the picture changes need to update the picture?