How did Dropbox implement these two features (changing icons & sending only difference in file changes.)

How did DropBox implement these two features (a high-level answer is fine):

1. Get the icon of a file to change once it's synchronised on Windows? (I am guessing similar answers would apply to the other 开发者_如何学编程OSs)

2. Send only the changes done to a file and not the whole file?

For 2, I am guessing that it worked by using something similar to diff on the client side, and sending the output...

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As for the first question, it's done using an icon overlay handler. There's an example in codeproject, and you can also take a look at the great TortoiseSVN source, which implements such a handler.

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Only sending modifications to a file is accomplished with something called a Rolling Checksum / Hash, an example of a program that can remotely diff a file and only send the differences is rsync.