For example.You can use "openURL" message in shared NSWorkspace provided by the API.
This is the official google documentation for the API.
To use DropBox services also,you have no other choice but to use the SDK provided by the Dropbox itslef namin "Dropbox for Developers"...
You can download the SDK and use the available REST API to access the Dropbox services.
And for the iCloud, lots of documentation is available from Apple itslef. Check it out
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