I would like to know how iPhone Apps like Seesmic or Instagram connect
to the Twitter account. They don't use any webview. I can do an app
connecting with Oauth through a webview but how do these apps manage
the connection?
Do these apps have some special permissions/way to connect ?
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Twitter
Native mobile applications can apply for xAuth access,
http://dev.twitter.com/pages/xauth which allows an application to exchange a
login password for an OAuth access token. Instagram Seesmic both use
xAuth to simplify this flow.
Depending on how deeply your application needs to use Twitter, I
