In order to avoid walking into people and vehicles when emailing while walking, we have this little app, Email 'n Walk.
Fire it up and you're presented with a live video feed from the rear-mounted camera, with white text displayed over a translucent black background – and you can enter a subject line and the body; when you hit Send, it passes your text to the Mail application for addressing.
As far as it goes, it works, but the angle at which you naturally hold your iPhone when composing a message means you're unlikely to get decent warning for cliffs/lamp-posts/dog mess.
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