Avoid placing modal views above your extension. Extensions are displayed within a modal view by default. While an alert might make sense above an extension, avoid layering additional modal views.
Use your main app to denote the progress of lengthy operations. An activity view should dismiss immediately after initiating sharing or an action. Time-consuming tasks should continue in the background, and your main app should provide some way to check the status of these tasks. Don’t use notifications for this. People don’t want to see a notification every time a task completes, although it’s fine to notify them if there’s a problem.
Use a template image for an action extension icon. A template image uses a mask to create an icon. Use black and white with appropriate transparency and antialiasing, and don’t include a drop shadow. Template images should be centered in an area measuring about 70px × 70px.
Tip Share extensions automatically use your app icon, instilling confidence that the extension is in fact provided by your app.