Working Range example

[rnystrom]

Changes in this pull request

Adding a working range example to the example app. This example:

• Displays a list of images downloaded from unsplash.it
• Create 20 uniquely-random sized objects
• When section controllers enter the range, create a data task to download the image
  • When finished, store image in downloadedImage
  • Set in cell if cell is visible
  • Don't create task if already downloaded or task created
• Cancel task when section controller is destroyed
• Show a spinner cell while downloading the image

Fixes #84

Pull request checklist

• All tests pass. Demo project builds and runs.
• I have reviewed the contributing guide

[rnystrom]

Screenshots of the demo:

Downloading images:

Images finished:

[facebook-github-bot]

@rnystrom updated the pull request - view changes

[facebook-github-bot]

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[jessesquires]

nice 🎉