Reload contents when activity start

[unixzii]

This is a workaround for the enhancement I proposed in #85 .

It basically reloads anything when activity is started, and find out whether the selections are valid. If an album user previously selected is not existed after reloading, we will navigate to the initial album.

For now, I simply clear all selections when contents changed.

[gejiaheng]

I've tested this PR. There is a severe issue. As the data reloads after MatisseActivity.onStart() got called, if you preview media content and come back, all data is reloaded with UI refreshed. You will be always on the top of the list. Users would have to scroll to the previous position again and again.

[unixzii]

Hum, that's really a bad bug, I'll fix it soon.

[unixzii]

@gejiaheng
I've fixed that issue, now any changes can be found out and only changed items will be refreshed. Additionally, users' selections will not lose.

[gejiaheng]

Under what conditions would the path be null?

[unixzii]

When users restarted the picker activity, we will reload the album list first, and then check out whether the previously selected items still exist, if users deleted the image, PathUtils will resolve nothing, that means we should move it out of selections.

[gejiaheng]

I tested this PR again and got a little confused. When user goes to preview page or home to launcher, then comes back, Matisse restarts the loader and the onCreateLoader() got called. Actually we don't need to recreate the loader, right?
And I don't figure out how you keep the list untouched from a user perspective.

[unixzii]

1. I recreate the loader to reload albums because we need to figure out what changed, don't forget the case that users switch out and delete something then come back. And we've discussed the problems with loader's built-in observing mechanism, so I choose to just reload.
2. I modified the logic of fragment management, and when users go back to our activity from other apps, I will just let the recycler view in existing fragment swap the cursor, so scrolling position and selections will not lose.

[gejiaheng]

Yeah, I understand most of the work you do. Do we have to recreate the loader? What if we don't recreate but just reload the loader?

I'm sorry I am like a little cautious about this PR, because Zhihu App is heavily dependent on this library.

[unixzii]

@gejiaheng
I really understand your worry, cuz we all want this library to be better. I reviewed my code again only just, and I might have made a mistake. Actually, we don't need to restart the loader. And in the newest commit, to be more moderate, I've changed it to Loader#forceLoad.

[gejiaheng]

I'll conduct some more tests. Thanks for your patience.