fixed issue where input bar only appeared behind keyboard (#605, #615)

[tditchek]

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PR Summary

Fixed the issue where the Input Bar was always hidden behind the keyboard. It was caused by the method slk_updateViewConstraints in SLKTextViewController.m being called every time viewSafeAreaInsetsDidChange was fired. This caused the problem by constraining the bottom of the InputBar to the bottom of a CGRectNull that was supposed to be representing the keyboard frame. I fixed it by adding a new iVar called _cachedKeyboardRect that would default to CGRectNull but store the keyboard frame when it's going to becomeFirstResponder. Then I added a method slk_updateViewConstraintsFromCachedKeyboard which sets the proper Input Bar constraints to the bottom of the _cachedKeyboardRect. I updated viewSafeAreaInsetsDidChange to call this method instead of the slk_updateViewConstraints that really should only apply when the keyboard isn't visible.

Related Issues

Fixes #615 and #605

[tditchek]

@dzenbot I believe this issue was caused by one of the changes added to support the iPhone X, so add it to the iPhone X / iOS 11 update. thanks!

[JustinBobo]

Any updates here?