iOS expand UITableViewCell on click with AutoLayout - ios

Question

I'm working on simple application and now I'm focused on expanding UITableViewCell after user tap that cell. It's iOS 8 app so I have set:
self.tableVIew.rowHeight = UITableViewAutomaticDimension
self.tableVIew.estimatedRowHeight = 50
cell constraints look like this:
If user tap cell this function is called:
func extend() {
self.contentView.removeConstraint(self.bottomConstraint)
let additionalView = UIView()
additionalView.setTranslatesAutoresizingMaskIntoConstraints(false)
additionalView.backgroundColor = UIColor.orangeColor()
self.contentView.addSubview(additionalView)
self.contentView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("H:[additionalView(50)]-5-|", options: nil, metrics: nil, views: ["additionalView" : additionalView]))
self.contentView.addConstraints(NSLayoutConstraint.constraintsWithVisualFormat("V:|-5-[additionalView(100)]-5#999-|", options: nil, metrics: nil, views: ["additionalView" : additionalView]))
}
self.bottomConstraint is costraint between green circle bottom and cell contentView bottom.
The question is:
Why this solution works only if there is priority set in constraint:
V:|-5-[additionalView(100)]-5#999-|
?
Without explicit priority I got errors:
(
"<NSLayoutConstraint:0x7a63f7a0 'UIView-Encapsulated-Layout-Height' V:[UITableViewCellContentView:0x7a737840(60)]>",
"<NSLayoutConstraint:0x7a644f40 V:|-(5)-[UIView:0x7a63f2a0] (Names: '|':UITableViewCellContentView:0x7a737840 )>",
"<NSLayoutConstraint:0x7a644fb0 V:[UIView:0x7a63f2a0(100)]>",
"<NSLayoutConstraint:0x7a644f70 V:[UIView:0x7a63f2a0]-(5)-| (Names: '|':UITableViewCellContentView:0x7a737840 )>"
)
Will attempt to recover by breaking constraint
<NSLayoutConstraint:0x7a644f70 V:[UIView:0x7a63f2a0]-(5)-| (Names: '|':UITableViewCellContentView:0x7a737840 )>

Answer 1

I have made a demo that gives you the exact behaviour as you want.
here is the link:
https://github.com/rushisangani/TableViewCellExpand
Please set constraint of your expand/ collapse views as described in demo.

Answer 2

'UIView-Encapsulated-Layout-Height' is the constraint for the row height created after the table view system has calculated what it should be. Simply changing the constraint inside isn't enough to force recalculation.
This isn't new unexpected behavior; while effects could sometimes be achieved by letting cells hang out of their tableview-supplied height, the table view data source delegate is meant to provide the "official" row height.
Apple's sample project Table View Animations and Gestures is illustrative. It hasn't been updated for iOS 8's self-sizing cells but it's worth a look regardless. In the APLTableViewController.m the tableViewController is the delegate for the cell's pitch gesture, because it needs to adjust the height. Its handlePinch: calls updateForPinchScale: atIndexPath: which does the expansion like you want.
The Table View Controller doesn't try to change the row height directly. First, it sets its own custom data source object (called sectionInfo) to provide an updated value for the row height [sectionInfo replaceObjectInRowHeightsAtIndex:indexPath.row withObject:#(newHeight)];
. Then the Table View Controller does the following:
BOOL animationsEnabled = [UIView areAnimationsEnabled];
[UIView setAnimationsEnabled:NO];
[self.tableView beginUpdates];
[self.tableView endUpdates];
[UIView setAnimationsEnabled:animationsEnabled];
Turning off all UIView animations is because this is a pinch, so it should change instantly without jerkyness. Since you only want a one-time change, you should experiment with different animation settings to get the effect you want.
They key is the pair of beginUpdates / endUpdates calls that apparently triggers the system to recalculate the row heights. It's possible that with the new iOS self-sizing cells, simply changing the constraints and calling these methods might be enough; you could also try sandwiching reloadRowsAtIndexPaths:withRowAnimation:UITableViewRowAnimationNone

Answer 3

set the height priority of one of your views to 999 instead 1000.