Detected a case where constraints ambiguously suggest a height of zero

问题:

After updating to Xcode 6.1 beta 2 when I run my app that contains tableview cells, the debug assistant says:

Warning once only: Detected a case where constraints ambiguously suggest a height of zero for a tableview cell's content view. We're considering the collapse unintentional and using standard height instead. 

Before, when I used Xcode 5 on this project, I would get a few errors but those have gone away since I upgraded. I have no other errors or warnings now. I have already tried adjusting the sizes of all the tableview cells and also tried using standard height but I still get the same warning:

Warning once only: Detected a case where constraints ambiguously suggest a height of zero for a tableview cell's content view. We're considering the collapse unintentional and using standard height instead. 

I have also read through all similar topics on this but none of their solutions help. When I test the app with the simulator, the app runs fine except the pictures that are supposed to be in the tableView cells aren't there.

回答1:

Three things have managed to silence this warning so far. You can pick up the most convenient for you. Nothing pretty though.

• To set up default cell's height in viewDidLoad

  self.tableView.rowHeight = 44; 

• Go to storyboard and change row height on your tableview to something different than 44.

• To implement tableview's delegate method heightForRowAtIndexPath

  - (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath {     return 44; } 

Weird.

回答2:

You're encountering the side effect of a fantastic new feature in iOS8's Tableviews: Automatic Row Heights.

In iOS 7, you either had rows of a fixed size (set with tableView.rowHeight), or you'd write code to calculate the height of your cells and you'd return that in tableView:heightForRowAtIndexPath. Writing code for the calculation of a cell's height could be quite complex if you had numerous views in your cell and you had different heights to consider at different font sizes. Add in Dynamic Type and the process was a pain in the ass.

In iOS 8, you can still do the above, but now the height of the rows can be determined by iOS, provided that you've configured the content of your cell using Auto Layout. This is huge benefit for developers, because as the dynamic font size changes, or the user modifies the text size using Accessibility Settings, your UI can be adaptive to the new size. It also means if you have a UILabel that can have multiple rows of text, your cell can now grow to accommodate those when the cells needs to, and shrink when it does not, so there isn't any unnecessary whitespace.

The warning message you're seeing is telling you that there aren't enough constraints in your cell for Auto Layout to inform the tableview of the height of the cell.

To use dynamic cell height, which, along with the techniques already mentioned by other posters, will also get rid of this message, you need to ensure your cell has sufficient constraints to bind the UI items to the top and bottom of the cell. If you've used Auto Layout before, you are probably accustomed to setting Top + Leading constraints, but dynamic row height also requires bottom constraints.

The layout pass works like this, which occurs immediately before a cell is displayed on screen, in a just-in-time manner:

1. Dimensions for content with intrinsic sizes is calculated. This includes UILabels and UIImageViews, where their dimensions are based on the text or UIImages they contain, respectively. Both of these views will consider their width to be a known (because you've set constraints for trailing/leading edges, or you set explicit widths, or you used horizontal constraints that eventually reveal a width from side to side). Let's say a label has a paragraph of text ("number of lines" is set to 0 so it'll auto-wrap), it can only be 310 points across, so it's determined to be 120pt high at the current font size.

2. The UI is laid out according to your positioning constraints. There is a constraint at the bottom of the label that connects to the bottom margin of the cell. Since the label has grown to be 120 points tall, and since it's bound to the bottom of the cell by the constraint, it must push the cell "down" (increasing the height of the cell) to satisfy the constraint that says "bottom of the label is always standard distance from the bottom of the cell.

The error message you reported occurs if that bottom constraint is missing, in which case there is nothing to "push" the bottom of the cell away from the top of the cell, which is the ambiguity that's reported: with nothing to push the bottom from the top, the cell collapses. But Auto Layout detects that, too, and falls back to using the standard row height.

For what it's worth, and mostly to have a rounded answer, if you do implement iOS 8's Auto Layout-based dynamic row heights, you should implement tableView:estimatedHeightForRowAtIndexPath:. That estimate method can use rough values for your cells, and it'll be called when the table view is initially loaded. It helps UIKit draw things like the scrollbar, which can't be drawn unless the tableview knows how much content it can scroll through, but does't need totally accurate sizes, since it's just a scrollbar. This lets the calculation of the actual row height be deferred until the moment the cell is needed, which is less computationally intensive and lets your UITableView be presented quicker.

回答3:

To resolve this without a programmatic method, adjust the row height of the table view in the Size Inspector from the storyboard.

回答4:

I got this warning today. Here is what made it disappear for me(in interface builder)

1.Set the row height field for the table view to something other than 44 2 Set the row height field for the tableView cell to something other than 44

I did not have to make any changes in code

回答5:

I had this problem after creating a custom UITableViewCell and adding my subviews to the cell instead of its contentView.

回答6:

This is an autolayout issue. Make sure that your subviews have all the constraints. For me, the bottom constraint was missing for the Title Label in the cell. When I added that, the warning went away and everything showed up perfectly.

回答7:

I too experienced this warning with moving to Xcode 6 GM. I was only getting the warning when I rotated the device back to its original position.

I am using custom UITableViewCells. The storyboard table view is set to my custom size (100.0 in my case). While the table cells render properly as they have in previous releases, I did not like warning message.

In addition to the above ideas, I added this

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath {     return 100.0; } 

Screen renders... responds to rotation and no more warning messages.

回答8:

In xcode 6.0.1 I had removed this warnings specifying the row height using:

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath {     return 44.0; } 

回答9:

In the storyboard set the cell Row height field with the same value as Row height in tableView (both with the same value worked for me).

If you add heightForRowAtIndexPath function to your code it may induce a performance issue because it will be called for each cell so be careful.

回答10:

You may also see this message if your only constraints are set to align all items vertically and you don't have/want a height specified for the cell. If you set a top/bottom constraint on the item the warning will disappear.

回答11:

I had this problem when my labels and views in the custom tableViewCell were constrained to the customCell, not its Content View. When I cleared the constraints and connected them to cells Content View the problem was solved.

回答12:

I had the same error message, make sure all your outlets are valid like table view and tableview Constraints

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