Subview disappears with auto layout

Question

I just added a UIView as subview to the main UIView on interface builder (basic single view application). Without setting any constraints, my subview disappears.

subview's frame = (0 0; 320 0);

Why is that?

If I try to add some constraints like for trailing space, leading space, top space and bottom space to be fixed, still my view disappears.

How can I solve this?

Thank you.

Just to clarify thing a little I created a test project (single view application), and added 2 subview to the main view like in the image. I didn't change any default constraint. And you can see the error in the log of the image.

Logs:

**2013-01-19 17:16:02.435 Test[8871:c07] Unable to simultaneously satisfy constraints.
    Probably at least one of the constraints in the following list is one you don't want. Try this: (1) look at each constraint and try to figure out which you don't expect; (2) find the code that added the unwanted constraint or constraints and fix it. (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 
(
    "<NSAutoresizingMaskLayoutConstraint:0x106178c0 h=--- v=--- V:[UIWindow:0x9917040(480)]>",
    "<NSLayoutConstraint:0x106159e0 UIView:0x991a5a0.bottom == UIWindow:0x9917040.bottom>",
    "<NSLayoutConstraint:0x991ab00 V:|-(518)-[BottomView:0x9919c90]   (Names: '|':UIView:0x991a5a0 )>",
    "<NSLayoutConstraint:0x10615960 V:|-(20)-[UIView:0x991a5a0]   (Names: '|':UIWindow:0x9917040 )>",
    "<NSLayoutConstraint:0x991aa80 BottomView:0x9919c90.bottom == UIView:0x991a5a0.bottom>"
)

Will attempt to recover by breaking constraint 
<NSLayoutConstraint:0x991aa80 BottomView:0x9919c90.bottom == UIView:0x991a5a0.bottom>

Break on objc_exception_throw to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.**

Constraints:

Also here is the result on the simulator:

Answer 1

It is good practice to understand those logs but if you are going to use Autolayout you are going to have to read up on this. Everyone says it is simply but I personally have not found it simple.

Apples Programming Guide For AutoLayout

Please read this guide especially the debugging section.

As a very very general rule if you are going to add a view then you need to turn off autoresizingmasks (Spring and struts) for the view. Add the view as a subview and give it 2 or 3 constraints. In your case above you would give it a constaint that it should have a left or leading space to superview of 0. A top space to superview of 0 and a width of 320.

EDIT; Here is an example of adding a view; note you do not need to create a frame. The constraints may be a little strange. The first puts the view in the centre of the superview. The second gives it a width of 200. The next method is the vertical constraint which puts the view at the bottom and makes it 2 high.

    UIView *sView = [[UIView alloc] init];
[sView setTranslatesAutoresizingMaskIntoConstraints:NO];
[superView addSubview:sView];

[superView addConstraint:[NSLayoutConstraint constraintWithItem:sView
                                                 attribute:NSLayoutAttributeCenterX
                                                 relatedBy:NSLayoutRelationEqual
                                                    toItem:superView
                                                 attribute:NSLayoutAttributeCenterX
                                                multiplier:1.0
                                                  constant:0]];

[superView addConstraint:[NSLayoutConstraint constraintWithItem:sView
                                                             attribute:NSLayoutAttributeWidth
                                                             relatedBy:NSLayoutRelationEqual
                                                                toItem:Nil
                                                             attribute:NSLayoutAttributeWidth
                                                            multiplier:1.0
                                                              constant:200]];

[superView addConstraints:[NSLayoutConstraint
                                  constraintsWithVisualFormat:@"V:[sView(2)]|"
                                  options:0
                                  metrics:nil
                                  views:NSDictionaryOfVariableBindings(sView)]];