Autolayout和设备定位

Question

I am developing an application which supports portrait and landscape modes . I am using auto layout to arrange my views . As i have been reading many posts and i have realized that developers commonly use one among the following approaches.

1. First approach:

Implement the UIViewController:updateConstraints method and update constraints according to device orientation.

2. Second approach:

Implement the UIViewController:viewWillLayoutSubviews method and update constraints according to device orientation.

Could anyone please tell me what is the best approach to use ? I have been searching for a best practice to combine autorotation and auto layout and nothing yet. Thanks.

Answer 1

I would use this UIViewController's method:

- (void)willAnimateRotationToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation duration:(NSTimeInterval)duration;

that is called upon rotation, and call -setNeedsUpdateConstraints from there. If you need to do additional calculations or manage the contrainsts add

- (void)updateViewConstraints
{
    [super updateViewConstraints];
    // do calculations if needed, like set constants
}

Answer 2

The best approach is to use neither. Instead, configure your constraints correctly so that no programmatic changes are required on rotation. The Auto Layout runtime will maintain the views in position as you already have specified.

Updating constraints other than changing the value of .constant is a real performance hit and should be avoided.

Using viewWillLayoutSubviews is not necessary. Auto Layout methods are updateViewConstraints (for the view controller), and updateConstraints (in the views).

Answer 3

I believe that the best approach is to update the constraints in -(void)updateViewConstraints by checking the device orientation. There is no need to call setNeedsUpdateConstraints in willAnimateRotationToInterfaceOrientation because it is automatically called by the iOs when the device orientation changes. Thank you all for the great effort.

Answer 4

for ios8+

From the documentation for:

- (void)willTransitionToTraitCollection:(UITraitCollection *)newCollection
          withTransitionCoordinator:(id<UIViewControllerTransitionCoordinator>)coordinator

A standard view controller might use this method to change the constraints on the views it manages.