Can I use autolayout to provide different constrai

问题:

Is it possible to change the constraints when the device is rotated? How might this be achieved?

A simple example might be two images which, in portrait, are stacked one above the other, but in landscape are side-by-side.

If this is not possible, how else might I accomplish this layout?

I am constructing my views and constraints in code and not using interface builder.

回答1:

Edit: Using the new concept of Size Classes introduced in Xcode 6, you can easily setup different constraints for specific size classes in Interface Builder. Most devices (e.g. all current iPhones) have a Compact vertical size class in landscape mode.

This is a much better concept for general layout decisions than determining the device's orientation.

That being said, if you really need to know the orientation, UIDevice.currentDevice().orientation is the way to go.

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Original post:

Override the updateViewConstraints method of UIViewController to provide layout constraints for specific situations. This way, the layout is always set up the correct way according to situation. Make sure they form a complete set of constraints with those created within the storyboard. You can use IB to set up your general constraints and mark those subject to change to be removed at runtime.

I use the following implementation to present a different set of constraints for each orientation:

-(void)updateViewConstraints {
    [super updateViewConstraints];

    // constraints for portrait orientation
    // use a property to change a constraint's constant and/or create constraints programmatically, e.g.:
    if (!self.layoutConstraintsPortrait) {
        UIView *image1 = self.image1;
        UIView *image2 = self.image2;
        self.layoutConstraintsPortrait = [[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-[image1]-[image2]-|" options:NSLayoutFormatDirectionLeadingToTrailing metrics:nil views:NSDictionaryOfVariableBindings(image1, image2)] mutableCopy];
        [self.layoutConstraintsPortrait addObject:[NSLayoutConstraint constraintWithItem:image1 attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem: image1.superview attribute:NSLayoutAttributeCenterY multiplier:1 constant:0]];
        [self.layoutConstraintsPortrait addObject:[NSLayoutConstraint constraintWithItem:image2 attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:image2.superview attribute:NSLayoutAttributeCenterY multiplier:1 constant:0]];
    }

    // constraints for landscape orientation
    // make sure they don't conflict with and complement the existing constraints
    if (!self.layoutConstraintsLandscape) {
        UIView *image1 = self.image1;
        UIView *image2 = self.image2;
        self.layoutConstraintsLandscape = [[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-[image1]-[image2]-|" options:NSLayoutFormatDirectionLeadingToTrailing metrics:nil views:NSDictionaryOfVariableBindings(image1, image2)] mutableCopy];
        [self.layoutConstraintsLandscape addObject:[NSLayoutConstraint constraintWithItem:image1 attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:image1.superview attribute:NSLayoutAttributeCenterY multiplier:1 constant:0]];
        [self.layoutConstraintsLandscape addObject:[NSLayoutConstraint constraintWithItem:image2 attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem: image2.superview attribute:NSLayoutAttributeCenterY multiplier:1 constant:0]];
    }

    BOOL isPortrait = UIInterfaceOrientationIsPortrait(self.interfaceOrientation);
    [self.view removeConstraints:isPortrait ? self.layoutConstraintsLandscape : self.layoutConstraintsPortrait];
    [self.view addConstraints:isPortrait ? self.layoutConstraintsPortrait : self.layoutConstraintsLandscape];        
}

Now, all you need to do is trigger a constraint update whenever the situation changes. Override willAnimateRotationToInterfaceOrientation:duration: to animate the constraint update on orientation change:

- (void)willAnimateRotationToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation duration:(NSTimeInterval)duration {
    [super willAnimateRotationToInterfaceOrientation:toInterfaceOrientation duration:duration];

    [self.view setNeedsUpdateConstraints];

}

回答2:

The approach I am using (for better or worse) is to define both sets of constraints (portrait and landscape) in the storyboard editor.

To avoid the storyboard warnings of hell, I place all of one set at a priority of 999, just so it doesn't show red everywhere.

Then I add all of the constraints to outlet collections:

@property (strong, nonatomic) IBOutletCollection(NSLayoutConstraint) NSArray *portraitConstraints;
@property (strong, nonatomic) IBOutletCollection(NSLayoutConstraint) NSArray *landscapeConstraints;

Finally, I implement my ViewControllers's viewWillLayout method:

- (void) viewWillLayoutSubviews {
    [super viewWillLayoutSubviews];
    for (NSLayoutConstraint *constraint in self.portraitConstraints) {
        constraint.active = (UIApplication.sharedApplication.statusBarOrientation == UIDeviceOrientationPortrait);
    }
    for (NSLayoutConstraint *constraint in self.landscapeConstraints) {
        constraint.active = (UIApplication.sharedApplication.statusBarOrientation != UIDeviceOrientationPortrait);
    }
}

This seems to work. I really wish you could set the default active property in the the storyboard editor.

回答3:

I am following the same approach as yours (no nib files or storyboards). You have to update your constraints in updateViewConstraints method (by checking the device orientation). There is no need to call setNeedsUpdateConstraints in updateViewConstraints because as soon as you change the device orientation the last method is called automatically.

回答4:

You can save your constraints, to a property or variable as portrait and landscape versions then set and remove them on rotate.

I have done this making my constraints in xib for initial view, assigning them to outlets in the view controller. On rotate I create the alternate restraints, remove the outlets but retain them, insert the alternates.

Reverse the process on rotate back.

回答5:

My idea is to handle orientation by changing constraint priorities.
Supposing priorities will be:

• landscape: 910 active / 10 inactive.
• portrait: 920 active / 20 inactive.

Step 1: Create landscape (or portrait) design in Storyboard with constraints.
Step 2: For constraints, that must be active only for landscape mode set priority to 10.
Step 3: Add constraints for portrait mode and set their priority to 920.

In willAnimateRotationToInterfaceOrientation add code:

for (NSLayoutConstraint *constraint in myView.constraints) {
    if (UIInterfaceOrientationIsLandscape(UIApplication.sharedApplication.statusBarOrientation)) {
        if (constraint.priority == 10)  constraint.priority = 910;
        if (constraint.priority == 920) constraint.priority = 20;
    } else {
        if (constraint.priority == 20)  constraint.priority = 920;
        if (constraint.priority == 910) constraint.priority = 10;
    }
}

Advantage of this approach - easy adjusting in Interface Builder. When we need to switch to any orientation, we select all constraints by priority and change them simultaneously (910->10, 20->920):

Interface will rebuilded automatically.