如何为子视图控制器设置topLayoutGuide位置

Question

我正在实现一个与UINavigationController非常相似的自定义容器,除了它不包含整个控制器堆栈.它有一个UINavigationBar,它被约束到容器控制器的topLayoutGuide,它恰好是顶部的20px,这没关系.

当我添加一个子视图控制器并将其视图放入层次结构时,我希望在IB中看到它的topLayoutGuide并用于布置子视图控制器的视图子视图以显示在导航栏的底部.需要注意的是相关文档中要做的事情:

具体而言,此属性的值是查询此属性时返回的对象的length属性的值.此值受视图控制器或其封闭容器视图控制器(如导航或标签栏控制器)的约束,如下所示:

• 不在容器视图控制器内的视图控制器将此属性限制为指示状态栏的底部(如果可见),
  或者指示视图控制器视图的上边缘.
• 容器视图控制器中的视图控制器不会设置此属性的值.相反,容器视图控制器约束值以指示:
  • 如果导航栏可见,则导航栏的底部
  • 如果只显示状态栏,则状态栏的底部
  • 如果状态栏和导航栏都不可见,则视图控制器视图的上边缘

但我不太明白如何"约束它的价值"因为topLayoutGuide和它的长度属性都是只读的.

我试过这个代码来添加一个子视图控制器:

[self addChildViewController:gamePhaseController];
UIView *gamePhaseControllerView = gamePhaseController.view;
gamePhaseControllerView.translatesAutoresizingMaskIntoConstraints = NO;
[self.contentContainer addSubview:gamePhaseControllerView];

NSArray *horizontalConstraints = [NSLayoutConstraint constraintsWithVisualFormat:@"|-0-[gamePhaseControllerView]-0-|"
                                                                         options:0
                                                                         metrics:nil
                                                                           views:NSDictionaryOfVariableBindings(gamePhaseControllerView)];

NSLayoutConstraint *topLayoutGuideConstraint = [NSLayoutConstraint constraintWithItem:gamePhaseController.topLayoutGuide
                                                                            attribute:NSLayoutAttributeTop
                                                                            relatedBy:NSLayoutRelationEqual
                                                                               toItem:self.navigationBar
                                                                            attribute:NSLayoutAttributeBottom
                                                                           multiplier:1 constant:0];
NSLayoutConstraint *bottomLayoutGuideConstraint = [NSLayoutConstraint constraintWithItem:gamePhaseController.bottomLayoutGuide
                                                                               attribute:NSLayoutAttributeBottom
                                                                               relatedBy:NSLayoutRelationEqual
                                                                                  toItem:self.bottomLayoutGuide
                                                                               attribute:NSLayoutAttributeTop
                                                                              multiplier:1 constant:0];
[self.view addConstraint:topLayoutGuideConstraint];
[self.view addConstraint:bottomLayoutGuideConstraint];
[self.contentContainer addConstraints:horizontalConstraints];
[gamePhaseController didMoveToParentViewController:self];

_contentController = gamePhaseController;

在IB中,我为gamePhaseController指定"Under Top Bars"和"Under Bottom Bars".其中一个视图特别限制在顶部布局指南中,无论如何在设备上它看起来是容器导航栏底部20px ...

使用此行为实现自定义容器控制器的正确方法是什么？

Answer 1

据我所知,经过数小时的调试后,布局指南是只读的,并从用于基于约束的布局的私有类派生.覆盖访问器什么都不做(即使它们被调用),而且这些都只是让人烦恼.

Answer 2

(更新:现在可以作为cocoapod,请参阅https://github.com/stefreak/TTLayoutSupport)

一个有效的解决方案是删除Apple的布局约束并添加自己的约束.我为此做了一个小类.

这是代码 - 但我建议cocoapod.它有单元测试,更有可能是最新的.

//
//  UIViewController+TTLayoutSupport.h
//
//  Created by Steffen on 17.09.14.
//

#import <UIKit/UIKit.h>

@interface UIViewController (TTLayoutSupport)

@property (assign, nonatomic) CGFloat tt_bottomLayoutGuideLength;

@property (assign, nonatomic) CGFloat tt_topLayoutGuideLength;

@end

-

#import "UIViewController+TTLayoutSupport.h"
#import "TTLayoutSupportConstraint.h"
#import <objc/runtime.h>

@interface UIViewController (TTLayoutSupportPrivate)

// recorded apple's `UILayoutSupportConstraint` objects for topLayoutGuide
@property (nonatomic, strong) NSArray *tt_recordedTopLayoutSupportConstraints;

// recorded apple's `UILayoutSupportConstraint` objects for bottomLayoutGuide
@property (nonatomic, strong) NSArray *tt_recordedBottomLayoutSupportConstraints;

// custom layout constraint that has been added to control the topLayoutGuide
@property (nonatomic, strong) TTLayoutSupportConstraint *tt_topConstraint;

// custom layout constraint that has been added to control the bottomLayoutGuide
@property (nonatomic, strong) TTLayoutSupportConstraint *tt_bottomConstraint;

// this is for NSNotificationCenter unsubscription (we can't override dealloc in a category)
@property (nonatomic, strong) id tt_observer;

@end

@implementation UIViewController (TTLayoutSupport)

- (CGFloat)tt_topLayoutGuideLength
{
    return self.tt_topConstraint ? self.tt_topConstraint.constant : self.topLayoutGuide.length;
}

- (void)setTt_topLayoutGuideLength:(CGFloat)length
{
    [self tt_ensureCustomTopConstraint];

    self.tt_topConstraint.constant = length;

    [self tt_updateInsets:YES];
}

- (CGFloat)tt_bottomLayoutGuideLength
{
    return self.tt_bottomConstraint ? self.tt_bottomConstraint.constant : self.bottomLayoutGuide.length;
}

- (void)setTt_bottomLayoutGuideLength:(CGFloat)length
{
    [self tt_ensureCustomBottomConstraint];

    self.tt_bottomConstraint.constant = length;

    [self tt_updateInsets:NO];
}

- (void)tt_ensureCustomTopConstraint
{
    if (self.tt_topConstraint) {
        // already created
        return;
    }

    // recording does not work if view has never been accessed
    __unused UIView *view = self.view;
    // if topLayoutGuide has never been accessed it may not exist yet
    __unused id<UILayoutSupport> topLayoutGuide = self.topLayoutGuide;

    self.tt_recordedTopLayoutSupportConstraints = [self findLayoutSupportConstraintsFor:self.topLayoutGuide];
    NSAssert(self.tt_recordedTopLayoutSupportConstraints.count, @"Failed to record topLayoutGuide constraints. Is the controller's view added to the view hierarchy?");
    [self.view removeConstraints:self.tt_recordedTopLayoutSupportConstraints];

    NSArray *constraints =
        [TTLayoutSupportConstraint layoutSupportConstraintsWithView:self.view
                                                     topLayoutGuide:self.topLayoutGuide];

    // todo: less hacky?
    self.tt_topConstraint = [constraints firstObject];

    [self.view addConstraints:constraints];

    // this fixes a problem with iOS7.1 (GH issue #2), where the contentInset
    // of a scrollView is overridden by the system after interface rotation
    // this should be safe to do on iOS8 too, even if the problem does not exist there.
    __weak typeof(self) weakSelf = self;
    self.tt_observer = [[NSNotificationCenter defaultCenter] addObserverForName:UIDeviceOrientationDidChangeNotification
                                                                         object:nil
                                                                          queue:[NSOperationQueue mainQueue]
                                                                     usingBlock:^(NSNotification *note) {
        __strong typeof(self) self = weakSelf;
        [self tt_updateInsets:NO];
    }];
}

- (void)tt_ensureCustomBottomConstraint
{
    if (self.tt_bottomConstraint) {
        // already created
        return;
    }

    // recording does not work if view has never been accessed
    __unused UIView *view = self.view;
    // if bottomLayoutGuide has never been accessed it may not exist yet
    __unused id<UILayoutSupport> bottomLayoutGuide = self.bottomLayoutGuide;

    self.tt_recordedBottomLayoutSupportConstraints = [self findLayoutSupportConstraintsFor:self.bottomLayoutGuide];
    NSAssert(self.tt_recordedBottomLayoutSupportConstraints.count, @"Failed to record bottomLayoutGuide constraints. Is the controller's view added to the view hierarchy?");
    [self.view removeConstraints:self.tt_recordedBottomLayoutSupportConstraints];

    NSArray *constraints =
    [TTLayoutSupportConstraint layoutSupportConstraintsWithView:self.view
                                              bottomLayoutGuide:self.bottomLayoutGuide];

    // todo: less hacky?
    self.tt_bottomConstraint = [constraints firstObject];

    [self.view addConstraints:constraints];
}

- (NSArray *)findLayoutSupportConstraintsFor:(id<UILayoutSupport>)layoutGuide
{
    NSMutableArray *recordedLayoutConstraints = [[NSMutableArray alloc] init];

    for (NSLayoutConstraint *constraint in self.view.constraints) {
        // I think an equality check is the fastest check we can make here
        // member check is to distinguish accidentally created constraints from _UILayoutSupportConstraints
        if (constraint.firstItem == layoutGuide && ![constraint isMemberOfClass:[NSLayoutConstraint class]]) {
            [recordedLayoutConstraints addObject:constraint];
        }
    }

    return recordedLayoutConstraints;
}

- (void)tt_updateInsets:(BOOL)adjustsScrollPosition
{
    // don't update scroll view insets if developer didn't want it
    if (!self.automaticallyAdjustsScrollViewInsets) {
        return;
    }

    UIScrollView *scrollView;

    if ([self respondsToSelector:@selector(tableView)]) {
        scrollView = ((UITableViewController *)self).tableView;
    } else if ([self respondsToSelector:@selector(collectionView)]) {
        scrollView = ((UICollectionViewController *)self).collectionView;
    } else {
        scrollView = (UIScrollView *)self.view;
    }

    if ([scrollView isKindOfClass:[UIScrollView class]]) {
        CGPoint previousContentOffset = CGPointMake(scrollView.contentOffset.x, scrollView.contentOffset.y + scrollView.contentInset.top);

        UIEdgeInsets insets = UIEdgeInsetsMake(self.tt_topLayoutGuideLength, 0, self.tt_bottomLayoutGuideLength, 0);
        scrollView.contentInset = insets;
        scrollView.scrollIndicatorInsets = insets;

        if (adjustsScrollPosition && previousContentOffset.y == 0) {
            scrollView.contentOffset = CGPointMake(previousContentOffset.x, -scrollView.contentInset.top);
        }
    }
}

@end

@implementation UIViewController (TTLayoutSupportPrivate)

- (NSLayoutConstraint *)tt_topConstraint
{
    return objc_getAssociatedObject(self, @selector(tt_topConstraint));
}

- (void)setTt_topConstraint:(NSLayoutConstraint *)constraint
{
    objc_setAssociatedObject(self, @selector(tt_topConstraint), constraint, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (NSLayoutConstraint *)tt_bottomConstraint
{
    return objc_getAssociatedObject(self, @selector(tt_bottomConstraint));
}

- (void)setTt_bottomConstraint:(NSLayoutConstraint *)constraint
{
    objc_setAssociatedObject(self, @selector(tt_bottomConstraint), constraint, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (NSArray *)tt_recordedTopLayoutSupportConstraints
{
    return objc_getAssociatedObject(self, @selector(tt_recordedTopLayoutSupportConstraints));
}

- (void)setTt_recordedTopLayoutSupportConstraints:(NSArray *)constraints
{
    objc_setAssociatedObject(self, @selector(tt_recordedTopLayoutSupportConstraints), constraints, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (NSArray *)tt_recordedBottomLayoutSupportConstraints
{
    return objc_getAssociatedObject(self, @selector(tt_recordedBottomLayoutSupportConstraints));
}

- (void)setTt_recordedBottomLayoutSupportConstraints:(NSArray *)constraints
{
    objc_setAssociatedObject(self, @selector(tt_recordedBottomLayoutSupportConstraints), constraints, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (void)setTt_observer:(id)tt_observer
{
    objc_setAssociatedObject(self, @selector(tt_observer), tt_observer, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (id)tt_observer
{
    return objc_getAssociatedObject(self, @selector(tt_observer));
}

-

//
//  TTLayoutSupportConstraint.h
//
//  Created by Steffen on 17.09.14.
//

#import <UIKit/UIKit.h>

@interface TTLayoutSupportConstraint : NSLayoutConstraint

+ (NSArray *)layoutSupportConstraintsWithView:(UIView *)view topLayoutGuide:(id<UILayoutSupport>)topLayoutGuide;

+ (NSArray *)layoutSupportConstraintsWithView:(UIView *)view bottomLayoutGuide:(id<UILayoutSupport>)bottomLayoutGuide;

@end

-

//
//  TTLayoutSupportConstraint.m
// 
//  Created by Steffen on 17.09.14.
//

#import "TTLayoutSupportConstraint.h"

@implementation TTLayoutSupportConstraint

+ (NSArray *)layoutSupportConstraintsWithView:(UIView *)view topLayoutGuide:(id<UILayoutSupport>)topLayoutGuide
{
    return @[
             [TTLayoutSupportConstraint constraintWithItem:topLayoutGuide
                                                 attribute:NSLayoutAttributeHeight
                                                 relatedBy:NSLayoutRelationEqual
                                                    toItem:nil
                                                 attribute:NSLayoutAttributeNotAnAttribute
                                                multiplier:1.0
                                                  constant:0.0],
             [TTLayoutSupportConstraint constraintWithItem:topLayoutGuide
                                                 attribute:NSLayoutAttributeTop
                                                 relatedBy:NSLayoutRelationEqual
                                                    toItem:view
                                                 attribute:NSLayoutAttributeTop
                                                multiplier:1.0
                                                  constant:0.0],
             ];
}

+ (NSArray *)layoutSupportConstraintsWithView:(UIView *)view bottomLayoutGuide:(id<UILayoutSupport>)bottomLayoutGuide
{
    return @[
             [TTLayoutSupportConstraint constraintWithItem:bottomLayoutGuide
                                                 attribute:NSLayoutAttributeHeight
                                                 relatedBy:NSLayoutRelationEqual
                                                    toItem:nil
                                                 attribute:NSLayoutAttributeNotAnAttribute
                                                multiplier:1.0
                                                  constant:0.0],
             [TTLayoutSupportConstraint constraintWithItem:bottomLayoutGuide
                                                 attribute:NSLayoutAttributeBottom
                                                 relatedBy:NSLayoutRelationEqual
                                                    toItem:view
                                                 attribute:NSLayoutAttributeBottom
                                                multiplier:1.0
                                                  constant:0.0],
             ];
}

@end