我目前很难理解为什么以下单元测试在iPad 2上失败.自动布局似乎略微(0.5分)在超视图内部的错位置视图相对于两个布局约束所要求的精确对中.似乎特别奇怪的是,关键的测试(但最后的断言)通过iPhone 5,所以明显的舍入误差仅影响一个(iOS 6)平台.这里发生了什么？

更新1我已经改变了代码,以确保两个框架在宽度和高度方面都受到足够的约束,即使转换为AutotoresizingMaskIntoConstraints为NO,建议作为可能的相关补救措施here.但是,这显然不会改变这种情况.

@H_502_4@#import "BugTests.h" @implementation BugTests - (void)testCenteredLayout { UIView *superview = [[UIView alloc] initWithFrame:CGRectMake(0,768,88)]; superview.autoresizingMask = UIViewAutoresizingFlexibleTopMargin | UIViewAutoresizingFlexibleBottomMargin; superview.translatesAutoresizingMaskIntoConstraints = YES; UILabel *view = [[UILabel alloc] initWithFrame:CGRectMake(0,0)]; view.text = @"Single Round against iPad."; view.autoresizingMask = UIViewAutoresizingFlexibleTopMargin | UIViewAutoresizingFlexibleBottomMargin; view.translatesAutoresizingMaskIntoConstraints = NO; [view addConstraint:[NSLayoutConstraint constraintWithItem:view attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:0.0 constant:206.0]]; [view addConstraint:[NSLayoutConstraint constraintWithItem:view attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:0.0 constant: 21.0]]; [superview addSubview:view]; [superview addConstraint:[NSLayoutConstraint constraintWithItem:superview attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:view attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:0.0]]; [superview addConstraint:[NSLayoutConstraint constraintWithItem:superview attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:view attribute:NSLayoutAttributeCenterY multiplier:1.0 constant:0.0]]; STAssertEquals(superview.center,CGPointMake(384,44),nil); // succeeds STAssertEquals(view.center,CGPointMake( 0,0),nil); // succeeds [superview setNeedsLayout]; [superview layoutIfNeeded]; STAssertTrue(!superview.hasAmbiguousLayout,nil); STAssertEquals(superview.frame.size,CGSizeMake(768,88),nil); // succeeds STAssertEquals(view.frame.size,CGSizeMake(206,21),nil); // succeeds STAssertEquals(superview.center,view.center,nil); // fails: why? STAssertEquals(view.center,44.5),nil); // succeeds: why? } @end

更新2我在另一个单元测试中孤立了另一个(显然)同一个问题的实例.这一次它涉及顶部(而不是中心)的约束,而这次分数点坐标似乎是触发器. (测试也可以在Retina设备上成功,例如y = 951,即一个奇点坐标).我已经检查了各种模拟器配置(我的物理iPad 2和iPhone 5旁边)的确发生似乎与没有一个Ratina显示. (再次感谢@ArkadiuszHolko领先.)

我目前从这些测试的感觉是,如果需要在Retina显示器上进行点精确自动布局,则必须避免奇异的高度和小数y坐标.但为什么？

@H_502_4@- (void)testNonRetinaAutoLayoutProblem2 { UIView *superview = [[UIView alloc] initWithFrame:CGRectMake(0,1004)]; superview.autoresizingMask = UIViewAutoresizingFlexibleRightMargin | UIViewAutoresizingFlexibleBottomMargin; superview.translatesAutoresizingMaskIntoConstraints = YES; CGFloat y = 950.5; // see e.g. pageControlTopConstraint UIView *view = [[UIView alloc] initWithFrame:CGRectMake(0,0)]; view.translatesAutoresizingMaskIntoConstraints = NO; [superview addConstraint:[NSLayoutConstraint constraintWithItem:view attribute:NSLayoutAttributeLeading relatedBy:NSLayoutRelationEqual toItem:superview attribute:NSLayoutAttributeLeading multiplier:1.0 constant:0.0]]; [superview addConstraint:[NSLayoutConstraint constraintWithItem:view attribute:NSLayoutAttributeTrailing relatedBy:NSLayoutRelationEqual toItem:superview attribute:NSLayoutAttributeTrailing multiplier:1.0 constant:0.0]]; [superview addConstraint:[NSLayoutConstraint constraintWithItem:view attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:superview attribute:NSLayoutAttributeTop multiplier:1.0 constant:y]]; [superview addConstraint:[NSLayoutConstraint constraintWithItem:view attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:0.0 constant:8]]; [superview addSubview:view]; [superview setNeedsLayout]; [superview layoutIfNeeded]; STAssertTrue(!superview.hasAmbiguousLayout,nil); STAssertTrue(!view.hasAmbiguousLayout,nil); STAssertEquals(superview.frame,CGRectMake(0,1004),nil); // succeeds STAssertEquals(view.frame,y,8),nil); // fails: why? STAssertEquals(view.frame,y + 0.5,nil); // succeeds: why? }