为什么正确的约束有错误和宽度约束使用砖石效果很好？答案

【问题标题】：why right constraint got error and width constraint work well using masonry?为什么正确的约束有错误和宽度约束使用砖石效果很好？
【发布时间】：2015-11-10 03:22:18
【问题描述】：

我使用砖石来布局我的 UI。

在我的viewController中，我在tableView中添加了一个headView，在headView中，添加了一个按钮。

当我向按钮添加右约束时，它在下面出现警告/错误：

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. 
(
    "<MASLayoutConstraint:0x7ff5fa9ead00 UIButton:0x7ff5fa9e4ae0.left == UIView:0x7ff5fa9dd780.left + 20>",
    "<MASLayoutConstraint:0x7ff5fa9eae80 UIButton:0x7ff5fa9e4ae0.right == UIView:0x7ff5fa9dd780.right - 20>",
    "<NSLayoutConstraint:0x7ff5fad3ac10 UIView:0x7ff5fa9dd780.width ==>"
)

Will attempt to recover by breaking constraint 
<MASLayoutConstraint:0x7ff5fa9eae80 UIButton:0x7ff5fa9e4ae0.right == UIView:0x7ff5fa9dd780.right - 20>

Make a symbolic breakpoint at UIViewAlertForUnsatisfiableConstraints to catch this in the debugger.
The methods in the UIConstraintBasedLayoutDebugging category on UIView listed in <UIKit/UIView.h> may also be helpful.

我的代码如下：

-(void)viewDidLoad{
    [super viewDidLoad];

    self.tableView = [[UITableView alloc] initWithFrame:CGRectZero style:UITableViewStylePlain];
    self.tableView.backgroundColor = [UIColor whiteColor];
    self.tableView.delegate = self;
    self.tableView.dataSource = self;
    self.tableView.showsHorizontalScrollIndicator = NO;
    self.tableView.showsVerticalScrollIndicator = NO;
    self.tableView.separatorStyle = UITableViewCellSeparatorStyleNone;
    [self.view addSubview:self.tableView];

    UIView * headView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, MainScreenWidth, 438)];
    headView.backgroundColor = [UIColor whiteColor];
    self.headView = headView;
    self.tableView.tableHeaderView = headView;


    UIButton * button = [UIButton buttonWithType:UIButtonTypeCustom];
    [self.headView addSubview:button];
    [button setTitle:@"a button" forState:UIControlStateNormal];
    [button addTarget:self action:@selector(clickThawButton:) forControlEvents:UIControlEventTouchUpInside];
    button.clipsToBounds = YES;
    button.layer.cornerRadius = 5.0f;
    [button mas_makeConstraints:^(MASConstraintMaker *make) {
        make.left.equalTo(@20);
        //make.right.equalTo(@-20);  // The error line
        make.width.equalTo(@(MainScreenWidth - 40)); //I slove the problem by this.
        make.top.equalTo(otherView.mas_bottom).offset(25);
        make.height.equalTo(@55);
    }];

}

虽然我通过约束宽度解决了这个问题，但我想知道为什么会发生这种情况。或者有什么建议吗？

【问题讨论】：

同时设置左右和宽度约束是否会出错？还是在设置左右约束时出错？
未设置在一起。我设置了左右，它得到了错误。我设置了 left,width 并且效果很好。
根据日志消息，<NSLayoutConstraint:0x7ff5fad3ac10 UIView:0x7ff5fa9dd780.width ==> 与right space with button 冲突，因为按钮大小不能为负数（小于零）。不知何故，button 的超级视图的宽度设置为低于 20，这使得按钮的宽度为负。尝试将 tableView 的 frameWidth 设置为 20 以上。

标签： ios objective-c autolayout masonry

【解决方案1】：

您的约束应如下所示：

[button mas_makeConstraints:^(MASConstraintMaker *make) {
        make.left.equalTo(self.headView).with.offset(20);
        make.right.equalTo(self.headView).with.offset(-20);
        make.top.equalTo(otherView.mas_bottom).offset(25);
        make.height.equalTo(@55);
 }];

你想用-20 偏移将按钮的右边缘捕捉到它的父视图，而不是用x 坐标等于-20 的点

【讨论】：