objective-c - iOS 使用Masonry库做动画时报错？

问题描述

问 题

1.实现过程:在VC viewDidload方法中 self.view addSubView:aLabel,viewWillLayoutSubviews 中使用Masonry添加label约束；另外界面有一个button,当当点击button时候做动画.

- (void)viewDidLoad {
    [super viewDidLoad];
    [self.view addSubview:self.nameLabel];
}
-(UILabel*)nameLabel{
    
    if (!_nameLabel) {
        _nameLabel = [[UILabel alloc]initWithFrame:CGRectZero];
        _nameLabel.lineBreakMode = NSLineBreakByTruncatingTail;
        _nameLabel.text = @"王二小";
        _nameLabel.backgroundColor = [UIColor greenColor];
        _nameLabel.textAlignment = NSTextAlignmentLeft;
    }
    
    return _nameLabel;
}

-(void)viewWillLayoutSubviews{
    [self addCContraints];
}

-(void)addCContraints{
    
    [self.nameLabel mas_makeConstraints:^(MASConstraintMaker *make) {
        make.left.equalTo(self.view.mas_left).offset(10);
        make.top.equalTo(self.view.mas_top).offset(10);
    }];
}


//点击button的响应时间.

-(void)animationAction{
    
    ///设置长度.
    [self.nameLabel mas_updateConstraints:^(MASConstraintMaker *make) {
        make.left.mas_equalTo(-30);
    }];

    [UIView animateWithDuration:0.3 animations:^{
        [self.nameLabel.superview layoutIfNeeded];
    }];
    
}

**"约束冲突信息:"**

2017-08-09 22:24:47.196344 NSString[13501:3837010] [LayoutConstraints] 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:0x1700b4640 UILabel:0x10dd09830.left == UIView:0x10de0c4d0.left - 30>",
    "<MASLayoutConstraint:0x1700b5ae0 UILabel:0x10dd09830.left == UIView:0x10de0c4d0.left + 10>"
)

Will attempt to recover by breaking constraint 
<MASLayoutConstraint:0x1700b5ae0 UILabel:0x10dd09830.left == UIView:0x10de0c4d0.left + 10>

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.

跟了下Masonry执行过程也没发现什么问题.有朋友遇到过这种问题么？感谢在先.

解决方案

首先, self.View显示时会调用viewWillLayoutSubviews,然后点击button时,执行animateWithDuration, 由于修改了约束, 控制器还会执行viewWillLayoutSubviews一次.
也就是说从显示到点击, 一共viewWillLayoutSubviews会执行两次.

虽然mas_updateConstraints不会添加新的left约束, 但是由于mas_updateConstraints执行后再调用layoutIfNeeded, 那么self.nameLabel内部会有left约束=-30(mas_updateConstraints)和left约束=View+10(layoutIfNeeded调用viewWillLayoutSubviews再次添加)同时存在, 所以系统报约束重复警告!

不要在viewWillLayoutSubviews上执行添加约束方法, 因为会执行多次,可以改在viewDidLoad上添加.

- (void)viewDidLoad {
    [super viewDidLoad];
    [self.view addSubview:self.nameLabel];
    [self addCContraints];
}

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