学习随记 - AutoLayout

由于iOS运行在不同设备之上，因而作为代码搬运工的我们经常需要对不同的屏幕尺寸进行适配。我们常用的方法有NSLayoutConnstraints类、xib或storybroad、Masonry等第三方库。

xib或story broad

xib或storyborad.png

NSLayoutConstraints类

1. 常用创建方法

    // 创建按钮
    _button1 = [UIButton new];
    [_button1 setTitle:@"点我" forState:0];
    [_button1 setTitleColor:[UIColor blackColor] forState:0];
    [_button1 addTarget:self action:@selector(updateConstraints) forControlEvents:UIControlEventTouchUpInside];
    // 关闭自动约束，必须关闭
    _button1.translatesAutoresizingMaskIntoConstraints = NO;
    [self.view addSubview:_button1];

    _button2 = [UIButton new];
    [_button2 setTitleColor:[UIColor blackColor] forState:0];
    [_button2 setTitle:@"就不点" forState:0];
    // 关闭自动约束
    _button2.translatesAutoresizingMaskIntoConstraints = NO;
    [self.view addSubview:_button2];

    // 创建约束
    //当约束与其他试图相关时，参数multiplier不能为0
    NSLayoutConstraint * topConstraint = [NSLayoutConstraint constraintWithItem:_button1 attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1 constant:30];
    NSLayoutConstraint * leftConstraint = [NSLayoutConstraint constraintWithItem:_button1 attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1 constant:30];
    NSLayoutConstraint * widthConstraint = [NSLayoutConstraint constraintWithItem:_button1 attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:0 multiplier:0 constant:70];
    NSLayoutConstraint * heightConstraint = [NSLayoutConstraint constraintWithItem:_button1 attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:0 multiplier:0 constant:40];
    
    NSLayoutConstraint * top1Constraint = [NSLayoutConstraint constraintWithItem:_button2 attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1 constant:130];
    NSLayoutConstraint * left1Constraint = [NSLayoutConstraint constraintWithItem:_button2 attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1 constant:30];
    NSLayoutConstraint * width1Constraint = [NSLayoutConstraint constraintWithItem:_button2 attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:0 multiplier:0 constant:70];
    NSLayoutConstraint * height1Constraint = [NSLayoutConstraint constraintWithItem:_button2 attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:0 multiplier:0 constant:40];
    
    // 添加自身相关约束
    [_button1 addConstraints:@[widthConstraint, heightConstraint]];
    [_button2 addConstraints:@[width1Constraint, height1Constraint]];
    // 添加间距等约束
    [self.view addConstraints:@[topConstraint, leftConstraint, top1Constraint, left1Constraint]];

constraintWithItem.png

2. 通过指定格式创建约束

    NSDictionary * viewsDic = @{@"button1":_button1, @"button2":_button2};

    NSArray * hConstraints = [NSLayoutConstraint constraintsWithVisualFormat:@"H:|-(30)-[button1]-(30)-[button2(button1)]-(30)-|" options:0 metrics:@{@"width":@(70)} views:viewsDic];
    NSArray * vConstraints = [NSLayoutConstraint constraintsWithVisualFormat:@"V:|-(30)-[button1(height)]-(30)-[button2(40)]" options:0 metrics:@{@"height":@(40)} views:viewsDic];
    
    // button1和button2左对齐
//    CGFloat W = self.view.frame.size.width;
//    NSArray * hConstraints = [NSLayoutConstraint constraintsWithVisualFormat:@"H:|-(30)-[button1(width)]" options:0 metrics:@{@"width":@(W - 60)} views:viewsDic];
//    NSArray * h2Constraints = [NSLayoutConstraint constraintsWithVisualFormat:@"H:|-(30)-[button2(width)]" options:0 metrics:@{@"width":@(W - 60)} views:viewsDic];
//    NSArray * vConstraints = [NSLayoutConstraint constraintsWithVisualFormat:@"V:|-(30)-[button1(height)]-(30)-[button2(40)]" options:0 metrics:@{@"height":@(40)} views:viewsDic];
    
//    [self.view addConstraints:h2Constraints];
    [self.view addConstraints:hConstraints];
    [self.view addConstraints:vConstraints];

constraintWithFormat.png

例：

NSArray * hConstraints = [NSLayoutConstraint constraintsWithVisualFormat:@"H:|-(30)-[button1]-(30)-[button2(button1)]-(30)-|" options:0 metrics:@{@"width":@(70)} views:viewsDic];

这句代码中@"H:|-(30)-[button1]-(30)-[button2(button1)]-(30)-|" "所表示的意思是：_button1与父视图的左边间距是30，_button1的宽度为width(width的值即后面字典中的值)；_button2与_button1的左间距是0，宽度和_button1的宽度相等。

参数解释及看法

• format：可视样式字符串，具体字符释义如下：
  "H:" 、"V:" ：表示这是水平或垂 直方向的约束
  "|"：表示父视图的边界
  "-()-"：表示间距
  "[]"：表示在水平上的宽度或垂直上的高度
• options：format中约束的规则，常置为0
• metrics：字符串中各数值的字典。key为字符串中使用名称，value为代表的值(NSNumber)
• views：相同层级的视图的字典。key为字符串中使用的名称，value为对应的视图
• 看法：由于format中只有H、V作为约束方向，所以一次只能创建一条水平线或垂线上的控件之间的约束；设置对齐等效果时代码繁琐，不如Masonry、constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:方便，所以这种创建方法不应广泛使用

Masonry等第三方库

使用第三方进行手写约束，目前我就使用过Masonry。所以一下代码为Masonry的布局代码。

    __weak typeof(self) weakSelf = self;
    
    [_button1 mas_makeConstraints:^(MASConstraintMaker *make) {
        make.top.equalTo(weakSelf.view.mas_top).offset(30);
        make.left.equalTo(weakSelf.view.mas_left).offset(30);
        make.width.mas_equalTo(70);
        make.height.equalTo(@40);
    }];
    
    [_button2 mas_makeConstraints:^(MASConstraintMaker *make) {
        make.top.equalTo(weakSelf.button1.mas_bottom).offset(30);
        make.left.equalTo(weakSelf.button1.mas_left);
        make.width.equalTo(weakSelf.button1.mas_width);
        make.height.equalTo(weakSelf.button1.mas_height);
    }];

constraintWithMasonry.png
如上代码表示button1与父视图的左上间距都为30，宽为70，高为40；其中宽高都为常量，但我们使用了两个语法：mas_equalTo()括号中需要传入的值为基本类型，而equalTo()中需要传入的值为id类型。

更新约束

1. NSLayoutConstraint

[UIView animateWithDuration:1 animations:^{
            for (NSLayoutConstraint * constraint in weakSelf.view.constraints) {
                if (constraint.firstItem == weakSelf.button1 && constraint.firstAttribute == NSLayoutAttributeTop) {
                    constraint.constant = 100;
                    break;
                }
            }
            [weakSelf.view updateConstraintsIfNeeded];
        }];

找到你需要改变的约束对象，然后改变其constant值，最后调用updateConstraintsIfNeeded。

2. Masonry

  [UIView animateWithDuration:1 animations:^{
            [weakSelf.button1 mas_updateConstraints:^(MASConstraintMaker *make) {
                make.top.equalTo(weakSelf.view.mas_top).offset(100);
            }];
        }];

Masonry更新约束需调用mas_updateConstraints:，在block中重新设置需要改变的约束即可。

参考资料

• 使用Auto Layout中的VFL(Visual format language)--代码实现自动布局
• Masonry Github仓库
• NSLayoutConstraintDemo
• 注意：本文所有代码都是在XCode 8 的环境下编写的；若有不对之处欢迎指出。