由于iOS運行在不同設備之上，因而作為代碼搬運工的我們經常需要對不同的屏幕尺寸進行適配。我們常用的方法有NSLayoutConnstraints類、xib或storybroad、Masonry等第三方庫。

xib或story broad

xib或storyborad.png

NSLayoutConstraints類

1. 常用創建方法

    // 創建按鈕
    _button1 = [UIButton new];
    [_button1 setTitle:@"點我" forState:0];
    [_button1 setTitleColor:[UIColor blackColor] forState:0];
    [_button1 addTarget:self action:@selector(updateConstraints) forControlEvents:UIControlEventTouchUpInside];
    // 關閉自動約束，必須關閉
    _button1.translatesAutoresizingMaskIntoConstraints = NO;
    [self.view addSubview:_button1];

    _button2 = [UIButton new];
    [_button2 setTitleColor:[UIColor blackColor] forState:0];
    [_button2 setTitle:@"就不點" forState:0];
    // 關閉自動約束
    _button2.translatesAutoresizingMaskIntoConstraints = NO;
    [self.view addSubview:_button2];

    // 創建約束
    //當約束與其他試圖相關時，參數multiplier不能為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];
    
    // 添加自身相關約束
    [_button1 addConstraints:@[widthConstraint, heightConstraint]];
    [_button2 addConstraints:@[width1Constraint, height1Constraint]];
    // 添加間距等約束
    [self.view addConstraints:@[topConstraint, leftConstraint, top1Constraint, left1Constraint]];

constraintWithItem.png

2. 通過指定格式創建約束

    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左對齊
//    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];

這句代碼中@"H:|-(30)-[button1]-(30)-[button2(button1)]-(30)-|" "所表示的意思是：_button1與父視圖的左邊間距是30，_button1的寬度為width(width的值即后面字典中的值)；_button2與_button1的左間距是0，寬度和_button1的寬度相等。

參數解釋及看法

• format：可視樣式字符串，具體字符釋義如下：
  "H:" 、"V:" ：表示這是水平或垂 直方向的約束
  "|"：表示父視圖的邊界
  "-()-"：表示間距
  "[]"：表示在水平上的寬度或垂直上的高度
• options：format中約束的規則，常置為0
• metrics：字符串中各數值的字典。key為字符串中使用名稱，value為代表的值(NSNumber)
• views：相同層級的視圖的字典。key為字符串中使用的名稱，value為對應的視圖
• 看法：由于format中只有H、V作為約束方向，所以一次只能創建一條水平線或垂線上的控件之間的約束；設置對齊等效果時代碼繁瑣，不如Masonry、constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:方便，所以這種創建方法不應廣泛使用

Masonry等第三方庫

使用第三方進行手寫約束，目前我就使用過Masonry。所以一下代碼為Masonry的布局代碼。

    __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

更新約束

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];
        }];

找到你需要改變的約束對象，然后改變其constant值，最后調用updateConstraintsIfNeeded。

2. Masonry

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

Masonry更新約束需調用mas_updateConstraints:，在block中重新設置需要改變的約束即可。

參考資料

• 使用Auto Layout中的VFL(Visual format language)--代碼實現自動布局
• Masonry Github倉庫
• NSLayoutConstraintDemo
• 注意：本文所有代碼都是在XCode 8 的環境下編寫的；若有不對之處歡迎指出。