iOS编程(双语版)-视图-Autolayout代码初步

一谈到Autolayout,初学者肯定想到的是IB中使用拖拽啊，pin啊各种鼠标操作来进行添加各种约束。

今天我们要聊得是如何利用代码来添加视图间的约束。

我们来看一个例子：

(Objective-C代码)

UIView* v1 = [[UIView alloc] initWithFrame:CGRectMake(, , , )];
v1.backgroundColor = [UIColor colorWithRed: green:. blue: alpha:];
UIView* v2 = [UIView new];
v2.backgroundColor = [UIColor colorWithRed:. green: blue: alpha:];
UIView* v3 = [UIView new];
v3.backgroundColor = [UIColor colorWithRed: green: blue: alpha:];
[mainview addSubview: v1];
[v1 addSubview: v2];
[v1 addSubview: v3];

v2.translatesAutoresizingMaskIntoConstraints = NO;
v3.translatesAutoresizingMaskIntoConstraints = NO;

[v1 addConstraint:
    [NSLayoutConstraint
        constraintWithItem:v2 attribute:NSLayoutAttributeLeft
        relatedBy:
        toItem:v1 attribute:NSLayoutAttributeLeft
        multiplier: constant:]];

[v1 addConstraint:
    [NSLayoutConstraint
        constraintWithItem:v2 attribute:NSLayoutAttributeRight
        relatedBy:
        toItem:v1 attribute:NSLayoutAttributeRight
        multiplier: constant:]];

[v1 addConstraint:
    [NSLayoutConstraint
        constraintWithItem:v2 attribute:NSLayoutAttributeTop
        relatedBy:
        toItem:v1 attribute:NSLayoutAttributeTop
        multiplier: constant:]];

[v2 addConstraint:
    [NSLayoutConstraint
        constraintWithItem:v2 attribute:NSLayoutAttributeHeight
        relatedBy:
        toItem:nil attribute:
        multiplier: constant:]];

[v3 addConstraint:
    [NSLayoutConstraint
        constraintWithItem:v3 attribute:NSLayoutAttributeWidth
        relatedBy:
        toItem:nil attribute:
        multiplier: constant:]];

[v3 addConstraint:
    [NSLayoutConstraint
        constraintWithItem:v3 attribute:NSLayoutAttributeHeight
        relatedBy:
        toItem:nil attribute:
        multiplier: constant:]];

[v1 addConstraint:
    [NSLayoutConstraint
        constraintWithItem:v3 attribute:NSLayoutAttributeRight
        relatedBy:
        toItem:v1 attribute:NSLayoutAttributeRight
        multiplier: constant:]];

[v1 addConstraint:
    [NSLayoutConstraint
        constraintWithItem:v3 attribute:NSLayoutAttributeBottom
        relatedBy:
        toItem:v1 attribute:NSLayoutAttributeBottom
        multiplier: constant:]];

(Swift代码 iOS9)

let v1 = UIView(frame:CGRectMake(, , , ))
v1.backgroundColor = UIColor(red: , green: 0.4, blue: , alpha: )
let v2 = UIView()
v2.backgroundColor = UIColor(red: 0.5, green: , blue: , alpha: )
let v3 = UIView()
v3.backgroundColor = UIColor(red: , green: , blue: , alpha: )
mainview.addSubview(v1)
v1.addSubview(v2)
v1.addSubview(v3)

v2.translatesAutoresizingMaskIntoConstraints = false
v3.translatesAutoresizingMaskIntoConstraints = false

v1.addConstraint(
    NSLayoutConstraint(item: v2,
        attribute: .Leading,
        relatedBy: .Equal,
        toItem: v1,
        attribute: .Leading,
        multiplier: , constant: )
)

v1.addConstraint(
    NSLayoutConstraint(item: v2,
        attribute: .Trailing,
        relatedBy: .Equal,
        toItem: v1,
        attribute: .Trailing,
        multiplier: , constant: )
)

v1.addConstraint(
    NSLayoutConstraint(item: v2,
        attribute: .Top,
        relatedBy: .Equal,
        toItem: v1,
        attribute: .Top,
        multiplier: , constant: )
)

v2.addConstraint(
    NSLayoutConstraint(item: v2,
        attribute: .Height,
        relatedBy: .Equal,
        toItem: nil,
        attribute: .NotAnAttribute,
        multiplier: , constant: )
)

v3.addConstraint(
    NSLayoutConstraint(item: v3,
        attribute: .Width,
        relatedBy: .Equal,
        toItem: nil,
        attribute: .NotAnAttribute,
        multiplier: , constant: )
)

v3.addConstraint(
    NSLayoutConstraint(item: v3,
        attribute: .Height,
        relatedBy: .Equal,
        toItem: nil,
        attribute: .NotAnAttribute,
        multiplier: , constant: )
)

v1.addConstraint(
    NSLayoutConstraint(item: v3,
        attribute: .Trailing,
        relatedBy: .Equal,
        toItem: v1,
        attribute: .Trailing,
        multiplier: , constant: )
)

v1.addConstraint(
    NSLayoutConstraint(item: v3,
        attribute: .Bottom,
        relatedBy: .Equal,
        toItem: v1,
        attribute: .Bottom,
        multiplier: , constant: )
)

运行效果:

(竖屏)

(横屏)

看了以上代码后，你肯定要疯了，那么多约束。。。

下面，我们就来逐一分析：

我们先来看一下这段代码

OC

v3 = [[UIView alloc] initWithFrame:CGRectMake(v1.bounds.size.width-,
    v1.bounds.size.height-,
    , )];

Swift

let v3 = UIView(frame:CGRectMake(
    v1.bounds.width-, v1.bounds.height-, , ))

这段代码很清楚地表达了：v3是宽高各20，并且位置在v1的右下角，其原点距离v1的右下角

坐标x,y各偏移20，也就是我们上图中看到的大红色矩形。

约束的API语句有时候是很冗长的，看上去很难懂。

为此，Apple发明了可视化格式(Visual Format)来便于理解。

看看下面的这个例子：

@"V:|[v2(10)]"

上面的表达式中，V:表示是垂直方向上的约束，同理，H:表示水平方向上约束。

管道符|代表父视图。

中括号内是要添加约束的视图变量名。

所以，这里的约束清晰地表达： v2和父视图顶端对齐，并且v2的高度是10。