随着iPhone设备出现了不同的尺寸，iOS6.0以后storyboard和xib文件新增了一个Use Auto Layout选项，用来实现自动布局。当你勾选上这个选项，以前传统的布局方式将不能正常工作。一旦使用了自动布局,就要忘记 Frame 的概念!
要判断UIView是否可以使用自动布局，可以使用如下方法：
if ([self.view respondsToSelector:@selector(addConstraints:)])
{
//自动布局
}
else
{
//传统布局
}
可以通过-layoutIfNeeded和-setNeedsUpdateConstraints两个方法来刷新约束的改变，使UIView重新布局。layoutIfNeeded是调整布局，也就是view的位置，一般是对subviews作用。setNeedsDisplay涉及到redraw，也就是重绘，一般都是对receiver作用。
layoutIfNeeded
使用此方法强制立即进行layout,从当前view开始，此方法会遍历整个view层次(包括superviews)请求layout。因此，调用此方法会强制整个view层次布局。
setNeedsUpdateConstraints
当一个自定义view的某个属性发生改变，并且可能影响到constraint时，需要调用此方法去标记constraints需要在未来的某个点更新，系统然后调用updateConstraints.
添加约束一般要遵循下面的规则：
对于两个同层级view之间的约束关系，添加到他们的父view上：
对于两个不同层级view之间的约束关系，添加到他们最近的共同父view上：
对于有层次关系的两个view之间的约束关系，添加到层次较高的父view上：
下面的例子演示了通过设定控件边界距离，从而确定控件尺寸：
UIButton *button = [UIButton buttonWithType:UIButtonTypeRoundedRect];
[button setTitle:@"测试按钮" forState:UIControlStateNormal];
[button sizeToFit];
[button setBackgroundColor:[UIColor yellowColor]];
button.translatesAutoresizingMaskIntoConstraints = NO;
[self.view addSubview:button];
NSLayoutConstraint *constraint;
//上边距
constraint = [NSLayoutConstraint
constraintWithItem:button
attribute:NSLayoutAttributeTop
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeTop
multiplier:1.0f
constant:50.0f];
[self.view addConstraint:constraint];
//左边距
constraint = [NSLayoutConstraint
constraintWithItem:button
attribute:NSLayoutAttributeLeading
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeLeading
multiplier:1.0f
constant:100.0f];
[self.view addConstraint:constraint];
//右边距
constraint = [NSLayoutConstraint
constraintWithItem:button
attribute:NSLayoutAttributeTrailing
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeTrailing
multiplier:1.0f
constant:-100.0f];
[self.view addConstraint:constraint];
//下边距
constraint = [NSLayoutConstraint
constraintWithItem:button
attribute:NSLayoutAttributeBottom
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeBottom
multiplier:1.0f
constant:-350.0f];
[self.view addConstraint:constraint];
下面的例子设定了控件在父容器中水平垂直居中：
UIButton *button = [UIButton buttonWithType:UIButtonTypeRoundedRect];
[button setTitle:@"测试按钮" forState:UIControlStateNormal];
[button sizeToFit];
[button setBackgroundColor:[UIColor yellowColor]];
button.translatesAutoresizingMaskIntoConstraints = NO;
[self.view addSubview:button];
NSLayoutConstraint *constraint;
//水平居中
constraint = [NSLayoutConstraint
constraintWithItem:button
attribute:NSLayoutAttributeCenterX
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeCenterX
multiplier:1.0f
constant:0.0f];
[self.view addConstraint:constraint];
//垂直居中
constraint = [NSLayoutConstraint
constraintWithItem:button
attribute:NSLayoutAttributeCenterY
relatedBy:NSLayoutRelationEqual
toItem:self.view
attribute:NSLayoutAttributeCenterY
multiplier:1.0f
constant:0.0f];
[self.view addConstraint:constraint];
下面的例子直接设定了控件的尺寸：
UIButton *button = [UIButton buttonWithType:UIButtonTypeRoundedRect];
[button setTitle:@"测试按钮" forState:UIControlStateNormal];
[button sizeToFit];
[button setBackgroundColor:[UIColor yellowColor]];
button.translatesAutoresizingMaskIntoConstraints = NO;
[self.view addSubview:button];
NSLayoutConstraint *constraint;
//设置宽度
constraint = [NSLayoutConstraint constraintWithItem:button attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0f constant:130.0f];
[self.view addConstraint:constraint];
//设置高度
constraint = [NSLayoutConstraint constraintWithItem:button attribute:NSLayoutAttributeHeight relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:1.0f constant:70.0f];
[self.view addConstraint:constraint];
除了上面添加约束的方法，还有一种全新的方法：Visual Format Language(可视格式语言)，这种语言是对视觉描述的一种抽象。基本用法如下：
UIButton *buttonA = [UIButton buttonWithType:UIButtonTypeRoundedRect];
[buttonA setTitle:@"AAA" forState:UIControlStateNormal];
[buttonA sizeToFit];
[buttonA setBackgroundColor:[UIColor yellowColor]];
buttonA.translatesAutoresizingMaskIntoConstraints = NO;
[self.view addSubview:buttonA];
UIButton *buttonB = [UIButton buttonWithType:UIButtonTypeRoundedRect];
[buttonB setTitle:@"BBB" forState:UIControlStateNormal];
[buttonB sizeToFit];
[buttonB setBackgroundColor:[UIColor yellowColor]];
buttonB.translatesAutoresizingMaskIntoConstraints = NO;
[self.view addSubview:buttonB];
NSMutableArray *array = [NSMutableArray array];
//水平方向布局(从左向右)
[array addObjectsFromArray:[NSLayoutConstraint
constraintsWithVisualFormat:@"|-60-[buttonA(==90)]-30-[buttonB]"
options:NSLayoutFormatDirectionLeadingToTrailing
metrics:nil
views:NSDictionaryOfVariableBindings(buttonA,buttonB)]];
//垂直方向布局(从上向下)
[array addObjectsFromArray:[NSLayoutConstraint
constraintsWithVisualFormat:@"V:|-100-[buttonB]-50-[buttonA]"
options:NSLayoutFormatDirectionLeadingToTrailing
metrics:nil
views:NSDictionaryOfVariableBindings(buttonA,buttonB)]];
[self.view addConstraints:array];
constraintsWithVisualFormat方法的最后一个参数views需要提供一个字典，用来指明可视化字符串里出现控件名所对应的控件：
NSDictionary *viewsDictionary = NSDictionaryOfVariableBindings(buttonA,buttonB);
for(NSString *key in viewsDictionary)
{
NSLog(@"key:%@, value:%@",key,viewsDictionary[key]);
}
//key:buttonA, value:<UIButton: 0x8dc5850; frame = (0 0; 30 30); opaque = NO; layer = <CALayer: 0x8dc7270>>
//key:buttonB, value:<UIButton: 0x8dc8a90; frame = (0 0; 32 30); opaque = NO; layer = <CALayer: 0x8dc8b80>>
下面是两个具有代表性的语句示例：
@"|-50-[buttonA(80@100)]-[buttonB(90@200)]-50-|"
这条语句的含义是：“左右边距都为50，中间有两个按钮，相隔缺省宽度，一个控件宽度为80，约束优先级为100；另一个控件宽度为90，约束优先级为200”。实际运行后，发现buttonB的控件宽度为90，而buttonA的宽度为自适应宽度，并不是80像素；这是因为buttonB的约束优先级200大于buttonA的约束优先级，所以优先生效。可以把buttonA的优先级改的比buttonB大，就可以看到完全相反的结果。
@"V:[buttonA(80)]-20-[buttonB(==buttonA)]"
这条语句的含义是：“垂直方向有一个高度为80的buttonA，然后间隔20有一个和buttonA同样高度的buttonB”