Content Hugging和Content Compress

AutoLayout 引入的两个概念，Content Hugging 和 Content Compression Resistance 这两个属性对有intrinsic content size的控件（例如button，label）非常重要。通俗的讲，具有intrinsic content size的控件自己知道（可以计算）自己的大小，例如一个label，当你设置text，font之后，其大小是可以计算到的
view中关于Content Hugging 和 Content Compression Resistance的方法有：

- (UILayoutPriority)contentCompressionResistancePriorityForAxis:(UILayoutConstraintAxis)axis API_AVAILABLE(ios(6.0));
- (void)setContentCompressionResistancePriority:(UILayoutPriority)priority forAxis:(UILayoutConstraintAxis)axis

- (UILayoutPriority)contentHuggingPriorityForAxis:(UILayoutConstraintAxis)axis API_AVAILABLE(ios(6.0));
- (void)setContentHuggingPriority:(UILayoutPriority)priority forAxis:(UILayoutConstraintAxis)axis API_AVAILABLE(ios(6.0));

大概的意思就是设置优先级的。

Hugging priority 确定view有多大的优先级阻止自己变大。

Compression Resistance priority确定有多大的优先级阻止自己变小。

很抽象，其实content Hugging就是要维持当前view在它的optimal size（intrinsic content size），可以想象成给view添加了一个额外的width constraint，此constraint试图保持view的size不让其变大：

view.width <= optimal size

此constraint的优先级就是通过上面的方法得到和设置的，content Hugging默认为250.

Content Compression Resistance就是要维持当前view在他的optimal size（intrinsic content size），可以想象成给view添加了一个额外的width constraint，此constraint试图保持view的size不让其变小：
view.width >= optimal size
此默认优先级为750.

还是很抽象，好吧，下面举个例子就明白了。

- (void)viewDidLoad {
    [super viewDidLoad];    
    UIButton   *button1 = [UIButton buttonWithType:UIButtonTypeSystem];
    button1.backgroundColor = [UIColor redColor];
    button1.translatesAutoresizingMaskIntoConstraints = NO;
    [button1 setTitle:@"button 1 button 2" forState:UIControlStateNormal];
    [button1 sizeToFit];
    
    [self.view addSubview:button1];
    
    NSLayoutConstraint *constraint = [NSLayoutConstraint constraintWithItem:button1 attribute:NSLayoutAttributeLeading relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeading multiplier:1.0f constant:100.0f];
    [self.view addConstraint:constraint];
    
    constraint = [NSLayoutConstraint constraintWithItem:button1 attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1.0f constant:100.0f];
    [self.view addConstraint:constraint];
    
    constraint = [NSLayoutConstraint constraintWithItem:button1 attribute:NSLayoutAttributeTrailing relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTrailing multiplier:1.0f constant:-150.0f];
    constraint.priority = 751.0f;
    [self.view addConstraint:constraint];

}

代码很容易理解，创建一个button，设置其constraint，使其距离顶部100，距离左边100，距离右边150，注意右边的constraint的优先级我们设置的为751.0，此时autolayout系统会去首先满足此constraint，再去满足Content Compression Resistance（其优先级为750，小于751）。

当我们把751变为749的时候，效果如下：

image.png
这时就是先满足了Content Compression Resistance的constraint，因为其优先级高。

下面举的例子为Content Hugging的例子，代码如下：

button1 = [UIButton buttonWithType:UIButtonTypeSystem];  
button1.backgroundColor = [UIColor redColor];
button1.translatesAutoresizingMaskIntoConstraints = NO;  
[button1 setTitle:@"button 1 button 2" forState:UIControlStateNormal];  
[button1 sizeToFit];  
  
[self.view addSubview:button1];  
  
NSLayoutConstraint *constraint = [NSLayoutConstraint constraintWithItem:button1 attribute:NSLayoutAttributeLeading relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeading multiplier:1.0f constant:100.0f];  
[self.view addConstraint:constraint];  
  
constraint = [NSLayoutConstraint constraintWithItem:button1 attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1.0f constant:100.0f];  
[self.view addConstraint:constraint];  
  
constraint = [NSLayoutConstraint constraintWithItem:button1 attribute:NSLayoutAttributeTrailing relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTrailing multiplier:1.0f constant:-10.0f];  
constraint.priority = 251.0f;  
[self.view addConstraint:constraint]; 

image.png

当把优先级priority改为249的时候，效果如下：

image.png