Notes of learning AutoLayout

在XCode5中，如果我们添加一个Button或者Label，或者其他的什么标准View，而不设置任何constraints，IB会自动生成constraints，而这些constraints是fixed，无法根据intrinsic content size的变化而变化，这并不是我们想要的。比如，我在View中添加一个label, label的title是Label，另加一个Button，点击Button后改变label的title为很长的文字，并且调用invalidateInstrinsicContentSize,在执行前后，分别打印constraints，代码如下：

    NSLog(@"current constraints %@ ", [[self textLabel] constraintsAffectingLayoutForAxis:UILayoutConstraintAxisHorizontal]);
    NSLog(@"current constraints %@ ", [[self textLabel] constraintsAffectingLayoutForAxis:UILayoutConstraintAxisVertical]);

    [[self textLabel] setText:@"weuituwjaskdtuqwieotuasdjgakleutaiosgajksdgequwtioasdjgkasjdgkajskgd"];
    [[self textLabel] invalidateIntrinsicContentSize];
    NSLog(@"current constraints %@ ", [[self textLabel] constraintsAffectingLayoutForAxis:UILayoutConstraintAxisHorizontal]);
    NSLog(@"current constraints %@ ", [[self textLabel] constraintsAffectingLayoutForAxis:UILayoutConstraintAxisVertical]);

这时看到的输出是

2014-07-04 17:46:28.580 AutoLayoutExample[1110:70b] current constraints (
    "<NSIBPrototypingLayoutConstraint:0x8b95fb0 'IB auto generated at build time for view with fixed frame' H:|-(179)-[UILabel:0x8b93e80](LTR)   (Names: '|':UIView:0x8b95290 )>",
    "<NSIBPrototypingLayoutConstraint:0x8b960e0 'IB auto generated at build time for view with fixed frame' H:[UILabel:0x8b93e80(42)]>"
)
2014-07-04 17:46:28.582 AutoLayoutExample[1110:70b] current constraints (
    "<NSIBPrototypingLayoutConstraint:0x8b95d60 'IB auto generated at build time for view with fixed frame' V:|-(147)-[UILabel:0x8b93e80]   (Names: '|':UIView:0x8b95290 )>",
    "<NSIBPrototypingLayoutConstraint:0x8b96110 'IB auto generated at build time for view with fixed frame' V:[UILabel:0x8b93e80(21)]>"
) 

2014-07-04 17:46:28.586 AutoLayoutExample[1110:70b] current constraints (
    "<NSIBPrototypingLayoutConstraint:0x8b95fb0 'IB auto generated at build time for view with fixed frame' H:|-(179)-[UILabel:0x8b93e80](LTR)   (Names: '|':UIView:0x8b95290 )>",
    "<NSIBPrototypingLayoutConstraint:0x8b960e0 'IB auto generated at build time for view with fixed frame' H:[UILabel:0x8b93e80(42)]>"
)
2014-07-04 17:46:28.587 AutoLayoutExample[1110:70b] current constraints (
    "<NSIBPrototypingLayoutConstraint:0x8b95d60 'IB auto generated at build time for view with fixed frame' V:|-(147)-[UILabel:0x8b93e80]   (Names: '|':UIView:0x8b95290 )>",
    "<NSIBPrototypingLayoutConstraint:0x8b96110 'IB auto generated at build time for view with fixed frame' V:[UILabel:0x8b93e80(21)]>"
)

可以看出IB产生的是IB auto generated at build time for view with fixed frame，无论intrinsic content size如何变化，都无济于事。所以，在使用AutoLayout时，切记：每个元素都应该有能完全确定size和位置的constraints， 如果有一样不能确定，IB都会提示Error 或者Warning，而这些Error或者Warning也是必须解决的。

关于两种PlaceHolder

placeholder constraits 和placeholder instrinsic content size

placeholder constraits是说这个constraint只是个placeholder，在build时就会被去掉，这样可以阻止IB自动生成constraints；placeholder instrinsic content size是說在IB中设计时使用的size，在runtime时则需要调用intrinsicContentSize方法来确定。

下面说说intrinsicContentSize和constraint的关系：intrinsicContentSize和constraint是Auto Layout的支柱概念，缺一不可。同时有时intrinsicContentSize可以在某个方面也是一种constraint，通常unambiguous layout generally requires setting two attributes in each axis（也就是在一个维度上需要设置起点和长度两个信息）. When a view has an intrinsic content size, that size accounts for one of the two attributes.（如果一个view 有intrinsicContentSize，并且设计者想这样，intrinsicContentSize是可以起到那个长度的限制作用的） You can, for example, place a text-based control or an image view in the center of its superview, and its layout will not be ambiguous. The intrinsic content size plus the location combine for a fully specified placement. 能够这样的前提是1. 使用系统的标准View，2. 在IB中指定Intrinsic size时需要用Default（System defined），比如如果只指定位置信息，不指定size，使用system default intrinsic size，时，输出时这样的

2014-07-04 22:25:40.866 AutoLayoutExample[2237:70b] current constraints (
    "<NSLayoutConstraint:0x8a6bde0 H:|-(179)-[UILabel:0x8a6a010]   (Names: '|':UIView:0x8a6b400 )>",
    "<NSContentSizeLayoutConstraint:0x8b847a0 H:[UILabel:0x8a6a010(42)] Hug:251 CompressionResistance:750>"
)
2014-07-04 22:25:40.868 AutoLayoutExample[2237:70b] current constraints (
    "<NSLayoutConstraint:0x8a6bd30 V:[_UILayoutGuide:0x8a6b4b0]-(127)-[UILabel:0x8a6a010]>",
    "<_UILayoutSupportConstraint:0x8a67660 V:[_UILayoutGuide:0x8a6b4b0(20)]>",
    "<_UILayoutSupportConstraint:0x8a6a300 V:|-(0)-[_UILayoutGuide:0x8a6b4b0]   (Names: '|':UIView:0x8a6b400 )>",
    "<NSContentSizeLayoutConstraint:0x8b847e0 V:[UILabel:0x8a6a010(21)] Hug:251 CompressionResistance:750>"
)
2014-07-04 22:25:40.869 AutoLayoutExample[2237:70b] current constraints (
    "<NSLayoutConstraint:0x8a6bde0 H:|-(179)-[UILabel:0x8a6a010]   (Names: '|':UIView:0x8a6b400 )>",
    "<NSContentSizeLayoutConstraint:0x8b847a0 H:[UILabel:0x8a6a010(42)] Hug:251 CompressionResistance:750>"
)
2014-07-04 22:25:40.870 AutoLayoutExample[2237:70b] current constraints (
    "<NSLayoutConstraint:0x8a6bd30 V:[_UILayoutGuide:0x8a6b4b0]-(127)-[UILabel:0x8a6a010]>",
    "<_UILayoutSupportConstraint:0x8a67660 V:[_UILayoutGuide:0x8a6b4b0(20)]>",
    "<_UILayoutSupportConstraint:0x8a6a300 V:|-(0)-[_UILayoutGuide:0x8a6b4b0]   (Names: '|':UIView:0x8a6b400 )>",
    "<NSContentSizeLayoutConstraint:0x8b847e0 V:[UILabel:0x8a6a010(21)] Hug:251 CompressionResistance:750>"
)

如果只指定位置信息，但不指定size信息，但intrisic size是PlaceHolder,width 和height 都是None时输出是这样的：

2014-07-04 21:11:56.941 AutoLayoutExample[1790:70b] current constraints (
    "<NSLayoutConstraint:0x8ad9670 H:|-(179)-[UILabel:0x8ad7880]   (Names: '|':UIView:0x8ad8c90 )>",
    "<NSIBPrototypingLayoutConstraint:0x8ad9c10 'IB auto generated at build time for view with ambiguity' H:[UILabel:0x8ad7880(42@251)] priority:251>"
)
2014-07-04 21:11:56.942 AutoLayoutExample[1790:70b] current constraints (
    "<NSLayoutConstraint:0x8ad95c0 V:[_UILayoutGuide:0x8ad8d40]-(127)-[UILabel:0x8ad7880]>",
    "<_UILayoutSupportConstraint:0x8a6d3f0 V:[_UILayoutGuide:0x8ad8d40(20)]>",
    "<_UILayoutSupportConstraint:0x8aaf250 V:|-(0)-[_UILayoutGuide:0x8ad8d40]   (Names: '|':UIView:0x8ad8c90 )>",
    "<NSIBPrototypingLayoutConstraint:0x8ad9c40 'IB auto generated at build time for view with ambiguity' V:[UILabel:0x8ad7880(21@251)] priority:251>"
)
2014-07-04 21:11:56.943 AutoLayoutExample[1790:70b] current constraints (
    "<NSLayoutConstraint:0x8ad9670 H:|-(179)-[UILabel:0x8ad7880]   (Names: '|':UIView:0x8ad8c90 )>",
    "<NSIBPrototypingLayoutConstraint:0x8ad9c10 'IB auto generated at build time for view with ambiguity' H:[UILabel:0x8ad7880(42@251)] priority:251>"
)
2014-07-04 21:11:56.944 AutoLayoutExample[1790:70b] current constraints (
    "<NSLayoutConstraint:0x8ad95c0 V:[_UILayoutGuide:0x8ad8d40]-(127)-[UILabel:0x8ad7880]>",
    "<_UILayoutSupportConstraint:0x8a6d3f0 V:[_UILayoutGuide:0x8ad8d40(20)]>",
    "<_UILayoutSupportConstraint:0x8aaf250 V:|-(0)-[_UILayoutGuide:0x8ad8d40]   (Names: '|':UIView:0x8ad8c90 )>",
    "<NSIBPrototypingLayoutConstraint:0x8ad9c40 'IB auto generated at build time for view with ambiguity' V:[UILabel:0x8ad7880(21@251)] priority:251>"
)

size信息部分是由IB自动生成的，但是它们的priority只有251，而不是之前的1000（required）了。

只指定位置，对比Default 和 PlaceHolder的 constraints，可以看到，在使用Default时，IB给自动加了NSContentSizeLayoutConstraint；在使用Placeholder时，IB自动生成了NSIBPrototypingLayoutConstraint，因为我们使用了PlaceHolder，并且size都是0，所以IB已经无法知道size信息了，为了避免ambiguity就只能自动加了。

我们在使用custom view时，需要指定intrinsic size为placeholder，那我们应该怎么解决IB自动添加ambiguity的constraint的问题呢？

事实上，如果我们的view是个custom view的话，IB是不会自动添加ambity的constraint，IB会添加NSContentSizeLayoutConstraint，然后在runtime时调用custom view 的intrinsicContentSize来知道view应该有多大。

还有继续说说intrinsicContentSize，事实上它不是一个人在战斗，其他的constraint只有一个priority，但是intrinsicContentSize有两个，分别叫Content Hugging Priority和Content Compression Resistancy Priority。 Content Hugging Priority控制当view frame>intrinsicContentSize时，是不是要缩小view 的size；而Content Compression Resistancy Priority控制当view frame<intrinsicContentSize时, 要不要截断content，保持view frame size。

举例来说，如果我有个UILabel，开始Title为“Label”，设置Width constraint 为42@500，设置Content Hugging Priority为1000， 设置Content Compression Resistancy Priority为1。如果我在runtime时改动Title为“S”，那整个UILabel的size都被缩小到只能容下5，因为这时frame size大于IntrinsicContentSize，Content Hugging Priority为1000，要比width constraint高，所以应该是Content Hugging 起作用；如果我把Title改为“325ioqukejgakshgkjashdgjka”，那这个UILabel的size会变为42，内容被截断为“32...”，因为这时frame size比IntrinsicContentSize小，Content Compression Resistancy Priority只有1，width constraint起作用，将宽度设置为42。

一个使用AutoLayout的有趣Bug

删除CenterY之前，IB知道X和Y 位置信息，因此系统给加的是ContentSizeLayoutConstraint，但删除之后，IB无法确定其Y方向的信息，生成了PrototypingLayoutConstraint。但为什么一个只有251的priority的PrototypingLayoutConstraint不会clip label呢？那我们就需要更多的了解PrototypingLayoutConstraint了。

//Before
2014-07-05 23:29:40.068 AutoLayoutExample[2035:70b] current constraints (
    "<NSLayoutConstraint:0x8b44c20 H:|-(10)-[UILabel:0x8b450f0]   (Names: '|':UITableViewCellContentView:0x8b453e0 )>",
    "<NSContentSizeLayoutConstraint:0x8b37520 H:[UILabel:0x8b450f0(42)] Hug:251 CompressionResistance:750>"
) 

//After
2014-07-05 23:32:19.251 AutoLayoutExample[2062:70b] current constraints (
    "<NSLayoutConstraint:0x8b66a10 H:|-(10)-[UILabel:0x8b664d0]   (Names: '|':UITableViewCellContentView:0x8b66210 )>",
    "<NSIBPrototypingLayoutConstraint:0x8b521f0 'IB auto generated at build time for view with ambiguity' H:[UILabel:0x8b664d0(42@251)] priority:251>"
)