代码添加constraint，设置translatesAutoresizingMaskIntoConstraints为NO的原因

在用代码为控件添加NSLayoutConstraint的时候，有时会遇到以下错误：

Unable to simultaneously satisfy constraints.

Probably at least one of the constraints in the following list is one you don't want. Try this: (1) look at each constraint and try to figure out which you don't expect; (2) find the code that added the unwanted constraint or constraints and fix it. (Note: If you're seeing NSAutoresizingMaskLayoutConstraints that you don't understand, refer to the documentation for the UIView property translatesAutoresizingMaskIntoConstraints) 

这种错误在编译阶段又无法被查出。这时，我们在检查了自己的constraint之后，确认constraint没有逻辑错误，可能就是没有在对应的view添加以下代码

[view setTranslatesAutoresizingMaskIntoConstraints:NO];

那这一行代码有何作用呢？

在Programming IOS 7（第四版）有这么一段话：

Layout is performed in three primary ways:

Manual layout

       The superview is sent the layoutSubviews message whenever it is resized; so, to

       lay out subviews manually, provide your own subclass and overridelayout-

       Subviews. Clearly this could turn out to be a lot of work, but it means you can do

       anything you like.

Autoresizing

......

Autolayout

......

就是说layout有三种方式：Manual layout，Autoresizing，Autolayout。我们常用的可能就后面两种。

假设v1是一个不使用autolayout的view，而v2是一个使用autolayout的view，但v1成为v2的subview时，

v2需要四条隐含的constraint来确定v1的位置，这些约束都是从v1的frame转化而来：

This conversion is performed only if the view in question has its translates-

AutoresizingMaskIntoConstraints property set to YES. That is, in fact, the default if

the view came into existence either in code or by instantiation from a nib where “Use

Auto Layout” is not checked. The assumption is that if a view came into existence in

either of those ways, you want its frame and autoresizingMaskto act as its constraints

if it becomes involved in autolayout.

- (void)viewDidLoad {    [super viewDidLoad];        [self.view setTranslatesAutoresizingMaskIntoConstraints:NO];    self.v1 = [[UIView alloc]initWithFrame:CGRectMake(200, 200, 100, 100)];    self.v1.backgroundColor = [UIColor greenColor];        self.v2 = [UIView new];    self.v2.backgroundColor = [UIColor redColor];    [self.view addSubview:self.v2];    self.v2.translatesAutoresizingMaskIntoConstraints = NO;    //self.v1.translatesAutoresizingMaskIntoConstraints = NO;    NSDictionary * viewDic = [[NSDictionary alloc]initWithObjectsAndKeys:self.v2,@"v2", self.v1,@"v1",nil];    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-100-[v2]-100-|" options:0 metrics:nil views:viewDic]];    [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-100-[v2]-100-|" options:0 metrics:nil views:viewDic]];            [self.v2 addSubview:self.v1];    [self.v2 addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-100-[v1]-100-|" options:0 metrics:nil views:viewDic]];}

上面代码会产生以下错误(Unable to simultaneously satisfy constraints.) ,即不能满足所有约束。容易看出，添加在v2上的constraint没有问题。是v1的隐含constraint和后来添加的constriaint产生了冲突

但是v1还能显示，（这个问题还没想明白）：

但是去掉下面这一行的注释，再运行

//self.v1.translatesAutoresizingMaskIntoConstraints = NO;

v1无法显示：

显然，v1在translatesAutoresizingMaskIntoConstraints 设置为NO之后，没有添加隐含constraint，而代码添加的constraint又不足以确定v1的位置和大小，所以v1无法显示;

由于代码添加constraint很难检查出错误，所以我尽量避免代码编写constraint。但storyboard有时操作又比较麻烦，且不够灵活，有时就需要自己添加部分的代码来弥补storyboard的不足。

比如，我们要添加十几个按钮，这些按钮功能有基本一致，样式也一样，这时用storyboard编写就显得麻烦。以下代码向一个view中添加了12个按钮。并设置了他们的constraint

<span style="font-size:14px;">- (void)viewDidLoad {    [super viewDidLoad];        [self.view setTranslatesAutoresizingMaskIntoConstraints:NO];    NSDictionary * keyDetail;    UIView * secondItem = self.view;    keyDetail = [[NSDictionary alloc]initWithObjectsAndKeys:                 @"F1",@"101",                 @"F2",@"102",                 @"F3",@"103",                 @"F4",@"104",                 @"F5",@"105",                 @"F6",@"106",                 @"F7",@"107",                 @"F8",@"108",                 @"F9",@"109",                 @"F10",@"110",                 @"F11",@"111",                 @"F12",@"112",                 nil];    CGFloat keyHeightParameter = 30.0;    CGFloat keyWidthParameter = 10;    CGFloat keyspaceHorizontal = keyWidthParameter*keyDetail.count/(keyDetail.count+1);    CGFloat keyspaceVertical = 200.0;    int i;    for(i = 1; i <= keyDetail.count; i++)    {        UIButton * key = [[UIButton alloc]init];        key.tag = i+100 ;                [key setTranslatesAutoresizingMaskIntoConstraints:NO];                [key setTitle:[keyDetail valueForKey:[NSString stringWithFormat:@"%i",i+100]] forState:UIControlStateNormal];        [key setTitleColor:[UIColor colorWithRed:49.0/255.0 green:69.0/255.0 blue:116.0/255.0 alpha:1.0] forState:UIControlStateNormal];        [key setTitleColor:[UIColor colorWithRed:49.0/255.0 green:69.0/255.0 blue:116.0/255.0 alpha:1.0] forState:UIControlStateSelected];        key.titleLabel.font = [UIFont systemFontOfSize:14.0];        [key setBackgroundColor:[UIColor whiteColor]];        key.layer.borderColor = [UIColor grayColor].CGColor;        key.layer.borderWidth = 0.5;        key.layer.cornerRadius = 10;                [self.view addSubview:key];        if (secondItem == self.view)        {            [self.view addConstraint:[NSLayoutConstraint constraintWithItem:key attribute:NSLayoutAttributeLeading                                                                  relatedBy:NSLayoutRelationEqual                                                                     toItem:secondItem                                                                  attribute:NSLayoutAttributeLeading                                                                 multiplier:1 constant:keyspaceHorizontal]];        }        else        {            [self.view addConstraint:[NSLayoutConstraint constraintWithItem:key attribute:NSLayoutAttributeLeading                                                                  relatedBy:NSLayoutRelationEqual                                                                     toItem:secondItem                                                                  attribute:NSLayoutAttributeTrailing                                                                 multiplier:1 constant:keyspaceHorizontal]];        }                [self.view addConstraint:[NSLayoutConstraint constraintWithItem:key attribute:NSLayoutAttributeTop                                                              relatedBy:NSLayoutRelationEqual                                                                 toItem:self.view attribute:NSLayoutAttributeTop                                                             multiplier:1.0 constant:keyspaceVertical]];        [self.view addConstraint:[NSLayoutConstraint constraintWithItem:key                                                              attribute:NSLayoutAttributeWidth                                                              relatedBy:NSLayoutRelationEqual                                                                 toItem:self.view attribute:NSLayoutAttributeWidth                                                             multiplier:1.0/keyDetail.count constant:-keyWidthParameter]];        [key addConstraint:[NSLayoutConstraint constraintWithItem:key                                                        attribute:NSLayoutAttributeHeight                                                        relatedBy:NSLayoutRelationEqual                                                           toItem:nil                                                        attribute:NSLayoutAttributeNotAnAttribute                                                       multiplier:1.0 constant:keyHeightParameter]];                secondItem = key;    }}</span>

效果如下：

landscape：

portrait：

在这个过程中遇到一个错误：

        The view hierarchy is not prepared for the constraint:

        这种错误一般都是constrain与控件不对应，或者是subview没有加到superview中。