- (void)viewDidLoad {    [super viewDidLoad];        UIButton *btnTest = [UIButton buttonWithType:UIButtonTypeCustom];//不需要去刻意指定x,y的坐标，可以用CGRectZero    btnTest.backgroundColor = [UIColor redColor];    btnTest.layer.borderColor = [UIColor yellowColor].CGColor;    btnTest.layer.borderWidth = 2.0;    [self.view addSubview:btnTest];            [btnTest setTranslatesAutoresizingMaskIntoConstraints:NO];//将使用AutoLayout的方式布局    //btnTest顶部相对于self.view的顶部距离为100    NSLayoutConstraint *constraintTop = [NSLayoutConstraint constraintWithItem:btnTest attribute:NSLayoutAttributeTop relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeTop multiplier:1.0 constant:100];    //btnTest左侧相对于self.view的左侧距离为100    NSLayoutConstraint *constraintLeft = [NSLayoutConstraint constraintWithItem:btnTest  attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:1.0 constant:100];    //btnTest右侧相对于self.view的右侧距离为100    NSLayoutConstraint *constraintRight = [NSLayoutConstraint constraintWithItem:self.view  attribute:NSLayoutAttributeRight relatedBy:NSLayoutRelationEqual toItem:btnTest attribute:NSLayoutAttributeRight multiplier:1.0 constant:100];    //btnTest底部相对于self.view的底部距离为100    NSLayoutConstraint *constraintBottom = [NSLayoutConstraint constraintWithItem:self.view attribute:NSLayoutAttributeBottom relatedBy:NSLayoutRelationEqual toItem:btnTest attribute:NSLayoutAttributeBottom multiplier:1.0 constant:100];    //水平居中    NSLayoutConstraint *constraintXCenter = [NSLayoutConstraint constraintWithItem:btnTest attribute:NSLayoutAttributeCenterX relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterX multiplier:1.0 constant:00.0f];    //垂直居中     NSLayoutConstraint *constraintYCenter = [NSLayoutConstraint constraintWithItem:btnTest attribute:NSLayoutAttributeCenterY relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeCenterY multiplier:1.0 constant:00.0f];        //将约束添加到父视图中    [self.view addConstraint:constraintTop];    [self.view addConstraint:constraintLeft];    [self.view addConstraint:constraintRight];    [self.view addConstraint:constraintBottom];    [self.view addConstraint:constraintXCenter];    [self.view addConstraint:constraintYCenter];    }