自动布局 01 注意事项

//自动布局注意事项：

-(void)layoutTestFunc

{

    

    UIView * view0 = [[UIView alloc]init];

    

    //注意事项1:设置translatesAutoresizingMaskIntoConstraints为NO;

    view0.translatesAutoresizingMaskIntoConstraints = NO;

    //注意事项2:两个view在同一个hierarchy中才有可能添加相关约束。

    [self.view addSubview:view0];

    

    //注意事项3:当firstAttribute不为宽或者高时，multipler不能为0。

    /*

    NSLayoutConstraint * con = [NSLayoutConstraint constraintWithItem:view0 attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeLeft multiplier:0.0 constant:9];

    //[NSLayoutConstraint constraintWithItem:attribute:relatedBy:toItem:attribute:multiplier:constant:]: A multiplier of 0 or a nil second item together with a location for the first attribute creates an illegal constraint of a location equal to a constant. Location attributes must be specified in pairs'

    */

    

    //注意事项4:同上，当firstAttribute不为宽或者高时，secondItem不能为nil。

    //NSLayoutConstraint * con = [NSLayoutConstraint constraintWithItem:view0 attribute:NSLayoutAttributeLeft relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeLeft multiplier:1.0 constant:9];

    //当两个Attribute全是宽或高时multiplier才可以为0；

    NSLayoutConstraint * con0 = [NSLayoutConstraint constraintWithItem:view0 attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:self.view attribute:NSLayoutAttributeWidth multiplier:0.0 constant:9];

    NSLog(@"%@",con0);

    

    //当secondItem为nil时，secondAttribute应为NSLayoutAttributeNotAnAttribute

    NSLayoutConstraint * con1 = [NSLayoutConstraint constraintWithItem:view0 attribute:NSLayoutAttributeWidth relatedBy:NSLayoutRelationEqual toItem:nil attribute:NSLayoutAttributeNotAnAttribute multiplier:0.0 constant:9];

   

    

    [self.view addConstraint:con1];

    

    //包括："<NSLayoutConstraint:0x7fb5a84675e0 H:[UIView:0x7fb5a86528c0(9)]>"

    NSLog(@"%@",self.view.constraints);

    

    //空，注意虽然con理论上只涉及view0，但是view0的constraints并不包含约束con1；

    NSLog(@"%@",view0.constraints);

    

    

    UIView * view1 = [[UIView alloc]init];

    UIView * view2 = [[UIView alloc]init];

    UIView * view3 = [[UIView alloc]init];

    view1.translatesAutoresizingMaskIntoConstraints = NO;

    view2.translatesAutoresizingMaskIntoConstraints = NO;

    view3.translatesAutoresizingMaskIntoConstraints = NO;

    

    [view2 addSubview:view3];

    [view1 addSubview:view2];

    [self.view addSubview:view1];

    

    NSLayoutConstraint * con2 = [NSLayoutConstraint constraintWithItem:view2 attribute:NSLayoutAttributeLeading relatedBy:NSLayoutRelationEqual toItem:view3 attribute:NSLayoutAttributeLeading multiplier:1.0 constant:39];

    

    [self.view addConstraint:con2];

    

    NSLog(@"%@",self.view.constraints);

    //包括"<NSLayoutConstraint:0x7f82e0d2b090 UIView:0x7f82e0d2ab50.leading == UIView:0x7f82e0d2ae80.leading + 39>"

    

    //空，虽然约束是关于view2，与view3，view2月view3最近的父视图是view1，但是下面全为空。

    NSLog(@"%@",view1.constraints);

    //空

    NSLog(@"%@",view2.constraints);

    //空

    NSLog(@"%@",view3.constraints);

}