AutoLayout的实现

    UITableView *tableview = [[UITableView alloc] init];
    [superView addSubview:tableview];
    
    
    NSLayoutConstraint *cn = [NSLayoutConstraint constraintWithItem:tableview
                                                          attribute:NSLayoutAttributeCenterX
                                                          relatedBy:NSLayoutRelationEqual
                                                             toItem:superView
                                                          attribute:NSLayoutAttributeCenterX
                                                         multiplier:1.0
                                                           constant:0.0];
    [superView addConstraint:cn];
    
    cn = [NSLayoutConstraint constraintWithItem:tableview
                                      attribute:NSLayoutAttributeBottom
                                      relatedBy:NSLayoutRelationEqual
                                         toItem:superView
                                      attribute:NSLayoutAttributeBottom
                                     multiplier:1.0
                                       constant:-20.0];
    [superView addConstraint:cn];

    NSDictionary *views = NSDictionaryOfVariableBindings(_tableView);
    NSMutableArray *constraints = [NSMutableArray array];
    _tableView.translatesAutoresizingMaskIntoConstraints = NO;
    [constraints addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-[_tableView]-|" options:0 metrics:nil views:views]];
    [constraints addObjectsFromArray:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-[_tableView]-|" options:0 metrics:nil views:views]];
    [self.view addConstraints:constraints];