Setting auto layout constraints programmatically o

问题:

I am trying to add two UIView on ViewController. One of them is plain UIView and other is UITableView. I am not quite sure how to set height constraint on them in such away that UITableView will be of height required for its content and other view can resize itself based on UITableView height.

Below is the simple sketch and code I am using for setting up those constraints.

 NSDictionary *views = NSDictionaryOfVariableBindings(_infoView,_infoTableView);

[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"[_infoView]"
                                                                  options:0
                                                                  metrics:nil
                                                                    views:views]];

[self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[_infoView]-[_infoTableView]"
                                                                  options:0
                                                                  metrics:nil
                                                                    views:views]];

// Width constraint
[self.view addConstraint:[NSLayoutConstraint constraintWithItem:self.infoView
                                                 attribute:NSLayoutAttributeWidth
                                                 relatedBy:NSLayoutRelationEqual
                                                    toItem:self.view
                                                 attribute:NSLayoutAttributeWidth
                                                multiplier:1.0
                                                  constant:0]];

[self.view addConstraint:[NSLayoutConstraint constraintWithItem:self.infoTableView
                                                      attribute:NSLayoutAttributeWidth
                                                      relatedBy:NSLayoutRelationEqual
                                                         toItem:self.view
                                                      attribute:NSLayoutAttributeWidth
                                                     multiplier:1.0
                                                       constant:0]];

[self.view addConstraint:[NSLayoutConstraint constraintWithItem:self.infoView
                                                      attribute:NSLayoutAttributeTop
                                                      relatedBy:NSLayoutRelationEqual
                                                         toItem:self.view
                                                      attribute:NSLayoutAttributeTop
                                                     multiplier:1.0
                                                       constant:0]];

[self.view addConstraint:[NSLayoutConstraint constraintWithItem:self.infoTableView
                                                      attribute:NSLayoutAttributeBottom
                                                      relatedBy:NSLayoutRelationEqual
                                                         toItem:self.view
                                                      attribute:NSLayoutAttributeBottom
                                                     multiplier:1.0
                                                       constant:0]];

Any comments / feedback would be highly appreciated. Thanks.

回答1:

You are on the right track. The trick here is that you need to dynamically update the height of tableView to its contentSize.height so infoView will know it should either be vertically expanded or shrunk:

First you define a property that will hold the height constraint:

@property (nonatomic, strong) NSLayoutConstraint *tableViewHeightConstraint;

- (void)viewDidLoad {
  ...
  self.tableViewHeightConstraint = [NSLayoutConstraint constraintWithItem:self.tableView
                                                                attribute:NSLayoutAttributeHeight
                                                                relatedBy:NSLayoutRelationEqual
                                                                   toItem:nil
                                                                attribute:NSLayoutAttributeNotAnAttribute
                                                               multiplier:0.0
                                                                 constant:350.0];
}

Then you can update the height constraint right after tableView knows its contentSize (ex: viewDidAppear:animated):

- (void)viewDidAppear:(BOOL)animated {
  [super viewDidAppear:animated];

  self.tableViewHeightConstraint.constant = self.tableView.contentSize.height;
  [self.view layoutIfNeeded];
}  

I created a sample project for you so you can play around with it to have a better understanding about what I intend to do.