具有自动布局的UILabel高度动画

Question

I have an UILabel on my view and UIButton. When button touched UILabel should change height animated, depends on label content . I was trying this:

    - (void)viewDidLoad {
        self.textLabel= [[UILabel alloc] initWithFrame:CGRectZero];
        self.textLabel.numberOfLines=0;
        self.textLabel.font= [UIFont systemFontOfSize:14];
        self.textLabel.backgroundColor= [UIColor lightGrayColor];
        self.textLabel.text= @"short text";
        [self.view addSubview:self.textLabel];
        [self.textLabel setTranslatesAutoresizingMaskIntoConstraints:NO];
        [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-10-[_textLabel]-10-|"
        options:0
        metrics:nil
        views:NSDictionaryOfVariableBindings(_textLabel)]];


        self.button= [UIButton buttonWithType:UIButtonTypeSystem];
        [self.button addTarget:self action:@selector(buttonTouched:) forControlEvents:UIControlEventTouchUpInside];
        [self.button setTitle:@"Tap" forState:UIControlStateNormal];
        self.button.backgroundColor= [UIColor greenColor];
        [self.button setTranslatesAutoresizingMaskIntoConstraints:NO];
        [self.view addSubview:self.button];

        [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-10-[_button]-10-|"
                                   options:0
                                   metrics:nil
                                   views:NSDictionaryOfVariableBindings(_button)]];

        [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-50-[_textLabel(>=0)]-10-[_button(==20)]"
                                   options:0
                                   metrics:nil
                                    views:NSDictionaryOfVariableBindings(_textLabel,_button)]];

}

- (void)buttonTouched:(id)buttonTouched {
        self.shortText =!self.shortText;
        self.textLabel.text= self.shortText ?@"short text":@"long long long text\nlong long long text\nlong long long text\n";
        [UIView animateWithDuration:1.0
                         animations:^{
                             [self.view layoutIfNeeded];
                     }];
}

Answer 1

Right before the animation block, you need to call [self.view setNeedsUpdateConstraints] in order to trigger tell the view that the constraints need to be updated when you call layoutIfNeeded

So the new method:

- (void)buttonTouched:(id)buttonTouched {
    self.shortText =!self.shortText;
    self.textLabel.text= self.shortText ?@"short text":@"long long long text\nlong long long text\nlong long long text\n";
    [self.view setNeedsUpdateConstraints];
    [UIView animateWithDuration:1.0
                     animations:^{
                         [self.view layoutIfNeeded];
    }];
}

Answer 2

I have this example working no problem.

 @interface ViewController () @end @implementation ViewController { __weak UIView *_bgView; __weak UILabel *_label; } - (void)viewDidLoad { [super viewDidLoad]; // Do any additional setup after loading the view, typically from a nib. UIView *bgView = [[UIView alloc] initWithFrame:CGRectZero]; bgView.backgroundColor = [[UIColor redColor] colorWithAlphaComponent:0.5]; bgView.translatesAutoresizingMaskIntoConstraints = NO; [self.view addSubview:bgView]; [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|[bgView]|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(bgView)]]; [self.view addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:[bgView]|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(bgView)]]; _bgView = bgView; UILabel *label = [[UILabel alloc] initWithFrame:CGRectZero]; label.backgroundColor = [[UIColor greenColor] colorWithAlphaComponent:0.5]; label.text = @"ahoj"; label.numberOfLines = 99; label.textAlignment = NSTextAlignmentCenter; label.translatesAutoresizingMaskIntoConstraints = NO; [bgView addSubview:label]; [bgView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"H:|-8-[label]-8-|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(label)]]; [bgView addConstraints:[NSLayoutConstraint constraintsWithVisualFormat:@"V:|-8-[label]-8-|" options:0 metrics:nil views:NSDictionaryOfVariableBindings(label)]]; _label = label; [self performSelector:@selector(animate) withObject:nil afterDelay:1]; } - (void)animate { _label.text = @"ahoj, ahoj\\ntotalka ahoj"; [UIView animateWithDuration:1 animations:^{ [_bgView.superview layoutIfNeeded]; } completion:^(BOOL finished) { ; }]; } - (void)didReceiveMemoryWarning { [super didReceiveMemoryWarning]; // Dispose of any resources that can be recreated. } @end 

Answer 3

在故事板中定义高度约束，将其添加为IBOutlet，然后在动画块中更改其值。

Answer 4

Maybe if you use

[self.textLabel sizeToFit];

it works.