如何水平居中滚动视图的内容？

Question

In viewDidLoad I'm adding the buttons to the scroll view within the loop as follows:

for (int i = 0; i < self.topics.count; i++) {
    id obj = self.topics[i];
    if ([obj isKindOfClass:[TRBTaxonomy class]]) {
        TRBTaxonomy *topic = obj;

        UIButton *prevTopicButton = nil;
        if (i > 0) {
            prevTopicButton = buttons[i-1];
        }

        //Create topic divider
        UILabel *topicsDividerLabel =[[UILabel alloc] init];
        [topicsDividerLabel setBackgroundColor:[UIColor lightGrayColor]];
        [topicsDividerLabel setTranslatesAutoresizingMaskIntoConstraints:NO];

        UIButton *topicButton = [UIButton buttonWithType:UIButtonTypeSystem];
        topicButton.translatesAutoresizingMaskIntoConstraints = NO;
        [topicButton setTitle:topic.name forState:UIControlStateNormal];
        [topicButton setTitleColor:[UIColor whiteColor] forState:UIControlStateNormal];
        topicButton.titleLabel.font = [UIFont fontWithName:(NSString*)[[TRBUtils fonts] valueForKey:@"TopicsBarFontName"] size:15];
        topicButton.titleLabel.textAlignment = NSTextAlignmentLeft;
        [topicButton addTarget:self action:@selector(topicTapped:) forControlEvents:UIControlEventTouchUpInside];

        [self.scrollView addSubview:topicButton];
    }
}

Then, at the end of the loop I try to get the width of the scroll view content to place it in the center-horizontal within the scroll view:

CGFloat newContentOffsetX = (self.scrollView.contentSize.width/2) - (self.scrollView.bounds.size.width/2);
self.scrollView.contentOffset = CGPointMake(newContentOffsetX, 0);

but at this point self.scrollView.contentSize.width returns 0

When I get the content from viewDidAppear there is a content and everything works fine but there is a visible blink from moving the scroll view content from the left to the center.

How to get rid of this blink? Why self.scrollView.contentSize.width is zero on viewDidLoad and viewWillAppear but not in viewDidAppear?

Answer 1

Why self.scrollView.contentSize.width is zero on viewDidLoad and viewWillAppear but not in viewDidAppear?

When method viewDidLoad and viewWillAppear are called the view just hasn't appeared. So the contentSize is 0 shouldn't be strange.

How to get rid of this blink?

try to use this method

- (void)setContentOffset:(CGPoint)contentOffset animated:(BOOL)animated

CGFloat newContentOffsetX = (self.scrollView.contentSize.width/2) - (self.scrollView.bounds.size.width/2);
[self.scrollView setContentOffset:CGPointMake(newContentOffsetX, 0) animated:YES];