Question

I faced a strange problem, the scrollview does not scroll down, only scroll up. I have scrollview in my app, please look at my coding

.....

self.scrollView = [[UIScrollView alloc] initWithFrame: CGRectMake(0, 0, 320,427)];

[self.view addSubViews: self.scrollView];

UIView *blueView = [[UIView alloc] initWithFrame: CGRectMake(0, 47, 320, 320)];

blueView.backgroundColor = [UIColor blueColor];

[self.scrollView addSubViews: blueView];

self.scrollView.contentSize = CGSize(320, 640);

....

My problem is no matter what value I changed contentSize, my ScrollView only scroll up, not scroll down. I want user can move blueView to the top or bottom of iPhone screen from the original position.

do you have this problem?

Answer 1

The Problem

It looks like your issue is with how you’re orienting blueView within scrollView. You’re setting the frame of blueView to the CGRect (0, 47, 320, 320). When you set the frame like this, one of the things you’re implicitly saying is:

The top edge of blueView is 47 points below the top edge of scrollView.

That’s a perfectly valid thing to say, but it’s what’s causing the problem you describe. scrollView won’t scroll down because it is designed to start, by default, with the rect (0, 0, 320, 480) in view. The contentSize property only indicates the size of the content within the UIScrollView, not its positioning. When you set it, you’re basically telling scrollView:

Starting from your content origin, the content is 320 points wide and 640 points tall.

Thus, scrollView won’t scroll up because, as far as it knows, there’s no content above the coordinate (0, 0).

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The Solution

There are three steps you’ll need to take to get the functionality you want.

1. Set the contentSize to be just big enough to allow blueView to scroll all the way up and down.
2. Put blueView in the vertical center of scrollView.
3. Scroll the scrollView so that it is initially centered on blueView.

• Set the contentSize to be just big enough to allow blueView to scroll all the way up and down.

We’ll want to calculate the correct value of the contentSize property. It is of the type CGSize, so we need two parts: width and height. width is easy – since you don’t seem to want horizontal scrolling, just make it the width of the screen, 320. Height is a little more tricky. If you want blueView to just touch the top and bottom of the screen when scrolled up or down, you need to do some math. The correct total height will be double the height of the screen, minus the height of blueView. So:

scrollView.contentSize = CGSizeMake(320, 480 * 2.0 - blueView.frame.size.height);

• Put blueView in the vertical center of scrollView.

That’s easy; just set the center property of blueView:

blueView.center = CGPointMake(160, scrollView.contentSize.height / 2.0);

• Scroll the scrollView so that it is initially centered on blueView.

If you check the Apple UIScrollView documentation, you’ll see an instance method - (void)scrollRectToVisible:(CGRect)rect animated:(BOOL)animated. This is exactly what you need to scroll scrollView programmatically. The rect you want is the one centered on blueView, with the size of the iPhone screen. So:

CGRect targetRect = CGRectMake(0, scrollView.contentSize.height / 2.0 - 240,
                               320, 480);
[scrollView scrollRectToVisible:targetRect animated:NO];

Make sure you do this scrolling in viewWillAppear, so it’s ready right when the user sees the view.

That should be it. Let me know if you have any questions!