UITableView: scrolling programmatically the content view

Question

Hi i'm trying to forward the touches I receive from an UIView which is in front of an UITableView. But doins so I can't make the table scroll anymore (see here).

So now I'm trying to make tha tableview scroll programmatically and I am looking at setContentOffset:animated: and scrollRectToVisible:animated: with having success only with the first.
Here is my code in a class that subclasses the UITableView:

- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{
    NSLog(@"BEGTB");
    //previousXPan = 0.0;

    UITouch * touch = [touches anyObject];
    CGPoint tPoint = [touch locationInView:self]; 
    previousYPan = tPoint.y;

    [super touchesBegan:touches withEvent:event];
    //
    //[super touchesShouldBegin:touches withEvent:event inContentView:self];
    //[[self nextResponder] touchesBegan:touches withEvent:event];
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{
    UITouch * touch = [touches anyObject];
    CGPoint tPoint = [touch locationInView:self]; 


    NSLog(@"MOVTB");
    NSLog(@"BOUNDZ %f, %f, %f, %f", self.bounds.origin.x, self.bounds.origin.y, self.bounds.size.height, self.bounds.size.width);
    NSLog(@"CONTENT SIZE  %f, %f",  self.contentSize.height, self.contentSize.width);

    CGRect rc = [self bounds];
    NSLog(@"%f %f", rc.size.height, rc.size.width);
    CGRect newRect = CGRectMake(rc.origin.x, rc.origin.y + self.contentSize.height, self.contentSize.width, fabs(tPoint.y - previousYPan));
    NSLog(@"BOND RECT %f, %f, %f, %f", rc.origin.x, rc.origin.y, rc.size.height, rc.size.width);
    NSLog(@"NEW RECT %f, %f, %f, %f", newRect.origin.x, newRect.origin.y, newRect.size.height, newRect.size.width);
    //[self scrollRectToVisible:newRect animated:YES];
    NSLog(@"Content OFFSET %f",tPoint.y - previousYPan);
    [self setContentOffset:CGPointMake(rc.origin.x,(tPoint.y - previousYPan) *2.0 )animated:YES];
    previousYPan = tPoint.y;
    //[super touchesMoved:touches withEvent:event];
    //[[self nextResponder] touchesMoved:touches withEvent:event];
}

But the result is really laggy and buggy. Any better idea?

Is your UIView inside a uitableviewcell, which will make it easier by just looking inside the specific cell and forward the touch? — vodkhang, Aug 11 '10 at 13:05
@vodkhang: my UIView is a view op top of the tableview and it forwards the touch to the uitableviewcell touched which forwards to the uitable view (the code is on the link in the message). I'm putting this UIView and not using the UITableViewCell directly because I want to create an ImageView to be dragged across all the content area. — rano, Aug 11 '10 at 14:47
that might sound stupid, but NSLOG does suck up huge amounts of performance (especially if used within frequently called methods such as cellForRowAtIndexPath or things like that). try removing all the Log Calls and see if it is any better before looking much deeper into it. — samsam, Aug 11 '10 at 15:07
@samsam: nothing even commenting it all off. the performance is not such a problem but it is all buggy and weird, it bounces up and down and seems not to move sometimes, other times it goes up as a rocket — rano, Aug 11 '10 at 16:14
hmmm, then it might help to have a look at how often touchesMoved is being fired and maybe limit the calls to setContentOffset or at least find a way to let setContentOffset animate all the way through before calling setcontentOffset again (or maybe give it a try with animated:NO) this is all i can think of at the moment. cheers! — samsam, Aug 12 '10 at 07:03

score 0 · Accepted Answer · answered Aug 17 '10 at 06:45

I almost found a suitable solution (it is not like a normal scrolling but...).

First I determine the mamimum and minimum amount of pixels in the content area of the table view on the Y axis ( the height of the contentSize when the view has been fully loaded).

The I check the movement done incrementally but not referencing the position of the touch in the table view! I have to check it on a top layered UIView that covers it, because when you alter the content offset of the table you change dynamically its dimensions and you can end you having an acceleration effect .

Here is some code:

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{
    UITouch * touch = [touches anyObject];

    CGPoint tPoint = [touch locationInView:[viewController draggableAreaView]];



    CGRect rc = [self bounds];


    float totalOS = tPoint.y - previousYPan;
    if ((contentOffSetY >= minY && contentOffSetY <= maxY) ||
        (contentOffSetY < minY - 5 && totalOS < 0) ||
        (contentOffSetY > maxY + 5 && totalOS > 0))
    {

        if (totalOS > 0)
            totalOS += OFFSET;
        else if (totalOS < 0)
            totalOS -= OFFSET;
        [self setContentOffset:CGPointMake(rc.origin.x,self.contentOffset.y + totalOS)animated:NO];

    }
    else /*if (contentOffSetY < minY - 5 && totalOS < 0) */
    {
        [self resetOffset];
    }
    previousYPan = tPoint.y; 

}

- (void)resetOffset{

    CGRect rc = [self bounds];
    if(contentOffSetY < minY)
    {
        [self setContentOffset:CGPointMake(rc.origin.x, minY + 50)animated:YES];
    }
    else if (contentOffSetY > maxY)
    {
        [self setContentOffset:CGPointMake(rc.origin.x, maxY - 50)animated:YES];
    }
}

Some more tips:

here draggableAreaView is the top UIView from which I get an "absolute" position
OFFSET is a constant defined to increment linearly the speed of the scroolling (without it the touchesMoved: , called many times, gives a relative movement of just one pixel each time)
,minY and maxY are the precalculated limit values (in order to compute them I calculated the height of all the cells in the table for max and the height of the visible content view for min)
I added some displacement to minY and maxY in order to have a more visible animation

can you tell me what is minY, maxY, and contentOffSetY?? i think my problem is similar this problem — R. Dewi, Jan 12 '11 at 00:49

UITableView: scrolling programmatically the content view

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