Using autoResizingMask with CGRectZero

问题:

I am building a footer for a tableview's section. The height of the footer will be specified in heightForFooterInSection, so in viewForFooterInSection I would like to just add the subview and specify that the footer view should fill whatever footer height is specified (this footer size will be dynamic). So, I am using CGRectZero as the initial frame and telling the footer view to expand to fill its parent view.

- (UIView *)tableView:(UITableView *)tableView viewForFooterInSection:(NSInteger)section
{
    UIView *footerView = [[UIView alloc] initWithFrame:CGRectZero];
    footerView = UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;
    footerView = [UIColor greenColor];
    return footerView;
}

This works as expected - the footer of the table view is filled completely with the green view.

But now I want to add a UITextView to the footer. The text view should fill the same space, but leave a 5-point border:

{
    UIView *footerView = [[UIView alloc] initWithFrame:CGRectZero];
    footerView = UIViewAutoresizingFlexibleHeight | UIViewAutoresizingFlexibleWidth;
    footerView = [UIColor greenColor];

    UITextView *textView = [[UITextView alloc] initWithFrame:CGRectInset(footerView.frame, 5, 5)];
    textView.autoresizingMask = UIViewAutoresizingFlexibleWidth | UIViewAutoresizingFlexibleHeight;
    textView.backgroundColor = [UIColor redColor];

    [footerView addSubview:textView];
    return footerView;
}

Instead of filling the footer view (with a 5 point margin), the text view does not appear at all. It likely has a frame of CGRectZero (or maybe even -5 x -5?). If I set the inset to 0, 0, however, it expands as expected.

What is the explanation for this? And If I can't use an inset of CGRectZero for the initial frame, what am I expected to use when the frame of the footerView can not be known?

回答1:

CGRectInset will create a rectangle based on the existing rectangle. It doesn't refer to the footer view ever again: only when it is calculating it this one time. In this case, since you are trying to inset a rectangle with a zero size, this applies from the docs:

Discussion. The rectangle is standardized and then the inset parameters are applied. If the resulting rectangle would have a negative height or width, a null rectangle is returned.

Therefore, you are creating your label with a null rectangle.

I would create the footer with a "typical" size, then an appropriately sized label with the autoResizingMask that you want, and then set your footerView to zero if that is what you want it set to.

回答2:

I would guess that the TextView creates the content in the background, so at the moment of initialization it's empty. I usualy end up using [string sizeWithFont:constrainedToSize:lineBreakMode:]

CGSize size = [aString sizeWithFont:[UIFont systemFontOfSize:12.0] constrainedToSize:CGSizeMake(320,500) lineBreakMode:NSLineBreakByWordWrapping];
CGRect frame = CGRectMake(0, 0, size.width, size.height);