EsoErik

Tuesday, November 27, 2012

 

Centering a UIToolbar Button on iOS

I blew a couple hours figuring this out.  Hopefully it will save someone else's time.  The relevant code:
- (void)willRotateToInterfaceOrientation:(UIInterfaceOrientation)toInterfaceOrientation duration:(NSTimeInterval)duration
{
    isRotating = YES;
}

- (void)didRotateFromInterfaceOrientation:(UIInterfaceOrientation)fromInterfaceOrientation
{
    isRotating = NO;
    [self layoutToolbar];
}

- (void)viewDidLayoutSubviews
{
    _groupedTableBackground.frame = self.view.bounds;
    // If rotation is in progress, he system has not completed updating toolbar layout.  Because our adjustment of toolbar layout is based on the
    // layout made by the system, our adjustment of toolbar layout is postponed until the system indicates rotation is complete, at which point
    // the system has finished its layout.
    if(!isRotating)
    {
        [self layoutToolbar];
    }
}

- (void)layoutToolbar
{
    UIToolbar* tb = self.navigationController.toolbar;
    UIView* lb = [_saveButton valueForKey:@"view"];
    UIView* mb = [_makeCopyButton valueForKey:@"view"];
    UIView* rb = [_deleteButton valueForKey:@"view"];
    CGRect lbf = [tb convertRect:((UIView*)lb.subviews[0]).frame fromView:lb];
    CGRect mbf = [tb convertRect:((UIView*)mb.subviews[0]).frame fromView:mb];
    CGRect rbf = [tb convertRect:((UIView*)rb.subviews[0]).frame fromView:rb];
    CGRect tbb = tb.bounds;
    CGFloat lspacerCur = _leftCenteringSpacer.width;
    // The right edge of the left button
    CGFloat lbr = lbf.origin.x + lbf.size.width;
    // The distance the system leaves from the edge of a button aligned directly against the side of the toolbar
    CGFloat edgeBorder = lbf.origin.x;
    // The distance the system leaves between adjacent buttons aligned directly against each other
    CGFloat border = mbf.origin.x - lbr - lspacerCur - tbb.origin.x;
    // The width of the bar button item fixed space control between the left and middle buttons required order to center the middle button
    CGFloat lspacer = floorf(tbb.size.width / 2 - lbr - border - mbf.size.width / 2);
    if(lspacer < 0)
    {
        lspacer = 0;
    }
    else
    {
        // The width that the system will assign to the bar button item flexible space control between the middle and right buttons
        // after the width of the fixed space control to the left of the middle button has been set and the toolbar's layout method is
        // called.  We calculate this in order to ensure that centering does not cause the middle button to overlap the right button.
        CGFloat rspacer = (tbb.size.width - rbf.size.width - border - edgeBorder) - (lbr + border + lspacer + mbf.size.width);
        if(rspacer < 0)
        {
            lspacer = 0;
        }
    }
    if(lspacer != lspacerCur)
    {
        _leftCenteringSpacer.width = lspacer;
        [tb layoutSubviews];
    }
}
A UIViewController using this code to center the Make Copy button:
Without the centering code, the Make Copy button would be off-center to the right.  Note that I center the middle button by adjusting the width of a fixed space toolbar item placed between the left and middle buttons.  A flexible space toolbar item placed between the middle and right buttons keeps the right button aligned against the right edge of the toolbar.
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