UINavigationBar that Scrolls Away (Revised)

On this blog one of my most searched for posts is the one about a UINavigationBar that scrolls away.

That was written a while back.  I’ve recently had to revisit this topic again, so I thought I’d revise my implementation, which aims to remove many caveats and just make it easy to drop into your code and you’re done.

So I redid that code and made it slightly less hacky, and implemented it as a category on UIVIewController. If it doesn’t work for you the only thing I can think of are the new properties on UIViewController that pertain to layout. Mine had automaticallyAdjustScrollViewInsets to YES, and extend edges under top bars.

//  UIViewController+ScrollyNavBar.h
//
//  Created by Stephen O'Connor on 24/03/16.
//  MIT License
//

/*
 
 How to use this:
 
 import this into your (probably) table view controller
 
 make sure you set HS_navigationBarLayerDefaultPosition in viewDidLoad:
 
 self.HS_navigationBarLayerDefaultPosition = self.navigationController.navigationBar.layer.position;
 
 optionally set HS_scrollingNavigationBarThresholdHeight if you want to be able to scroll a bit before 
 the nav bar starts scrolling with it.  A typical use case would be if you want the bar to start
 scrolling with your first table view section, and not with the tableViewHeader.
 
 in viewWillAppear:, call:
 
 - (void)HS_updateNavigationBarPositionUsingScrollView:(UIScrollView*)scroller
 
 and, assuming your table view controller is still the UITableView's delegate, it's also
 a scroll view delegate.  In -scrollViewDidScroll:, call:
 
 - (void)HS_updateNavigationBarPositionUsingScrollView:(UIScrollView*)scroller
 
 as well.
 
 Done!
 
 
 */


#import <UIKit/UIKit.h>

@interface UIViewController (ScrollyNavBar)

@property (nonatomic, assign) CGFloat HS_scrollingNavigationBarThresholdHeight;  // defaults to 0.  I.e. think about tableViewHeader's height
@property (nonatomic, assign) CGPoint HS_navigationBarLayerDefaultPosition;

- (void)HS_updateNavigationBarPositionUsingScrollView:(UIScrollView*)scroller;

@end

Then then .m file:

// some theoretical knowledge here:  http://nshipster.com/associated-objects/

#import "UIViewController+ScrollyNavBar.h"
#import <objc/runtime.h>


@implementation UIViewController (ScrollyNavBar)

@dynamic HS_scrollingNavigationBarThresholdHeight;
@dynamic HS_navigationBarLayerDefaultPosition;

- (void)setHS_navigationBarLayerDefaultPosition:(CGPoint)HS_navigationBarLayerDefaultPosition {
    objc_setAssociatedObject(self,
                             @selector(HS_navigationBarLayerDefaultPosition),
                             [NSValue valueWithCGPoint:HS_navigationBarLayerDefaultPosition],
                             OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (CGPoint)HS_navigationBarLayerDefaultPosition {
    return [(NSValue*)objc_getAssociatedObject(self, @selector(HS_navigationBarLayerDefaultPosition)) CGPointValue];
}

- (void)setHS_scrollingNavigationBarThresholdHeight:(CGFloat)HS_scrollingNavigationBarThresholdHeight {
    objc_setAssociatedObject(self,
                             @selector(HS_scrollingNavigationBarThresholdHeight),
                             @(HS_scrollingNavigationBarThresholdHeight),
                             OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (CGFloat)HS_scrollingNavigationBarThresholdHeight {
    return [(NSNumber*)objc_getAssociatedObject(self, @selector(HS_scrollingNavigationBarThresholdHeight)) floatValue];
}

- (void)HS_updateNavigationBarPositionUsingScrollView:(UIScrollView*)scroller
{
    // get the navigation bar's underlying CALayer object
    CALayer *layer = self.navigationController.navigationBar.layer;
    CGFloat contentOffsetY = scroller.contentOffset.y;
    CGPoint defaultBarPosition = self.HS_navigationBarLayerDefaultPosition;
    CGFloat scrollingThresholdHeight = self.HS_scrollingNavigationBarThresholdHeight;
    
    // if the scrolling is not at the top and has passed the threshold, then set the navigationBar layer's position accordingly.
    if (contentOffsetY > -scroller.contentInset.top + scrollingThresholdHeight) {
        
        CGPoint newPos;
        newPos.x = layer.position.x;
        newPos.y = defaultBarPosition.y;
        newPos.y = newPos.y - MIN(contentOffsetY + scroller.contentInset.top - scrollingThresholdHeight, scroller.contentInset.top);
        
        layer.position = newPos;
        
    }
    else
    {
        layer.position = defaultBarPosition;  // otherwise we are at the top and the navigation bar should be seen as if it can't scroll away.
    }
}

@end

Boom! Pretty straightforward.

Note however, that I don’t really recommend this functionality because it’s prone to error, and it’s sort of hacking UINavigationBar.  It has a few related consequences:

  • UITableView section headers will still ‘stick’ to the bottom of the now invisible navigation bar.
  • You must use a translucent UINavigationBar for this to really work properly.

Now, for the first of those two points, I don’t have a solution and anyone who does, please leave me something in the comments.

For the second one, if you read the API docs of UINavigationBar closely, you’ll see:

@property(nonatomic, assign, getter=isTranslucent) BOOL translucent

Discussion

The default value is YES. If the navigation bar has a custom background image, the default is YES if any pixel of the image has an alpha value of less than 1.0, and NO otherwise.

So, with a little trickery, we can just change the Class on a UINavigationController to use a custom UINavigationBar class that sets an *almost* opaque image as the background image any time you change the barTintColor:


#import "UIImage+NotQuiteOpaque.h"

@interface HSNavigationBar : UINavigationBar
@end

@implementation HSNavigationBar

- (void)awakeFromNib
{
    [self setBarTintColor:self.barTintColor];
}

- (void)setTranslucent:(BOOL)translucent
{
    NSLog(@"This HSNavigationBar must remain translucent!");
    [super setTranslucent:YES];
}

- (void)setBarTintColor:(UIColor *)barTintColor
{
    UIImage *bgImage = [UIImage HS_stretchableImageWithColor:barTintColor];
    [self setBackgroundImage:bgImage forBarMetrics:UIBarMetricsDefault];
}

@end

And you’ll see here that the real trick is this category on UIImage that generates a stretchable image of the color specified, but it sets the top right corner’s pixel alpha value to 0.99. I assume that nobody will notice that slight translucency in one pixel at the top right of the screen.

@implementation UIImage (NotQuiteOpaque)

+ (UIImage*)HS_stretchableImageWithColor:(UIColor *)color
{
    
    UIImage *result;
    
    CGSize size = {5,5};
    CGFloat scale = 1;
    
    CGFloat width = size.width * scale;
    CGFloat height = size.height * scale;
    CGColorSpaceRef colorSpace = CGColorSpaceCreateDeviceRGB();
    
    size_t bitsPerComponent = 8;
    size_t bytesPerPixel    = 4;
    size_t bytesPerRow      = (width * bitsPerComponent * bytesPerPixel + 7) / 8;
    size_t dataSize         = bytesPerRow * height;
    
    unsigned char *data = malloc(dataSize);
    memset(data, 0, dataSize);
    
    CGContextRef context = CGBitmapContextCreate(data, width, height,
                                                 bitsPerComponent,
                                                 bytesPerRow, colorSpace,
                                                 (CGBitmapInfo)kCGImageAlphaPremultipliedLast | kCGBitmapByteOrder32Big);
    
    CGFloat r, g, b, a;
    UIColor *colorAtPixel = color;
    
    r = 0, g = 0, b = 0, a = 0;
    
    for (int x = 0; x < (int)width; x++)
    {
        for (int y = 0; y < (int)height; y++)
        {
            if (x == (int)width - 1 && y == 0) {
                colorAtPixel = [color colorWithAlphaComponent:0.99f];  // top right
            }
            
            [colorAtPixel getRed:&r green:&g blue:&b alpha:&a];
            
            
            int byteIndex = (int)((bytesPerRow * y) + x * bytesPerPixel);
            data[byteIndex + 0] = (int)roundf(r * 255);    // R
            data[byteIndex + 1] = (int)roundf(g * 255);  // G
            data[byteIndex + 2] = (int)roundf(b * 255);  // B
            data[byteIndex + 3] = (int)roundf(a * 255);  // A
            
            colorAtPixel = color;
        }
    }
    
    CGColorSpaceRelease(colorSpace);
    CGImageRef imageRef = CGBitmapContextCreateImage(context);
    result = [UIImage imageWithCGImage:imageRef scale:scale orientation:UIImageOrientationUp];
    CGImageRelease(imageRef);
    CGContextRelease(context);
    free(data);

    result = [result resizableImageWithCapInsets:UIEdgeInsetsMake(2, 2, 2, 2) resizingMode:UIImageResizingModeStretch];
    
    return result;
    
}

@end

So, there it is. It works, yes, but not perfectly, and the whole thing feels a bit hacky. I wonder if there’s a better way to do this, or if in the future Apple will start to update this code. Seeing as we see collapsing address bars on WebView controllers like in Safari, perhaps they might abstract this further to support UINavigationControllers.

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