Making UIToolbar and UINavigationBar’s background totally transparent

源自：http://atastypixel.com/blog/making-uitoolbar-and-uinavigationbars-background-totally-transparent/

I have an upcoming iPhone application, Cartographer, that is highly stylised and requires high customisation of the interface to achieve a convincing, beautiful vintage look. To make it work, I needed transparent toolbars and navigation bars for my UIViewController-based views.

The solution I came up with for this was to implement a category on UINavigationBar and UIToolbar, and overriding drawRect: with a method that does absolutely nothing. Then I can place my own textures behind the bar, and they’ll be seen, instead of the default bar background.

@interface UINavigationBar (TransparentAdditions)

@end

@implementation UINavigationBar (TransparentAdditions)

- (void)drawRect:(CGRect)rect {

    // Do nothing!

}

@end

For UIToolBar, if you’re using it within a UINavigationController, you’ll want to also overridedrawLayer:inContext:, as this appears to be used instead of drawRect: when used within a navigation controller, for some weird reason.

Note that this method will affect all bars in your app. If you only want some bars to be transparent, you’ll need to do a little objc-hocus-pocus. Thanks to Mike Ash for this solution on method replacement (read that article for the whys and hows). This technique replaces the default methods as before, but keeps track of the defaults. If you now set the tintColor of the bar to [UIColor clearColor], the bar will have a transparent background. Otherwise, it’ll just look the same as usual.

For UIToolbar (same principle for UINavigationBar):

#import <objc/runtime.h> // Keep track of default implementation

static void (*_origDrawRect)(id, SEL, CGRect);

static void (*_origDrawLayerInContext)(id, SEL, CALayer*, CGContextRef); 

// Override for drawRect:

static void OverrideDrawRect(UIToolbar *self, SEL _cmd, CGRect r) {

    if ( [[self tintColor] isEqual:[UIColor clearColor]] ) {

        // Do nothing

    } else {

        // Call default method

        _origDrawRect(self, _cmd, r);

    }

} 

// Override for drawLayer:inContext:

static void OverrideDrawLayerInContext(UIToolbar *self, SEL _cmd, CALayer *layer, CGContextRef context) {

    if ( [[self tintColor] isEqual:[UIColor clearColor]] ) {

        // Do nothing

    } else {

        // Call default method

        _origDrawLayerInContext(self, _cmd, layer, context);

    }

}  

@implementation UIToolbar (TransparentAdditions) 

+ (void)load {

    // Replace methods, keeping originals

    Method origMethod = class_getInstanceMethod(self, @selector(drawRect:));

    _origDrawRect = (void *)method_getImplementation(origMethod);

     if(!class_addMethod(self, @selector(drawRect:), (IMP)OverrideDrawRect, method_getTypeEncoding(origMethod)))

        method_setImplementation(origMethod, (IMP)OverrideDrawRect); 

    origMethod = class_getInstanceMethod(self, @selector(drawLayer:inContext:));

    _origDrawLayerInContext = (void *)method_getImplementation(origMethod); 

    if(!class_addMethod(self, @selector(drawLayer:inContext:), (IMP)OverrideDrawLayerInContext, method_getTypeEncoding(origMethod))

        method_setImplementation(origMethod, (IMP)OverrideDrawLayerInContext);

}

 @end

You can now add background texture to the bars in a number of ways. These are two I’ve used:

• By adding a CALayer to [bar layer] — but note that UINavigationBar will try to add elements at index 0, underneath your background. To make this work, I provided a subclassed CALayer (and overrode UINavigationBar’s +layer method) which only letsyou insert layers at index 0, via a custom method, and override insertLayer:atIndex:method, setting index to 1 if it’s 0. UIToolbar doesn’t require this.
• Or, by adding a CALayer to your view layer. Note that the view’s bounds do not cover the UINavigationBar; I had to offset the layer by the height of the bar in question (navigationBarLayer.frame = CGRectMake(0, -self.navigationController.navigationBar.frame.size.height, [barImage size].width, [barImage size].height);, for example), and setself.view.clipsToBounds = NO to allow the layer to be seen.

Of course, you can also draw the texture in drawRect:, instead. It’s entirely up to you. The advantage in using a CALayer is that it can overlap the view boundary, for effects like drop shadows.

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