Category Archives: Objective-C

Relation between MKMapRect and Spherical Mercator Resolution

MKMapRect projected coordinate system used in iOS MapKit  is not the same as Spherical Mercator used by Google, Bing and others. However there is a close linear relationship of the resolution in official spherical mercator and scale used by MapKit and that is   0.149291.

this number is constant for this relation :spherical mercator resolution (e.g. used by OpenLayers)  * zoomscale (used in MapKit)

spherical mercator resolution calculated as : spherical mercator width / view width

zoomscale in MapKit is calculated as : view width / mkMapRect.size.width

that is : spherical mercator width / mkMapRect.size.width = 0.149291  always.

So now any resolution from Spherical Mercator can be directly mapped into the MapKit internal coordinate system (mkMapRect) and vice versa without heavy calculations from mkMapRect to WGS84 and then to Spherical Mercator.

more about projection used in MapKit can be found here : LocationAwarenessPG


How to disable base Google Maps in MapKit

I have searched this and couldn’t find any answer, so here is my own research so far on this subject. You can disable MapKit base layers using 3 approaches:

#1 remove subview from MKMapView (quite bad as you will loose all overlays too)

#2 use undocumented function (bad too as this will be rejected by Apple approval process):

// get MKMapTileView from view hierarchy

UIView * mkMapTileView = [((UIView*) [ ((UIView*)[self.subviews objectAtIndex:0]).

subviews objectAtIndex:0]).subviews objectAtIndex:0];

// call undocumented method

if ( [mkMapTileView respondsToSelector:@selector(setDrawingEnabled:)]){

[mkMapTileView performSelector:@selector(setDrawingEnabled:) withObject:(id) NO];

#3 use method swizzle

inspired by http://www.mikeash.com/pyblog/friday-qa-2010-01-29-method-replacement-for-fun-and-profit.html

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

this method is fine as it is official method and enables you to ‘subclass’ a class that you don’t have access in compile time.

#import <objc/runtime.h>

// original method declaration

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

// set up subclass in runtime..in some entering method

UIView * mkMapTileView = [((UIView*) [ ((UIView*)[self.subviews objectAtIndex:0]). subviews objectAtIndex:0]).subviews objectAtIndex:0];

Method  origMethod = class_getInstanceMethod([mkMapTileView class], @selector(drawLayer:inContext:));

_origDrawLayerInContext = (void *)method_getImplementation(origMethod);

if(!class_addMethod([mkMapTileView class], @selector(drawLayer:inContext:), (IMP)OverrideDrawLayerInContext, method_getTypeEncoding(origMethod)))

method_setImplementation(origMethod, (IMP)OverrideDrawLayerInContext);

// implement our Override

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

// possibly call original method if you leave it empty base google maps will not be displayed. You can draw custom content here as well…

//  _origDrawLayerInContext(self, _cmd, layer, context);

}


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