Finding an NSView's current magnification.

Q:  How can I find out what the current scale of an NSView's coordinate space is, if it's been changed from the default?

A: How can I find out what the current scale of an NSView's coordinate space is, if it's been changed from the default?

NSView provides a method, -convertSize:fromView:, to convert sizes between the coordinate spaces of various views in a window. If you convert a known size from the window's base coordinate space to that of the NSView, the result is the current zoom level.

Another NSView method, -scaleUnitSquareToSize:, sets the magnification relative to the view's existing coordinate system. By using an Objective-C category to add a few methods to NSView, we can provide a convenient API to scale NSView instances in absolute terms.

Listing 1  ScaleUtilities category declaration

@interface NSView (ScaleUtilities) @property (assign) NSSize scale; - (void)resetScaling; @end

Listing 2  ScaleUtilities category implementation

@implementation NSView (ScaleUtilities)

static const NSSize unitSize = {1.0, 1.0};

// Returns the scale of the receiver's coordinate system, relative to the window's base coordinate system.
- (NSSize)scale;
{
    return [self convertSize:unitSize toView:nil];
}

// Sets the scale in absolute terms.
- (void)setScale:(NSSize)newScale;
{
    [self resetScaling]; // First, match our scaling to the window's coordinate system
    [self scaleUnitSquareToSize:newScale]; // Then, set the scale.
    [self setNeedsDisplay:YES]; // Finally, mark the view as needing to be redrawn
}

// Makes the scaling of the receiver equal to the window's base coordinate system.
- (void)resetScaling;
{
    [self scaleUnitSquareToSize:[self convertSize:unitSize fromView:nil]];
}

@end

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