objective c - Drawing selection box (rubberbanding, marching ants) in Cocoa, ObjectiveC

Question

I've currently implemented a simple selection box using mouse events and redrawing a rectangle on mouse drag. Here's my code:

-(void)drawRect:(NSRect)dirtyRect
{
if (!NSEqualRects(self.draggingBox, NSZeroRect))
{
    [[NSColor grayColor] setStroke];
    [[NSBezierPath bezierPathWithRect:self.draggingBox] stroke];
}
}

#pragma mark Mouse Events

- (void)mouseDown:(NSEvent *)theEvent
{
    NSPoint pointInView = [self convertPoint:[theEvent locationInWindow] fromView:nil];
    self.draggingBox = NSMakeRect(pointInView.x, pointInView.y, 0, 0);
    [self setNeedsDisplay:YES];
}

- (void)mouseDragged:(NSEvent *)theEvent
{
    NSPoint pointInView = [self convertPoint:[theEvent locationInWindow] fromView:nil];
    _draggingBox.size.width = pointInView.x - (self.draggingBox.origin.x);
    _draggingBox.size.height = pointInView.y - (self.draggingBox.origin.y);
    [self setNeedsDisplay:YES];
}

- (void)mouseUp:(NSEvent *)theEvent
{
    self.draggingBox = NSZeroRect;
    [self setNeedsDisplay:YES];
}

Ref: http://cocoadev.com/HowToCreateWalkingAnts

Questions:

Is this the most efficient way to do this? If the view was complex, would it be more efficient to draw a transparent view over the main view instead of continuously redrawing the view for the duration of the mouse drag (http://www.cocoabuilder.com/archive/cocoa/99877-drawing-selection-rectangle.html)? How is this done? I can't seem to find any examples.

Answer 1

You can use QuartzCore to animate the "marching ants" for you, if you want. This gets you completely out of the world of manually drawing the rubber-banded selection box, too. You just define the path that defines that box, and let Core Animation take care of drawing the box, as well as animating it.

In the XIB's "View Effects" Inspector, turn on "Core Animation", and then you can do something like:

#import <QuartzCore/QuartzCore.h>
#import "CustomView.h"

@interface CustomView ()

@property (nonatomic) NSPoint startPoint;
@property (nonatomic, strong) CAShapeLayer *shapeLayer;

@end

@implementation CustomView

#pragma mark Mouse Events

- (void)mouseDown:(NSEvent *)theEvent
{
    self.startPoint = [self convertPoint:[theEvent locationInWindow] fromView:nil];

    // create and configure shape layer

    self.shapeLayer = [CAShapeLayer layer];
    self.shapeLayer.lineWidth = 1.0;
    self.shapeLayer.strokeColor = [[NSColor blackColor] CGColor];
    self.shapeLayer.fillColor = [[NSColor clearColor] CGColor];
    self.shapeLayer.lineDashPattern = @[@10, @5];
    [self.layer addSublayer:self.shapeLayer];

    // create animation for the layer

    CABasicAnimation *dashAnimation;
    dashAnimation = [CABasicAnimation animationWithKeyPath:@"lineDashPhase"];
    [dashAnimation setFromValue:@0.0f];
    [dashAnimation setToValue:@15.0f];
    [dashAnimation setDuration:0.75f];
    [dashAnimation setRepeatCount:HUGE_VALF];
    [self.shapeLayer addAnimation:dashAnimation forKey:@"linePhase"];
}

- (void)mouseDragged:(NSEvent *)theEvent
{
    NSPoint point = [self convertPoint:[theEvent locationInWindow] fromView:nil];

    // create path for the shape layer

    CGMutablePathRef path = CGPathCreateMutable();
    CGPathMoveToPoint(path, NULL, self.startPoint.x, self.startPoint.y);
    CGPathAddLineToPoint(path, NULL, self.startPoint.x, point.y);
    CGPathAddLineToPoint(path, NULL, point.x, point.y);
    CGPathAddLineToPoint(path, NULL, point.x, self.startPoint.y);
    CGPathCloseSubpath(path);

    // set the shape layer's path

    self.shapeLayer.path = path;

    CGPathRelease(path);
}

- (void)mouseUp:(NSEvent *)theEvent
{
    [self.shapeLayer removeFromSuperlayer];
    self.shapeLayer = nil;
}

@end

This way, Core Animation takes care of the marching ants for you.