在NSView上绘图

Question

我试图使用鼠标事件在视图上绘制，但是xcode给出了"无效的上下文"。有谁能告诉我我做错了什么吗？

class DrawingView: NSView {

var gContext:CGContextRef?

var lastPoint = NSPoint(x:0, y:0)
var red: CGFloat = 0.0
var green: CGFloat = 0.0
var blue: CGFloat = 0.0
var brushWidth: CGFloat = 10.0
var opacity: CGFloat = 1.0
var swiped = false

func drawLineFrom(p1:NSPoint, p2:NSPoint){

    // CGContextBeginPath(gContext)
    // CGContextSetRGBStrokeColor(gContext, 0, 0, 0, 1)
    NSColor.redColor().set() // choose color
    CGContextSetLineWidth(gContext, 10.0)

    CGContextMoveToPoint(gContext, p1.x, p1.y)
    CGContextAddLineToPoint(gContext, p2.x, p2.y)

    CGContextStrokePath(gContext)

    // self.setNeedsDisplayInRect(self.bounds)
    needsDisplay = true

}

override func mouseUp(theEvent: NSEvent) {
    swiped = true

    let droppings = theEvent.locationInWindow
    let currentPoint:NSPoint = self.convertPoint(droppings, fromView: nil)
    self.drawLineFrom(lastPoint, p2: currentPoint)

    Swift.print("Up \(currentPoint)")
    lastPoint = currentPoint
}

override func mouseDown(theEvent: NSEvent) {
    // let position = theEvent.locationInWindow
    // Swift.print("Mouse Down: \(position.x) - \(position.y)")

    if theEvent.clickCount == 1{
        let point:NSPoint = self.convertPoint(theEvent.locationInWindow, fromView: self)
        Swift.print("Point >> \(point)")
        lastPoint = point
    }

}

override func drawRect(dirtyRect: NSRect) {
    super.drawRect(dirtyRect)

    // Drawing code here.
    let context = NSGraphicsContext.currentContext()?.CGContext

    let rawContext = NSGraphicsContext.currentContext()
    rawContext?.saveGraphicsState()

    self.gContext = context

    CGContextSetRGBStrokeColor(context, 0, 0, 0, 1)
    CGContextSetLineWidth(context, 10.0)

    CGContextMoveToPoint(context, 0, 0)
    CGContextAddLineToPoint(context, 200, 200)

    CGContextStrokePath(context)
}

以下是我的调试输出...

点 >> (247.71484375, 108.34375) Oct 25 15:40:07 OSX Basics[17113] : CGContextSetFillColorWithColor: 无效的上下文0x0。如果您想查看回溯，请设置CG_CONTEXT_SHOW_BACKTRACE环境变量。 Oct 25 15:40:07 OSX Basics[17113] : CGContextSetStrokeColorWithColor: 无效的上下文0x0。如果您想查看回溯，请设置CG_CONTEXT_SHOW_BACKTRACE环境变量。 向上 (394.96484375, 220.5078125)

- Carlos Farini

Answer 1

你不能在draw rect方法之外绘制视图内容。以下代码是错误的原因：

func drawLineFrom(p1:NSPoint, p2:NSPoint) {
    NSColor.redColor().set() // choose color
    // ....
}

您从鼠标抬起事件调用drawLineFrom（:)。 当没有上下文存在时，您尝试更改当前图形上下文的颜色。 要重新绘制视图，必须通过self.needsDisplay = true发出重新绘制请求，然后在draw rect方法中进行更改。

以下是示例代码：

class DrawingView: NSView {

    var point: CGPoint = CGPoint.zero

    override func mouseUp(theEvent: NSEvent) {
        // Convert the point from the window coordinate to the view coordinate
        self.point = self.convertPoint(theEvent.locationInWindow, fromView: nil) 
        // We ask to redraw the view
        self.needsDisplay = true 
        Swift.print("Up \(self.point)")
    }
}

override func drawRect(dirtyRect: NSRect) {
    super.drawRect(dirtyRect)

    // You have to draw inside this function and not outside
    // The context is only valid here and you cannot retain it to draw later
    let context = NSGraphicsContext.currentContext()?.CGContext
    // Draw here
}

- Jeremy Vizzini