在 NSView 上绘图答案

【问题标题】：Drawing on NSView在 NSView 上绘图
【发布时间】：2016-01-24 21:04:56
【问题描述】：

我正在尝试使用鼠标事件绘制视图，但 xcode 给了我“无效上下文”。谁能告诉我什么时候做错了？

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)
}

这是我的调试输出...

点>> (247.71484375, 108.34375) 10 月 25 日 15:40:07 OSX 基础知识 [17113]：CGContextSetFillColorWithColor：无效上下文 0x0。如果要查看回溯，请设置 CG_CONTEXT_SHOW_BACKTRACE 环境变量。 10 月 25 日 15:40:07 OSX 基础知识 [17113]：CGContextSetStrokeColorWithColor：无效上下文 0x0。如果要查看回溯，请设置 CG_CONTEXT_SHOW_BACKTRACE 环境变量。向上（394.96484375、220.5078125）

【问题讨论】：

标签： core-graphics swift2 xcode7 nsview cgcontext

【解决方案1】：

您不能在 draw rect 方法之外绘制视图的内容。错误是由于下面的代码：

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

您从鼠标向上事件调用 drawLineFrom(:)。当不存在上下文时，您尝试更改当前图形上下文的颜色。要重绘视图，您必须通过 self.needsDisplay = true 请求重绘，然后在 draw rect 方法中进行更改。

这是一个示例代码：

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
}

【讨论】：