对NSGraphicsContext.current所做的更改

Question

我正在从NSView的drawRect（）方法内部对当前的NSGraphicContext进行更改，一切正常。所做的任何更改都将通过随后对drawRect（）方法的调用而保持不变。

但是，我想做的是从另一个对象的方法中更改当前上下文。确实是从DrawRect（）方法内部调用了此方法。但是这些变化并不是持久的。

代码是这样的：

class SomeView:NSView {

  override func drawRect(dirtyRect: NSRect) {
    // myAObject is an instance of a Class i create, and stored inside SomeView
    // myAObject's exexcuteCode() method, makes changes to the current NSGraphicsContext
    myAObjectIntance.executeCode() 

    // In subsequent calls of drawRect, as the user keeps drawing using the 
    // mouse, another object, say myBObject's executeCode() method, draws,
    // lets say a rectangle.
    myBObjectIntance.executeCode()

    // Problem is, that any changes done to the current context by myAObjectIntance.executeCode()
    // seem to be lost.

    // If i change the context and draw inside the same method call, everything is OK.
    // If changes and drawing are done by different objects, changes are lost.
  }
}

从这两个方法myAObjectIntance.executeCode（）和myBObjectIntance.executeCode（）中，我使用如下代码获取当前的NSGraphicsContext：

NSGraphicsContext.current!.compositeOperation = .copy

或

NSGraphicsContext.current!.compositeOperation = .clear

我添加了一个更改前景色的小项目。在drawRect（）的顺序调用之间，颜色的更改未保留。NSGraphicsContextTest.zip

Answer 1

问题是您期望在对drawRect的多次调用之间保留您的图形和更改；那不会发生。每次调用drawRect时，都会重置上下文状态。那是设计使然。因此，每次调用drawRect时，都需要将上下文的状态设置为所需的状态，然后绘制以填充dirtyRect参数指定的区域。