为什么QGraphicsItem :: scenePos（）保持返回（0,0） [英] Why does QGraphicsItem::scenePos() keep returning (0,0)

问题描述

我一直在玩这段代码：

  QGraphicsLineItem * anotherLine = this-> addLine ，100,100）; 
 qDebug（）<< anotherLine-> scenePos（）; 
 
 QGraphicsLineItem * anotherLine2 = this-> addLine（80,10,300,300）; 
 qDebug（）<< anotherLine2-> scenePos（）; 
  

此指针指 QGraphicsScene 。在这两种情况下，我得到两个输出 QPointF（0,0）。
从阅读文档，我认为 scenePos code>应该返回场景中的线的位置，而不是它在其局部坐标系内的位置。我在做错了什么？

解决方案

仔细阅读了添加Line ，我意识到我一直做错了。根据文档：

请注意，项目的几何体是在项目坐标中提供的
，其
位置是初始化为（0,0）

所以如果我指定addLine（50,50，100，100）局部项坐标。我假设它将被视为场景坐标是错误的或毫无根据的。我应该做的是这

  //创建一个长度为100的行
 QGraphicsItem * anotherLine = addLine ，0,100,100）; 
 
 //将它移动到我想要它在场景中的位置
 anotherLine-> setPos（50,50）; 
  

所以如果我通过在场景中绘制来添加一行，我需要将它的中心重置为0,0）然后使用setPos（）将它移动到我想要它在场景中的位置。

希望这可以帮助任何绊倒同样的问题。 / p>

I have been toying with this piece of code:

QGraphicsLineItem * anotherLine = this->addLine(50,50, 100, 100);
qDebug() << anotherLine->scenePos();

QGraphicsLineItem * anotherLine2 = this->addLine(80,10, 300, 300);
qDebug() << anotherLine2->scenePos();

Where the this pointer refers to a QGraphicsScene. In both cases, I get QPointF(0,0) for both output.
From reading the document, I thought scenePos() is supposed to return the position of the line within the scene, not where it is within its local coordinate system. What am I doing wrong?

解决方案

After reading the QT 4.5 documentation carefully on addLine, I realize what I have been doing wrong. According to the doc:

Note that the item's geometry is provided in item coordinates, and its position is initialized to (0, 0)

So if I specify addLine(50,50, 100, 100), I am actually modifying its local item coordinate. The assumption I made that it will be treated as a scene coordinate is wrong or unfounded. What I should be doing is this

// Create a line of length 100
QGraphicsItem * anotherLine = addLine(0,0, 100, 100); 

// move it to where I want it to be within the scene
anotherLine->setPos(50,50);

So if I am adding a line by drawing within the scene, I need to reset its centre to (0,0) then use setPos() to move it to where I want it to be in the scene.

Hope this helps anyone who stumble upon the same problem.

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