使用 e.HasMorePages 打印新页面

Question

我有点困惑如何使用HasMorePages 属性。我正在尝试通过 YPosition 方法打印更多页面，但这会导致打印页面的无限循环。

这是我的代码：

        private float YPosition()
        {
            return this.TopMargin + ((float)this.LinesCount * this.Font.GetHeight(this.Graphics) + (float)this.ImagesHeight);
        }

        private void TicketPrintPage(object sender, PrintPageEventArgs e)
        {
            e.Graphics.PageUnit = GraphicsUnit.Millimeter;
            this.Graphics = e.Graphics;
            foreach (Tuple<Object, LineTypes> tuple in this.Objects)
            {
                switch (tuple.Item2)
                {
                    case LineTypes.LINE:
                        this.Graphics.DrawString((String)tuple.Item1, this.Font, this.SolidBrush, this.LeftMargin, this.YPosition(), new StringFormat());
                        this.LinesCount++;
                        break;
                    case LineTypes.IMAGE:
                        Image Image = (Image)tuple.Item1;

                        // Center Image, using PaperSize
                        Graphics GraphicsImage = Graphics.FromImage(Image);
                        RectangleF RectangleF = e.MarginBounds;
                        RectangleF.Offset(-e.PageSettings.HardMarginX, -e.PageSettings.HardMarginY);

                        float InchX = RectangleF.X / 100f + (RectangleF.Width / 100f - (float)Image.Width / GraphicsImage.DpiX) / 2f;
                        Int32 MillimeterX = (Int32)Math.Ceiling(InchX / 0.039370);

                        this.Graphics.DrawImage(Image, new Point((int)this.LeftMargin + (MillimeterX / 2), (int)this.YPosition()));
                        double a = (double)Image.Height / 58.0 * 15.0;
                        this.ImagesHeight += (int)Math.Round(a) + 3;
                        break;
                }
                if ((YPosition() * 4) >= e.PageSettings.PrintableArea.Height)
                {
                    e.HasMorePages = true;
                    return;
                }
                else
                {
                    e.HasMorePages = false;
                }
            }
        }

YPosition代表页面中每一行或图片的高度。

如果所有对象都被处理，如何防止打印的无限循环并停止？

Answer 1

您需要使用while 循环和Enumerator，而不是for 循环。枚举器保持您正在处理的对象的状态，并将其作为成员存储在表单实例上。使用PrintDocument 类的BeginPrint 和EndPrint 事件来初始化和清理枚举器。

// this is the variable that holds the enumerator instance 
// once printing has started
IEnumerator<Tuple<Object, LineTypes>> ObjectsEnumerator;

// this is the event raised by printdocument at the start of printing
private void printDocument1_BeginPrint(object sender, System.Drawing.Printing.PrintEventArgs e)
{
    this.ObjectsEnumerator = Objects.GetEnumerator();
}

由于我们现在有一个枚举器，PrintPage 实现将使用它。它调用MoveNext 并将结果存储在HasMorePages 中。其余代码与您的代码类似，但请确保将页面上位置的计算保持在方法的本地。不要（ab）为此使用实例变量。请注意对 Current 的调用是 while 循环中的第一条语句。

private void printDocument1_PrintPage(object sender, System.Drawing.Printing.PrintPageEventArgs e)
{
    int linesCount = 0;
    float yPosition = 10;
    // take the next item from the list by calling MoveNext
    // and assign the outcome to e.HasMorePages
    while(e.HasMorePages = this.ObjectsEnumerator.MoveNext())
    {
        var tuple = this.ObjectsEnumerator.Current;
        switch (tuple.Item2)
        {
            case LineTypes.LINE:
                // draw magic
                e.Graphics.DrawString(tuple.Item1.ToString(), font, solidBrush, new PointF(10,yPosition));
                yPosition += 300;
                linesCount++;
                break;
            case LineTypes.IMAGE:
                Image Image = (Image)tuple.Item1;
                // e.Graphics.DrawImage(....);
                // calculate new yPosition
                yPosition += Image.Height;
                break;
        }
        // if we reach the end of the page
        if (yPosition >= e.PageSettings.PrintableArea.Height)
        {
            //we break out of the while loop
            // e.HasMorePages is already set
            break;
        }
    }
}

使用此代码，您可以迭代您的集合，如果页面已满，则退出循环，并在没有更多要打印的项目时停止迭代和打印。

如果打印完成，则调用 Endprint，其任务是清理枚举器。

private void printDocument1_EndPrint(object sender, System.Drawing.Printing.PrintEventArgs e)
{
    if (this.ObjectsEnumerator != null)
    {
        this.ObjectsEnumerator.Dispose();
        this.ObjectsEnumerator = null;
    }
}