如何在 uwp 打印中断开打印事件处理程序? [英] How to disconnect print event handler in uwp printing?
问题描述
我需要在多页 UWP 应用中打印单个 xaml 页面.我只能打印此页面，但是当离开该页面并尝试导航回该页面时，会引发以下异常:
I need to print a single xaml-page within a multiple page UWP-app. I only get as far as printing this page, but when leaving that page and trying to navigate back to it the following exception is thrown:
对于 PrintTaskRequested 事件，一次只能注册一个处理程序(翻译).
For the PrintTaskRequested-event only one handler at a time can be registered (translated).
MS 指令说您必须在离开打印页面时断开打印事件处理程序的连接.奇怪的是，我在 Stackoverflow 上发现了 2 个简短易懂的 uwp 打印示例，它们被标记为答案，但缺少断开连接的方法，因此在添加第二页时两者都像我一样坏了.
The MS-instruction says that you have to disconnect the printing event handlers when you leave the printing page. Strange enough I found 2 short and understandable uwp-printing examples here on Stackoverflow which are marked as answers, but lacked a method to disconnect, so both broke like mine when adding a second page.
MS 打印示例对我这个初学者来说太复杂了.我尝试使用示例代码中的片段构建我的代码，但我迷失在错误中.
The MS Print Sample is too complex for me as a beginner. I tried to build my code using pieces from the Sample code, but I got lost in errors.
这是我目前的测试代码.我删除了所有不需要的部分 - 只是为了预览和打印页面:
This is my testing code so far. I removed every piece that is not needed - just to preview and print a page:
using Microsoft.Toolkit.Uwp.Helpers;
namespace Print_190905
{
public sealed partial class MainPage : Page
{
private PrintManager printMan;
private PrintDocument printDoc;
private IPrintDocumentSource printDocSource;
private PrintHelper printHelper;
public MainPage()
{
this.InitializeComponent();
}
protected override void OnNavigatedTo(NavigationEventArgs e)
{
printMan = PrintManager.GetForCurrentView();
printMan.PrintTaskRequested += PrintTaskRequested;
printDoc = new PrintDocument();
printDocSource = printDoc.DocumentSource;
printDoc.GetPreviewPage += GetPreviewPage;
}
private async void PrintButton_Click(object sender, RoutedEventArgs e)
{
await PrintManager.ShowPrintUIAsync();
}
private void PrintTaskRequested(PrintManager sender, PrintTaskRequestedEventArgs args)
{
var printTask = args.Request.CreatePrintTask("Print", PrintTaskSourceRequrested);
}
private void PrintTaskSourceRequrested(PrintTaskSourceRequestedArgs args)
{
args.SetSource(printDocSource);
}
private void GetPreviewPage(object sender, GetPreviewPageEventArgs e)
{
printDoc.SetPreviewPage(e.PageNumber, this.Print_Area);
}
private void CmdZurueck_Click(object sender, RoutedEventArgs e)
{
this.Frame.Navigate(typeof(Seite_2));
}
}
为了避免返回打印页面时的错误，我从打印示例中添加了这段:
To avoid the error when returning to the print-page, I added this piece from the Print Sample:
protected override void OnNavigatedFrom(NavigationEventArgs e)
{
if (printHelper != null)
{
printHelper.Dispose();
}
}
但是:printHelper 始终为 Null，因此永远不会调用Dispose"方法.代码中没有分配值的地方，我无法找到打印示例中printHelper 的值分配的位置.
But: printHelper is always Null, so the "Dispose"-method is never called. There is no place in the code where a value is assigned, and I could not find out where in the Print Sample the value for printHelper is assigned.
当我在 PrintSample 中设置断点时，值为(PrintSample.PrintHelper)".
When I set a breakpoint in the PrintSample, the value is "(PrintSample.PrintHelper)".
这是什么意思?我可以以某种方式为 printHelper 分配一个有效值吗?我花了好几个小时才走到那一步.因此，如果有人可以提供帮助，我将不胜感激！谢谢！
What does that mean? Can I assign a valid value to printHelper somehow? It took me many hours to get that far. Therefore I would be very grateful if somebody could help! Thanks!
推荐答案
printMan.PrintTaskRequested -= PrintTaskRequested;
和
printDoc.GetPreviewPage -= GetPreviewPage;
在 OnNavigatedFrom 中.
within OnNavigatedFrom.
每次进入页面都会重新注册上述功能.但是，离开页面时它不会取消注册.所以你进入的页面越多，你注册的功能就越多.
The above functions are re-registered each time you enter the page. However it does not get unregistered when leaving the page. So the more you enter the page, the more functions you have registered.
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