Android PDFDocument正在生成空白文档

Question

我正在使用Android Studio并试图让我的应用程序生成PDF。我已经设置了一个活动来生成我希望我的PDF具有的内容。如果我让它在屏幕上显示，这工作正常。该活动的onCreate部分低于：

protected void onCreate(Bundle savedInstanceState) { 
    super.onCreate(savedInstanceState); 

    // Connect to database 
    connectToDatabase(); 

    // Set view 
    setContentView(R.layout.activity_export_pattern); 

    // find various views and set up their contents 
    // (I've left this bit out as it's quite long, but it doesn't contain 
    // anything controversial) 

    // Now try to export to PDF. 
    // Return success or failure to calling activity 
    // (basically just displays a toast indicating success/failure) 
    try { 
     if(exportToPDF()) { 
      Intent returnIntent = new Intent(); 
      setResult(Activity.RESULT_OK, returnIntent); 
      finish(); 
     }else { 
      Intent returnIntent = new Intent(); 
      setResult(Activity.RESULT_CANCELED, returnIntent); 
      finish(); 
     } 
    } catch (IOException e) { 
     e.printStackTrace(); 
     Intent returnIntent = new Intent(); 
     setResult(Activity.RESULT_CANCELED, returnIntent); 
     finish(); 
    } 
} 

如果我省略了最后一部分（在try-catch），就让它在屏幕上显示，它工作正常。屏幕显示我期望它显示的内容。

但是，为了让它创建一个PDF，我使用try-catch与调用exportToPDF，它包含以下代码（这基本上是Android文档中的代码，并有一些更改，如注释所示下面）：

public boolean exportToPDF() { 
     // Create PDF document 
     PdfDocument document = new PdfDocument(); 
     // Create page description 
     // Line below changed as Android Studio was highlighting an error; 
     // instead of Rect, I have just put numbers. 
     // I've varied the numbers from 100 up to 1000, to no effect 
     PdfDocument.PageInfo pageInfo = new PdfDocument.PageInfo.Builder(720, 720, 1).create(); 
     // Start page 
     PdfDocument.Page page = document.startPage(pageInfo); 
     // Changed the line below from content = getContentView, as that was causing an error 
     // pageContent is the id of the overall LinearLayout in the XML file 
     // If I break after this in the debugger, content seems to have found the correct view and be populated with the appropriate page elements 
     View content = this.findViewById(R.id.pageContent); 
     // Added the line below after finding it suggested on here in another question 
     // Doesn't seem to make any difference 
     content.layout(0, 0, 200, 200); 
     if (content != null) { 
      content.draw(page.getCanvas()); 
     } 
     // Finish page 
     document.finishPage(page); 

     // Write document content to external storage 
     // I'm using a FileOutputStream instead of BufferedOutputStream as given in the documentation, but, since this does at least produce a file, I don't think this is the source of the problem 
     String filename = this.item.getName() + ".pdf"; 
     File file = new File(Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_DOWNLOADS) + "/" + filename); 
     FileOutputStream fileOut = null; 
     try { 
      fileOut = new FileOutputStream(file); 
      document.writeTo(fileOut); 
      fileOut.close(); 
      document.close(); 
      return true; 
     } catch (IOException e) { 
      e.printStackTrace(); 
      if(fileOut != null) fileOut.close(); 
      document.close(); 
      return false; 
     } 
    } 

因此，运行这将产生一个pdf在正确的目录中，正确命名，但它是空白的。我不知道还有什么可以尝试，并会感激任何帮助！谢谢！

Answer 1

在活动中将视图写入PDF onCreate（）时，它将无法工作，并且因为您的视图返回0高度和宽度。您需要等待活动窗口附加，然后将视图写入pdf。您可以尝试从您的活动的onWindowFocusChanged（）方法中调用exportToPDF。

public void onWindowFocusChanged(boolean hasFocus) { 
    // TODO Auto-generated method stub 
    super.onWindowFocusChanged(hasFocus); 
try { 
     if(exportToPDF()) { 
      Intent returnIntent = new Intent(); 
      setResult(Activity.RESULT_OK, returnIntent); 
      finish(); 
     }else { 
      Intent returnIntent = new Intent(); 
      setResult(Activity.RESULT_CANCELED, returnIntent); 
      finish(); 
     } 
    } catch (IOException e) { 
     e.printStackTrace(); 
     Intent returnIntent = new Intent(); 
     setResult(Activity.RESULT_CANCELED, returnIntent); 
     finish(); 
    } 

    } 

您还可以使用ViewTreeObserver

View content = this.findViewById(R.id.pageContent); 
    ViewTreeObserver vto = content.getViewTreeObserver(); 
      vto.addOnGlobalLayoutListener(new OnGlobalLayoutListener() { 
       @Override 
       public void onGlobalLayout() { 
       try { 
      if(exportToPDF()) { 
       Intent returnIntent = new Intent(); 
       setResult(Activity.RESULT_OK, returnIntent); 
       finish(); 
      }else { 
       Intent returnIntent = new Intent(); 
       setResult(Activity.RESULT_CANCELED, returnIntent); 
       finish(); 
      } 
     } catch (IOException e) { 
      e.printStackTrace(); 
      Intent returnIntent = new Intent(); 
      setResult(Activity.RESULT_CANCELED, returnIntent); 
      finish(); 
     } 
content.getViewTreeObserver().removeGlobalOnLayoutListener(this); 
    } 
      });