我正在尝试制作一个从互联网下载pdf文件并将其保存到“下载”文件夹的应用。下载pdf后，而不是打开第三方pdf应用程序，我希望它在应用程序本身中呈现。（我已经完成了下载并保存在Downloads文件夹中）。为此，我必须使用PDFRendered。但是我面临的问题是我是新手，在看了一些教程之后，我能够正确渲染pdf，但前提是该文件存在于资产文件夹中。下载的pdf文件位于“下载”文件夹中。有什么方法可以使用Downloads文件夹中的pdf并在应用程序中呈现它，而不是在资产文件夹中预定义pdf文件？
我尝试下载文件并将其保存在“下载”文件夹中，但已成功完成，但由于它不在资源文件夹中，因此无法使用该文件进行渲染。
//Code which is successfully rendering pdf from assets folder. How to
//use it to render pdf from Download folder?
public class PdfRender extends AppCompatActivity {
@BindView(R.id.pdf_image) ImageView imageViewPdf;
@BindView(R.id.button_pre_doc) FloatingActionButton prePageButton;
@BindView(R.id.button_next_doc) FloatingActionButton nextPageButton;
private static final String FILENAME = Environment.DIRECTORY_DOWNLOADS+"/a.pdf";
private int pageIndex;
private PdfRenderer pdfRenderer;
private PdfRenderer.Page currentPage;
private ParcelFileDescriptor parcelFileDescriptor;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_pdf_render);
ButterKnife.bind(this);
pageIndex = 0;
}
@RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
@Override
protected void onStart() {
super.onStart();
try {
openRenderer(getApplicationContext());
showPage(pageIndex);
} catch (IOException e) {
e.printStackTrace();
}
}
@RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
@Override
public void onStop() {
try {
closeRenderer();
} catch (IOException e) {
e.printStackTrace();
}
super.onStop();
}
@RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
@OnClick(R.id.button_pre_doc)
public void onPreviousDocClick(){
showPage(currentPage.getIndex() - 1);
}
@RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
@OnClick(R.id.button_next_doc)
public void onNextDocClick(){
showPage(currentPage.getIndex() + 1);
}
@RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
private void openRenderer(Context context) throws IOException {
// In this sample, we read a PDF from the assets directory.
File file = new File(context.getCacheDir(), FILENAME);
if (!file.exists()) {
// Since PdfRenderer cannot handle the compressed asset file directly, we copy it into
// the cache directory.
InputStream asset = context.getAssets().open(FILENAME);
FileOutputStream output = new FileOutputStream(file);
final byte[] buffer = new byte[1024];
int size;
while ((size = asset.read(buffer)) != -1) {
output.write(buffer, 0, size);
}
asset.close();
output.close();
}
parcelFileDescriptor = ParcelFileDescriptor.open(file, ParcelFileDescriptor.MODE_READ_ONLY);
// This is the PdfRenderer we use to render the PDF.
if (parcelFileDescriptor != null) {
pdfRenderer = new PdfRenderer(parcelFileDescriptor);
}
}
@RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
private void closeRenderer() throws IOException {
if (null != currentPage) {
currentPage.close();
}
pdfRenderer.close();
parcelFileDescriptor.close();
}
@RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
private void showPage(int index) {
if (pdfRenderer.getPageCount() <= index) {
return;
}
// Make sure to close the current page before opening another one.
if (null != currentPage) {
currentPage.close();
}
// Use `openPage` to open a specific page in PDF.
currentPage = pdfRenderer.openPage(index);
// Important: the destination bitmap must be ARGB (not RGB).
Bitmap bitmap = Bitmap.createBitmap(currentPage.getWidth(), currentPage.getHeight(),
Bitmap.Config.ARGB_8888);
// Here, we render the page onto the Bitmap.
// To render a portion of the page, use the second and third parameter. Pass nulls to get
// the default result.
// Pass either RENDER_MODE_FOR_DISPLAY or RENDER_MODE_FOR_PRINT for the last parameter.
currentPage.render(bitmap, null, null, PdfRenderer.Page.RENDER_MODE_FOR_DISPLAY);
// We are ready to show the Bitmap to user.
imageViewPdf.setImageBitmap(bitmap);
updateUi();
}
/**
* Updates the state of 2 control buttons in response to the current page index.
*/
@RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
private void updateUi() {
int index = currentPage.getIndex();
int pageCount = pdfRenderer.getPageCount();
prePageButton.setEnabled(0 != index);
nextPageButton.setEnabled(index + 1 < pageCount);
}
@RequiresApi(api = Build.VERSION_CODES.LOLLIPOP)
public int getPageCount() {
return pdfRenderer.getPageCount();
}
}
更新-：我尝试对Rendering应用的工作代码进行更改，并编辑openRenderer（）进行更改，并提供如下文件名和位置，但即使检测到pdf文件也无法打开（使用file.exists（））签入日志。有什么办法找到解决这个问题的方法吗？
`private void openRenderer(Context context) throws IOException {
File file = new File("storage/emulated/0/Download/" + FILENAME);
if (!file.exists()) {
Log.e("testit", "Not exists");
FileInputStream asset = new FileInputStream(file);
FileOutputStream output = new FileOutputStream(file);
final byte[] buffer = new byte[1024];
int size;
while ((size = asset.read(buffer)) != -1) {
output.write(buffer, 0, size);
}
asset.close();
output.close();
}
parcelFileDescriptor = ParcelFileDescriptor.open(file,ParcelFileDescriptor.MODE_READ_ONLY);
// This is the PdfRenderer we use to render the PDF.
if (parcelFileDescriptor != null) {
pdfRenderer = new PdfRenderer(parcelFileDescriptor);
}
}`
我在日志中收到以下错误
`java.lang.NullPointerException: Attempt to invoke virtual method 'void android.graphics.pdf.PdfRenderer.close()' on a null object reference`
可以获取预期结果，并从互联网上下载pdf。单击下载按钮，下载后立即在应用程序本身中呈现pdf，用户无需使用任何第三方pdf应用程序或webview（适用于5.0或之后的用户）即可查看应用程序本身中的pdf。