附近的连接：为什么“下载”文件夹中的有效负载文件为空？

Question

I'm following the File transfer example on the Nearby Connections Exchange page (the "more complex example" code snippet).

I can send an image and receive it on another device in the Download/Nearby folder. The image is sent successfully since if I were to change the file name to give it an appropriate extension (eg .jpg), I can open the image in a photo gallery app.

        private void processFilePayload(long payloadId) {
            Payload filePayload = completedFilePayloads.get(payloadId);
            String filename = filePayloadFilenames.get(payloadId);
            if (filePayload != null && filename != null) {
                completedFilePayloads.remove(payloadId);
                filePayloadFilenames.remove(payloadId);

                // Retrieve received file from Downloads folder
                Payload.File payloadFile2 = filePayload.asFile();
                File payloadJavaFile = payloadFile2.asJavaFile();

                if (payloadJavaFile == null) {
                    Log.d(TAG, "Payload java file is null in processFilePayload()");
                } else {
                    payloadJavaFile.renameTo(new File(payloadJavaFile.getParentFile(), filename));
                }
            }
        }

Why is the payloadJavaFile variable null? From looking at Payload.class, I know that the result of asJavaFile() is a nullable File and that, from the asJavaFile() method description , calling asJavaFile() in processFilePayload() from within onPayloadReceived() (as is done in the example on the API page) may lead to the File not having received all of the payload's contents yet. However, I also call processFilePayload() from within onPayloadTransferUpdate() after verifying the success of the PayloadTransferUpdate, and so shouldn't the payload have received all of its contents by this stage (and not be null when calling asJavaFile() on the payload object)?

My code is almost the same as the documentation for both sending and receiving the image and file name.

Answer 1

Payload.asJavaFile() will be null if the READ_EXTERNAL_STORAGE permission is not properly set up for your application. You need READ_EXTERNAL_STORAGE in the AndroidManifest.xml and you need to also request READ_EXTERNAL_STORAGE permission at runtime since it is a dangerous permission. ( https://developer.android.com/training/permissions/requesting )

To share files using Nearby Connections, your application will need all of the following:

AndroidManifest.xml

<!-- Required for Nearby Connections -->
<uses-permission android:name="android.permission.BLUETOOTH" />
<uses-permission android:name="android.permission.BLUETOOTH_ADMIN" />
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
<uses-permission android:name="android.permission.CHANGE_WIFI_STATE" />
<uses-permission android:name="android.permission.ACCESS_COARSE_LOCATION" />
<!-- Optional: needed to share files -->
<uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE"/>

MainActivity.java

ActivityCompat.requestPermissions(thisActivity,
                new String[]{               
                  Manifest.permission.ACCESS_COARSE_LOCATION,
                  Manifest.permission.READ_EXTERNAL_STORAGE
                },
                PERMISSIONS_REQUEST_CODE);

We will improve the documentation to make this requirement for sharing files more obvious.

Answer 2

I checked the source code. From what I can see, asJavaFile() is always set (and I'm not sure why it's marked as @Nullable).

Unfortunately, for me to look into it any further, I'd need you to provide me with a sample app that reproduces the bug you're seeing.