Flutter: how to let a file be opened by an external app (like Android's implicit intent)?

It's hard to get this to work correctly. Here are some hints that helped me to launch ACTION_VIEW intents from Flutter, with files downloaded with Flutter.

1) Register a FileProvider in the android manifest:

<provider
    android:name="android.support.v4.content.FileProvider"
    android:authorities="com.example.myapp.provider"
    android:exported="false"
    android:grantUriPermissions="true">
    <meta-data
        android:name="android.support.FILE_PROVIDER_PATHS"
        android:resource="@xml/provider_paths"/>
</provider>

provider_paths.xml:

    <?xml version="1.0" encoding="utf-8"?>
    <paths>
        <external-path name="external_files" path="." />
    </paths>

2) Create a custom platform channel that provides 2 methods (Code below is Kotlin):

getDownloadsDir: Should return the data dir where downloaded files should be placed. Try this one:

val downloadsDir = applicationContext.getExternalFilesDir(Environment.DIRECTORY_DOWNLOADS).path
result.success(downloadsDir);

previewFile that takes 2 string arguments: path (File.path in Dart) and type (e.g. "application/pdf"):

val file = File(path);
val uri = FileProvider.getUriForFile(this, "com.example.myapp.provider", file);

val viewFileIntent = Intent(Intent.ACTION_VIEW);
viewFileIntent.setFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP or Intent.FLAG_GRANT_READ_URI_PERMISSION);
viewFileIntent.setDataAndType(uri, type);
try {
  startActivity(viewFileIntent);
  result.success(null);
} catch (e: ActivityNotFoundException) {
  result.error(...);
}

The most important part is the creation of the FileProvider. url_launcher and android_intent will not work, you have to create your own platform channel. You can change the download path, but then you also have to find the correct provider/permissions settings.

Making this work on iOS is also possible, but out of scope of this question.

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If you are using the image_picker plugin:

The FileProvider conflicts with the current version of the image_picker (0.4.6) plugin, a fix will be released soon.