Category Archives: Uncategorized

PDF reader from URL hybrid approach

As many StackOverflow questions and answers hint at, there is no built-in support for reading PDF files on Android, which is kind of a shame, especially since iOS has native support for the format. The vision must have been that user’s will select their own favorite PDF reader app, and then read all other apps’ PDF files through that reader app.

However, as an app developer, you cannot make assumptions on which PDF reader apps exist on a user’s device – you cannot even assume that one is installed! Several StackOverflow posts suggest redirecting the user to Google Drive’s (formerly Google Docs) online HTML PDF viewer. This solution is kind of bulky though, and doesn’t work as well as using a native app (Like Adobe Reader or other PDF reader apps).

This inspired me to come up with a hybrid solution for reading that checks if a PDF reader app is installed and does the follwing:

  • If a reader is installed, download the PDF file to the device and start a PDF reader app
  • If no reader is installed, ask the user if he wants to view the PDF file online through Google Drive

NOTE! This solution uses the Android DownloadManager class, which was introduced in API9 (Android 2.3 or Gingerbread). This means that it doesn’t work on Android 2.2 or earlier.

The main method for the code is PDFTools.showPDFUrl( Context context, String pdfUrl );

The code can be found on GitHub or just right here:

public class PDFTools {
	private static final String GOOGLE_DRIVE_PDF_READER_PREFIX = "http://drive.google.com/viewer?url=";
	private static final String PDF_MIME_TYPE = "application/pdf";
	private static final String HTML_MIME_TYPE = "text/html";
	
	/**
	 * If a PDF reader is installed, download the PDF file and open it in a reader. 
	 * Otherwise ask the user if he/she wants to view it in the Google Drive online PDF reader.<br />
	 * <br />
	 * <b>BEWARE:</b> This method
	 * @param context
	 * @param pdfUrl
	 * @return
	 */
	public static void showPDFUrl( final Context context, final String pdfUrl ) {
		if ( isPDFSupported( context ) ) {
			downloadAndOpenPDF(context, pdfUrl);
		} else {
			askToOpenPDFThroughGoogleDrive( context, pdfUrl );
		}
	}

	/**
	 * Downloads a PDF with the Android DownloadManager and opens it with an installed PDF reader app.
	 * @param context
	 * @param pdfUrl
	 */
	@TargetApi(Build.VERSION_CODES.GINGERBREAD)
	public static void downloadAndOpenPDF(final Context context, final String pdfUrl) {
		// Get filename
		final String filename = pdfUrl.substring( pdfUrl.lastIndexOf( "/" ) + 1 );
		// The place where the downloaded PDF file will be put
		final File tempFile = new File( context.getExternalFilesDir( Environment.DIRECTORY_DOWNLOADS ), filename );
		if ( tempFile.exists() ) {
			// If we have downloaded the file before, just go ahead and show it.
			openPDF( context, Uri.fromFile( tempFile ) );
			return;
		}

		// Show progress dialog while downloading
		final ProgressDialog progress = ProgressDialog.show( context, context.getString( R.string.pdf_show_local_progress_title ), context.getString( R.string.pdf_show_local_progress_content ), true );
		
		// Create the download request
		DownloadManager.Request r = new DownloadManager.Request( Uri.parse( pdfUrl ) );
		r.setDestinationInExternalFilesDir( context, Environment.DIRECTORY_DOWNLOADS, filename );
		final DownloadManager dm = (DownloadManager) context.getSystemService( Context.DOWNLOAD_SERVICE );
		BroadcastReceiver onComplete = new BroadcastReceiver() {
			@Override
			public void onReceive(Context context, Intent intent) {
				if ( !progress.isShowing() ) {
					return;
				}
				context.unregisterReceiver( this );
				
				progress.dismiss();
				long downloadId = intent.getLongExtra( DownloadManager.EXTRA_DOWNLOAD_ID, -1 );
				Cursor c = dm.query( new DownloadManager.Query().setFilterById( downloadId ) );
				
				if ( c.moveToFirst() ) {
					int status = c.getInt( c.getColumnIndex( DownloadManager.COLUMN_STATUS ) );
					if ( status == DownloadManager.STATUS_SUCCESSFUL ) {
						openPDF( context, Uri.fromFile( tempFile ) );
					}
				}
				c.close();
			}
		};
		context.registerReceiver( onComplete, new IntentFilter( DownloadManager.ACTION_DOWNLOAD_COMPLETE ) );
		
		// Enqueue the request
		dm.enqueue( r );
	}
	
	/**
	 * Show a dialog asking the user if he wants to open the PDF through Google Drive
	 * @param context
	 * @param pdfUrl
	 */
	public static void askToOpenPDFThroughGoogleDrive( final Context context, final String pdfUrl ) {
		new AlertDialog.Builder( context )
			.setTitle( R.string.pdf_show_online_dialog_title )
			.setMessage( R.string.pdf_show_online_dialog_question )
			.setNegativeButton( R.string.pdf_show_online_dialog_button_no, null )
			.setPositiveButton( R.string.pdf_show_online_dialog_button_yes, new OnClickListener() {
				@Override
				public void onClick(DialogInterface dialog, int which) {
					openPDFThroughGoogleDrive(context, pdfUrl);	
				}
			})
			.show();
	}
	
	/**
	 * Launches a browser to view the PDF through Google Drive
	 * @param context
	 * @param pdfUrl
	 */
	public static void openPDFThroughGoogleDrive(final Context context, final String pdfUrl) {
		Intent i = new Intent( Intent.ACTION_VIEW );
		i.setDataAndType(Uri.parse(GOOGLE_DRIVE_PDF_READER_PREFIX + pdfUrl ), HTML_MIME_TYPE );
		context.startActivity( i );
	}
	/**
	 * Open a local PDF file with an installed reader
	 * @param context
	 * @param localUri
	 */
	public static final void openPDF(Context context, Uri localUri ) {
		Intent i = new Intent( Intent.ACTION_VIEW );
		i.setDataAndType( localUri, PDF_MIME_TYPE );
		context.startActivity( i );
	}
	/**
	 * Checks if any apps are installed that supports reading of PDF files.
	 * @param context
	 * @return
	 */
	public static boolean isPDFSupported( Context context ) {
		Intent i = new Intent( Intent.ACTION_VIEW );
		final File tempFile = new File( context.getExternalFilesDir( Environment.DIRECTORY_DOWNLOADS ), "test.pdf" );
		i.setDataAndType( Uri.fromFile( tempFile ), PDF_MIME_TYPE );
		return context.getPackageManager().queryIntentActivities( i, PackageManager.MATCH_DEFAULT_ONLY ).size() > 0;
	}

}

SQLite INSERT OR REPLACE through ContentProvider

Introduction

A lot of times in an app, you need to update some locally cached data stored in a database with newer data fetched from e.g. a webservice. A normal way (or so it was for me) to do this is with the “if-not-update-then-insert” pattern, like this:

if ( getContentProvider().update( uri, values, selection, selectionArgs ) == 0 ) {
    getContentProvider().insert( uri, values );
}

Or with many rows, using bulkInsert():

List<ContentValues> valuesToInsert = new ArrayList<ContentValues>();
for ( ContentValues values: myValues ) {
    if ( getContentProvider().update( uri, values, selection, selectionArgs ) == 0 ) {
        valuesToInsert.add( values );
    }
}
getContentProvider.bulkInsert( uri, valuesToInsert.toArray( new ContentValues[0] ) );

While this might seem like a solid way to do it, there are a few problems:

First of all, some of the code is boilerplate and could be reduced a bit, but the major issue here is…

Thread safety

If multiple threads are running this code simultaneously, it is possible to violate your database constraints:

For example if you have a unique key in your table that the update() tries to match with, and two threads executing the code, the following could happen:

Both threads have run all the update() calls without updating anything and are about to run the insert()/bulkInsert() method. This means that both threads think that it is safe to insert the values, because they have already checked that no constraints are violated when no updates took place. The threads are, however, not aware of eachother’s plans to insert data with the same unique key, which undoubtedly will result in a constraint violation, and possibly an exception like the following:

  
android.database.sqlite.SQLiteConstraintException: error code 19: constraint failed
	at android.database.sqlite.SQLiteStatement.native_execute(Native Method)
	at android.database.sqlite.SQLiteStatement.execute(SQLiteStatement.java:61)
	at android.database.sqlite.SQLiteDatabase.insertWithOnConflict(SQLiteDatabase.java:1677)
	at android.database.sqlite.SQLiteDatabase.insertOrThrow(SQLiteDatabase.java:1547)

Why not just synchronize it?

An easy solution for this thread safety problem is simply to wrap the code in a synchronized block, like this:

synchronized( this ) {
    List<ContentValues> valuesToInsert = new ArrayList<ContentValues>();
    for ( ContentValues values: myValues ) {
        if ( getContentProvider().update( uri, values, selection, selectionArgs ) == 0 ) {
            valuesToInsert.add( values );
        }
    }
    getContentProvider.bulkInsert( uri, valuesToInsert.toArray( new ContentValues[0] ) );
}

This works, and no constraint violations occur anymore. But now we added more code and slowed down the execution of the code by locking it to one thread at a time.

Better solution: Using SQLite’s INSERT OR REPLACE statement

The better solution to this is to use one of SQLite’s already builtin features, the INSERT OR REPLACE statement. However, the ContentProvider only has a insert() method and not a replace method, so we’ll have to work around this in some clever way:

Fortunately, the ContentValues objects can hold an arbitrary amount of parameters to the ContentProvider, so we’ll use that to indicate that we want to do an INSERT OR REPLACE instead of just a normal INSERT.

First we’ll define some static string constant that we would never use as column name:

public static final String SQL_INSERT_OR_REPLACE = "__sql_insert_or_replace__";

Next, we’ll pass that along with the other values in our ContentValues object:

ContentValues values = new ContentValues();
values.put( SQL_INSERT_OR_REPLACE, true );
// ... fill the ContentValues with the actual values ...

// ... notice we only need one line to insert/replace
getContentProvider().insert( uri, values );

Now we have passed the parameter on, so we’ll modify our ContentProvider to handle this:

public Uri insert(Uri uri, ContentValues values) {
    // ...

    boolean replace = false;
    if ( values.containsKey( SQL_INSERT_OR_REPLACE ) {
        replace = values.getAsBoolean( SQL_INSERT_OR_REPLACE );

        // Clone the values object, so we don't modify the original.
        // This is not strictly necessary, but depends on your needs
        values = new ContentValues( values );

        // Remove the key, so we don't pass that on to db.insert() or db.replace()
        values.remove( SQL_INSERT_OR_REPLACE );
    }

    long rowId;
    if ( replace ) {
        rowId = db.replace(tableName, null, values);
    } else {
        rowId = db.insert(tableName, null, values);
    }

    // ...

}

Conclusion

There you have it. We can now call SQLite’s INSERT OR REPLACE function through our ContentProvider,
and even takes even less code, since we only have to call insert(), and not update(). Neat huh? :-)