Implementing Android Custom Gesture and Pinch Recognition

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Detecting Common Gestures using the Android Gesture Detector ClassAn Introduction to Android Fragments


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The previous chapter looked at the steps involved in detecting a few common gestures from within an Android application. In practice, however, a gesture can conceivably involve just about any sequence of touch motions on the device display. In recognition of this fact, the Android SDK allows custom gestures to be defined by the application developer and used to trigger events when performed by the user. This is a multistage process, the details of which are the topic of this chapter.




The Android Gesture Builder Application

The Android SDK allows developers to design custom gestures that are then stored in a gesture file bundled with an application. Gestures are most easily designed using the Gesture Builder application which is bundled with the samples package supplied as part of the Android SDK. The creation of a gestures file involves launching the Gesture Builder application on a physical device or emulator session and then “drawing” the gestures that will need to be detected by the application. Once the gestures have been designed, the file containing the gesture data can be pulled off the SD card of the device or emulator and added to the application project. Within the application code, the file is then loaded into a GestureLibrary instance where it can be used to search for matches to gestures performed by the user.

The GestureOverlayView Class

In order to facilitate the detection of gestures within an application, the Android SDK provides the GestureOverlayView class. This is a transparent view that can be placed over other views in the user interface for the sole purpose of detecting gestures.


Detecting Gestures

Gestures are detected by loading the gestures file created using the Gestures Builder app and then registering a GesturePerformedListener event listener on an instance of the GestureOverlayView class. The enclosing class is then declared to implement both the OnGesturePerformedListener interface and the corresponding onGesturePerformed callback method required by that interface. In the event that a gesture is detected by the listener, the onGesturePerformed callback method is called by the runtime system.

Identifying Specific Gestures

When a gesture is detected, the onGesturePerformed callback method is called and passed as arguments a reference to the GestureOverlayView object on which the gesture was detected, together with a Gesture object containing information about the gesture.

With access to the Gesture object, the GestureLibrary can then be used to compare the detected gesture to those contained in the gestures file previously loaded into the application. The GestureLibrary reports the probability that the current gesture matches an entry in the gestures file by calculating a prediction score for each gesture. A prediction score of 1.0 or greater is generally accepted to be a good match for the gesture.

Adding SD Card Support to an AVD

Before the Gestures Builder app can be used to design gestures on an emulator, the virtual device needs to be configured with a virtual SD card onto which the gestures file will be saved (this is not necessary when using a physical Android device).

For the purposes of this example, the AVD created in the chapter entitled Creating an Android Virtual Device (AVD) will be modified to add SD Card support. Within the Eclipse environment, therefore, select the Window -> AVD Manager menu option. Within the resulting Android Virtual Device Manager screen, select an AVD (in this case the Nexus7 emulator) and click on the Edit button to display the editing dialog as illustrated in Figure 20-1.

Within the configuration dialog, enter a value of 10 MiB into the Size field of the SD Card section of the dialog, and then click on the OK button to commit the changes. The AVD now has virtual SD Card storage available to store the gestures file.


Adding an SD Card to an Android AVD

Figure 20-1


Building and Running the Gesture Builder Application

The Gesture Builder application is not bundled by default with the AVD emulator profile for most versions of the SDK. It is not, however,, nor is it pre-installed on most physical Android devices. If the utility is pre-installed, it will be listed along with the other Apps installed in the device or AVD instance. In the event that it is not installed, the source code for the utility isIt is, however, included amongst the SDK samples and consequently may be compiled and run on any Android device or emulator.

To install and build the GestureBuilder utility, launch theWithin the Eclipse environment and, select the Window -> Android SDK Manager menu option. Once the manager has loaded, locate the Samples for SDK package located beneath the Android 4.42.2 (API1917) section. If the package is not already installed, set the checkbox next to the package and click on the Install 1 Package button to initiate the installation.

Once the installation is complete, the SDK samples will be installed in the following directory (where <path_to_adt_installation> represents the location on your system where the ADT bundle was originally installed):

<path_to_adt_installation>/sdk/samples/android-19

The source code to the Gestures Builder application is located within this directory in a folder named GestureBuilder within the legacy sub-folder.

Returning to the Eclipse window, select the File -> Import… menu option and, within the resulting dialog, select Android -> Existing Android Code into Workspace before clicking Next. Click on the Browse button next to the Root Directory text box, navigate to and select the GestureBuilder folder within the SDK samples directory and click on Finish.

Once imported, the code for the Gesture Builder application will appear in the Package Explorer panel as GestureBuilderActivity. Right-click on the GestureBuilderActivity project and use the menu to install and run it on a device or emulator session attached to the development system.

Creating a Gestures File

Once the Gesture Builder activity has loaded, it should indicate that no gestures have yet been created. To create a new gesture, click on the Add gesture button located at the bottom of the device screen, enter the name Circle Gesture into the Name text box and then “draw” using a circular motion on the screen as illustrated in Figure 20-2. Assuming that the gesture appears as required (represented by the yellow line on the device screen), click on the Done button to add the gesture to the gestures file:


Creating an Android Gesture

Figure 20-2


After the gesture has been saved, the gestures builder will display a list of currently defined gestures, which, at this point, will consist solely of the new Circle Gesture.

Repeat the gesture creation process to add a further gesture to the file. This should involve a two-stroke gesture creating an X on the screen named X Gesture. When creating gestures involving multiple strokes be sure to allow as little time as possible between each stroke so that the builder knows that the strokes are part of the same gesture. Once this gesture has been added, the list within the Gestures Builder application should resemble that outlined in Figure 20-3:


New gestures created using the Android Gestures Builder application

Figure 20-3

Extracting the Gestures File from the SD Card

As each gesture was created within the Gesture Builder application, it was added to a file named gestures that, in turn, resides on the SD Card of the emulator or device on which the app was running. Before this file can be added to an Android project, however, it must first be pulled off the SD Card and saved to the local file system. This is most easily achieved by using the adb command-line tool. Open a Terminal or Command Prompt window and execute the following command:

adb devices

In the event that the adb command is not found, refer to Setting up an Android Development Environment for guidance on adding this to the PATH environment of your system.

Once executed, the command will list all active physical devices and AVD instances attached to the system. The following output, for example, indicates that a physical device and one AVD emulator are present on the system:

List of devices attached
emulator-5554   device
74CE000600000001        device

In order to pull the gestures file from the emulator in the above example and place it into the current working directory of the Terminal or Command Prompt window, the following command would need to be executed:

adb -s emulator-5554 pull /sdcard/gestures .

Once the gestures file has been created and pulled off the SD Card, it is ready to be added as a project resource file. The next step, therefore, is to create a new project.

Creating the Example Project

Launch Eclipse and create an Android Application Project named CustomGestures with the appropriate package name and SDK selections. As usual, request the creation of a blank activity and the use of the default launcher icons. On the New Blank Activity screen of the New Android Application wizard, set the Activity Name to CustomGesturesActivity and the Layout and Fragment names to activity_custom_gestures and fragment_custom_gestures respectively.

Within the Package Explorer panel, locate the new project, right-click on the res folder and select New -> Folder from the resulting menu. In the New Folder dialog, enter raw as the folder name and click on the Finish button. Using the appropriate file explorer for your operating system type, locate the gestures file pulled from the SD Card in the previous section and drag and drop it into the new raw folder.

When the File Operations dialog appears, make sure that the option to Copy files is selected prior to clicking on the OK button.

Designing the User Interface

This example application calls for a very simple user interface consisting of a LinearLayout view with a GestureOverlayView layered on top of it to intercept any gestures performed by the user. Locate the res -> layout -> fragment_custom_gestures.xml file and double click on it to load it into the main panel. Select the TextView object in the user interface layout and delete it.

By default, the ADT plugin has provided a RelativeLayout component. Right-click on this item in the Outline panel and, from the resulting menu, select the Change Layout… button. In the resulting Change Layout dialog, change the New Layout Type: menu to LinearLayout before clicking OK.

Switch to the XML layout using the fragment_custom_gestures.xml tab along the bottom edge of the Graphical Layout tool panel and remove any padding properties present in the file:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/LinearLayout1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".CustomGesturesActivity" >

</LinearLayout>

Return to the Graphical Layout view, select the Advanced section of the Palette and drag and drop a GestureOverlayView object onto the Layout canvas and resize it to fill the entire layout area. Change the ID of the GestureOverlayView object to gOverlay. When completed, the fragment_custom_gestures.xml file should read as follows:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/LinearLayout1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".CustomGesturesActivity" >

    <android.gesture.GestureOverlayView
        android:id="@+id/gOverlay"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_weight="1.07" 
    </android.gesture.GestureOverlayView>
</LinearLayout>

Save the layout file before continuing.

Loading the Gestures File

Now that the gestures file has been added to the project, the next step is to write some code so that the file is loaded when the activity starts up. For the purposes of this project, the code to achieve this will be placed in the onStart() method of the CustomGesturesActivity class which should be added to the CustomGesturesActivity.java source file as follows:

package com.example.customgestures;

import android.app.Activity;
import android.app.ActionBar;
import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.os.Build;
import android.gesture.GestureLibraries;
import android.gesture.GestureLibrary;
import android.gesture.GestureOverlayView;
import android.gesture.GestureOverlayView.OnGesturePerformedListener;

public class CustomGesturesActivity extends Activity implements OnGesturePerformedListener {

	private GestureLibrary gLibrary;
	
	@Override
	protected void onStart()
	{
		super.onStart();
		
		gLibrary = 
	              GestureLibraries.fromRawResource(this, 
							R.raw.gestures);
	            		if (!gLibrary.load()) {
	               			finish();
	            		}   

	}
.
.
.
}

The above code declares a GestureLibrary instance named gLibrary and then loads into it the contents of the gestures file located in the raw resources folder. The activity class has also been modified to implement the OnGesturePerformedListener interface, which requires the implementation of the onGesturePerformed callback method (which will be created in a later section of this chapter).

Registering the Event Listener

In order for the activity to receive notification that the user has performed a gesture on the screen, it is necessary to register the OnGesturePerformedListener event listener on the gLayout view, a reference to which can be obtained using the findViewById method as outlined in the following code fragment:

@Override
protected void onStart()
{
	super.onStart();
		
	gLibrary = GestureLibraries.fromRawResource(this, 
				R.raw.gestures);
	if (!gLibrary.load()) {
	          finish();
	}  
	            		
	GestureOverlayView gOverlay = (GestureOverlayView) 
					findViewById(R.id.gOverlay);
	   
	gOverlay.addOnGesturePerformedListener(this); 
}

Implementing the onGesturePerformed Method

All that remains before an initial test run of the application can be performed is to implement the OnGesturePerformed callback method that will be called when a gesture is performed on the GestureOverlayView instance:

package com.example.customgestures;

import android.app.Activity;
import android.app.ActionBar;
import android.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.os.Build;
import android.gesture.GestureLibraries;
import android.gesture.GestureLibrary;
import android.gesture.GestureOverlayView;
import android.gesture.GestureOverlayView.OnGesturePerformedListener;
import android.gesture.Prediction;
import android.widget.Toast;
import android.gesture.Gesture;
import java.util.ArrayList;

public class CustomGesturesActivity extends Activity implements OnGesturePerformedListener {

	private GestureLibrary gLibrary;
.
.
.	
     public void onGesturePerformed(GestureOverlayView overlay, Gesture 
                                             gesture) {
        ArrayList<Prediction> predictions = 
                 gLibrary.recognize(gesture);
        
        if (predictions.size() > 0 && predictions.get(0).score > 1.0) {
            
        	String action = predictions.get(0).name;
        	
            Toast.makeText(this, action, Toast.LENGTH_SHORT).show();
        }
    }
.
.
.
}

When a gesture on the gesture overlay view object is detected by the Android runtime, the onGesturePerformed method is called. Passed through as arguments are a reference to the GestureOverlayView object on which the gesture was detected and an object of type Gesture. The Gesture class is designed to hold the information that defines a specific gesture (essentially a sequence of timed points on the screen depicting the path of the strokes that comprise a gesture).

The Gesture object is passed through to the recognize() method of our gLibrary instance, the purpose of which is to compare the current gesture with each gesture loaded from the gestures file. Once this task is complete, the recognize() method returns an ArrayList object containing a Prediction object for each comparison performed. The list is ranked in order from the best match (at position 0) to the worst. Contained within each prediction object is the name of the corresponding gesture from the gestures file and a prediction score indicating how closely it matches the current gesture.

The code in the above method, therefore, takes the prediction at position 0 (the closest match) makes sure it has a score of greater than 1.0 and then displays a Toast message (an Android class designed to display notification pop ups to the user) displaying the name of the matching gesture.

Testing the Application

Build and run the application on either an emulator or a physical Android device and perform the circle and swipe gestures on the display. When performed, the toast notification should appear containing the name of the gesture that was performed. Note, however, that when attempting to perform the X Gesture that the gesture is not recognized. Also, note that when a gesture is recognized, it is outlined on the display with a bright yellow line whilst gestures about which the overlay is uncertain appear as a faded yellow line. Whilst useful during development, this is probably not ideal for a real world application. Clearly, therefore, there is still some more configuration work to do.

Configuring the GestureOverlayView

By default, GestureOverlayView is configured to display yellow lines during gestures and to recognize only single stroke gestures. Multi-stroke gestures can be detected by setting the android:gestureStrokeType property to multiple.

Similarly, the color used to draw recognized and unrecognized gestures can be defined via the android:gestureColor and android:uncertainGestureColor properties. For example, to hide the gesture lines and recognize multi-stroke gestures, modify the fragment_custom_gestures.xml file in the example project as follows:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/LinearLayout1"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".CustomGesturesActivity" >

    <android.gesture.GestureOverlayView
        android:id="@+id/gOverlay"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_weight="1.07" 
        android:gestureColor="#00000000" 
        android:uncertainGestureColor="#00000000"
        android:gestureStrokeType="multiple" >   
    </android.gesture.GestureOverlayView>

</LinearLayout>

On re-running the application, gestures should now be invisible.

Intercepting Gestures

The GestureOverlayView is, as previously described, a transparent overlay that may be positioned over the top of other views. This leads to the question as to whether events intercepted by the gesture overlay should then be passed on to the underlying views when a gesture has been recognized. This is controlled via the android:eventsInterceptionEnabled property of the GestureOverlayView. When set to true, the gesture events are not passed to the underlying views when a gesture is recognized. This can be a particularly useful setting when gestures are being performed over a view that might be configured to scroll in response to certain gestures. Setting this property to true will avoid gestures also being interpreted as instructions to the underlying view to scroll in a particular direction.

Detecting Pinch Gestures

Before moving on from touch handling in general and gesture recognition in particular, the last topic of this chapter is that of handling pinch gestures. Whilst it is possible to create and detect a wide range of gestures using the steps outlined in the previous sections of this chapter it is, in fact, not possible to detect a pinching gesture (where two fingers are used in a stretching and pinching motion, typically to zoom in and out of a view or image) using the techniques discussed.

The simplest method for detecting pinch gestures is to use the Android ScaleGestureDetector class. In general terms, detecting pinch gestures involves the following steps:

1. Declaration of a new class which implements the SimpleOnScaleGestureListener interface including the required onScale(), onScaleBegin() and onScaleEnd() callback methods.

2. Creation of an instance of the ScaleGestureDetector class, passing through an instance of the class created in step 1 as an argument.

3. Implementing the onTouchEvent() callback method on the enclosing activity which, in turn, calls the onTouchEvent() method of the ScaleGestureDetector class.

In the remainder of this chapter, we will create a very simple example designed to demonstrate the implementation of pinch gesture recognition.

A Pinch Gesture Example Project

Within Eclipse, create an Android Application Project named PinchExample with the appropriate package name and SDK selections. Request the creation of a blank activity and the use of the default launcher icons. On the New Blank Activity screen of the New Android Application wizard, set the Activity Name to PinchExampleActivity and the Layout and Fragment names to activity_pinch_example and fragment_pinch_example.

Within the fragment_pinch_example.xml file, locate the TextView object and change the ID to myTextView. Also change the ID of the RelativeLayout view to RelativeLayout1, making sure to save the file before continuing.

Locate and load the PinchExampleActivity.java file into the Eclipse editor and modify the file as follows to override the onStart() lifecycle method:

package com.example.pinchexample;

import android.app.Activity;
import android.app.ActionBar;
import android.app.Fragment;
import android.os.Bundle;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;
import android.os.Build;
import android.view.MotionEvent;
import android.view.ScaleGestureDetector;
import android.view.ScaleGestureDetector.SimpleOnScaleGestureListener;
import android.widget.RelativeLayout;
import android.widget.TextView;

public class PinchExampleActivity extends Activity {

	TextView scaleText;
	ScaleGestureDetector scaleGestureDetector;
.
.
.
	@Override 
	protected void onStart()
	{
		super.onStart();
		
		scaleText = (TextView)
			findViewById(R.id.myTextView);
			      
			scaleGestureDetector = new 
				ScaleGestureDetector(this, 
		               new MyOnScaleGestureListener());

			RelativeLayout myLayout = (RelativeLayout)  
                                 findViewById(R.id.RelativeLayout1);
			      
			myLayout.setOnTouchListener(
		        	new RelativeLayout.OnTouchListener() {
		        		public boolean onTouch(View v, 
					  MotionEvent m) {	           	                                    scaleGestureDetector.
							onTouchEvent(m);
		        		    return true;
		        		}
		        	}
		        );

	}
		
	public class MyOnScaleGestureListener extends
            SimpleOnScaleGestureListener {

	    @Override
	    public boolean onScale(ScaleGestureDetector detector) {
	
	       float scaleFactor = detector.getScaleFactor();
	 
	       if (scaleFactor > 1) {
		         scaleText.setText("Zooming Out");
	        } else {
		         scaleText.setText("Zooming In");
	        }
	       return true;
	   }
	
	   @Override
	   public boolean onScaleBegin(ScaleGestureDetector detector) {
	        return true;
	   }
	
	   @Override
	   public void onScaleEnd(ScaleGestureDetector detector) {
	
	   }
	}
.
.
.
}

The code begins by declaring TextView and ScaleGestureDetector variables. A new class named MyOnScaleGestureListener is declared which extends the Android SimpleOnScaleGestureListener class. This interface requires that three methods (onScale(), onScaleBegin() and onScaleEnd()) be implemented). In this instance the onScale() method identifies the scale factor and displays a message on the text view indicating the type of pinch gesture detected.

Within the onCreate() method, a reference to the text view object is obtained and assigned to the scaleText variable. Next, a new ScaleGestureDetector instance is created, passing through a reference to the enclosing activity and an instance of our new MyOnScaleGestureListener class as arguments. Finally, an OnTouchListener is implemented for the class, which simply calls the corresponding onTouchEvent() method of the ScaleGestureDetector object, passing through the MotionEvent object as an argument.

Compile and run the application on a physical Android device and perform pinching gestures on the screen, noting that the text view displays either the zoom in or zoom out message depending on the pinching motion.

Summary

A gesture is essentially the motion of points of contact on a touch screen involving one or more strokes and can be used as a method of communication between user and application. Android allows gestures to be designed using the Gestures Builder application. Once created, gestures can be saved to a gestures file and loaded into an activity at application runtime using the GestureLibrary.

Gestures can be detected on areas of the display by overlaying existing views with instances of the transparent GestureOverlayView class and implementing an OnGesturePerformedListener event listener. Using the GestureLibrary, a ranked list of matches between a gesture performed by the user and the gestures stored in a gestures file may be generated, using a prediction score to decide whether a gesture is a close enough match.

Pinch gestures may be detected through the implementation of the ScaleGestureDetector class, an example of which was also provided in this chapter.


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PreviousTable of ContentsNext
Detecting Common Gestures using the Android Gesture Detector ClassAn Introduction to Android Fragments