Implementing Custom Gesture and Pinch Recognition on Android 6
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The previous chapter looked at the steps involved in detecting what are referred to as “common gestures” from within an Android application. In practice, however, a gesture can conceivably involve just about any sequence of touch motions on the display of an Android device. In recognition of this fact, the Android SDK allows custom gestures of just about any nature 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 which are then stored in a gesture file bundled with an Android application package. These custom gesture files are most easily created 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, either on a physical device or emulator, and “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 an instance of the GestureLibrary class where it can be used to search for matches to any gestures performed by the user on the device display.
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 Gesture 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, a call to the onGesturePerformed callback method is triggered by the Android 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 gesture performed by the user 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 between a gesture stored in the file and that performed by the user on the device display.
Building and Running the Gesture Builder Application
The Gesture Builder application is bundled by default with the AVD emulator profile for most versions of the SDK. It is not, however, 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 is included amongst the standard Android SDK samples and consequently may be imported as an Android Studio project and compiled and run on any Android device or emulator.
To install and build the GestureBuilder utility, begin by installing the SDK samples. To do this, open the Android SDK Manager by selecting the Tools -> Android -> SDK Manager menu bar option from the project main window.
Once the SDK settings dialog has loaded click on the Launch Standalone SDK Manager link in the bottom left hand corner of the panel to open the full SDK Manager tool. Once the standalone SDK Manager is running, locate the Samples for SDK package located beneath the section for the Android version for which you are currently developing (for example Android 6.0 (API 23)). 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 as highlighted in Figure 23-1:
[[Image:]]
Figure 23-1
Once the installation is complete, the SDK samples will be installed in the following directory (where <path_to_installation> represents the location on your system where the SDK was originally installed as shown at the top of the SDK Manager window):
<path_to_installation>/sdk/samples/android-23
The source code for the Gesture Builder application is located within this directory in a folder named GestureBuilder within the legacy sub-folder.
The GestureBuilder project is based on Android 5.0.1 (API 21) so use the SDK Manager tool once again to ensure that that this version of the Android SDK is installed before proceeding.
From the Android Studio main window for an existing project, select the File -> New -> Import Project… menu option and, within the resulting dialog, navigate to and select the GestureBuilder folder within the SDK samples directory and click on OK. Note that on some Windows systems it may be necessary to click on the “Show Hidden Files and Directories” button in the file navigator toolbar as shown in Figure 23 2 in order to display the folder containing the SDK samples:
[[Image:]]
Figure 23-2
Confirm the destination directory and click on Next followed by Finish to accept the default settings on the final screen. At this point, Android Studio will import the project into the designated folder and convert it to match the Android Studio project file and build structure.
Once imported, install and run the GestureBuilder utility on an Android device attached to the development system. <google>BUY_ANDROID_STUDIO6</google>
Creating a Gestures File
Once the Gesture Builder application 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” a gesture using a circular motion on the screen as illustrated in Figure 23-3. 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:
[[Image:]]
Figure 23-3
After the gesture has been saved, the Gesture Builder app 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 Gesture Builder application should resemble that outlined in Figure 23-4:
[[Image:]]
Figure 23-4
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 located on the SD Card of the emulator or device on which the app was running. Before this file can be added to an Android Studio 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, therefore, and execute the following command:
adb devices
In the event that the adb command is not found, refer to Setting up an Android Studio Development Environment for guidance on adding this to the PATH environment variable 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 both a physical device and one AVD emulator have been detected on the development computer system:
List of devices attached HT4CTJT01906 device emulator-5554 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 .
Alternatively, the gestures file can be pulled from a device connected via adb using the following command (where the –d flag is used to indicate a physical device):
adb -d pull /sdcard/gestures .
Once the gestures file has been created and pulled off the SD Card, it is ready to be added to an Android Studio project as a resource file. The next step, therefore, is to create a new project.
Creating the Example Project
Create a new project in Android Studio, entering CustomGestures into the Application name field and ebookfrenzy.com as the Company Domain setting before clicking on the Next button.
On the form factors screen, enable the Phone and Tablet option and set the minimum SDK setting to API 8: Android 2.2 (Froyo). Continue to proceed through the screens, requesting the creation of an empty activity named CustomGesturesActivity with a corresponding layout file named activity_custom_gestures. <google>BUY_ANDROID_STUDIO6</google> Click on the Finish button to initiate the project creation process.
Adding the Gestures File to the Project
Within the Android Studio Project tool window, locate and right-click on the res folder (located under app) and select New -> Directory from the resulting menu. In the New Directory dialog, enter raw as the folder name and click on the OK button. Using the appropriate file explorer utility for your operating system type, locate the gestures file previously pulled from the SD Card and copy and paste it into the new raw folder in the Project tool window.
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 app -> res -> layout -> activity_custom_gestures.xml file and double click on it to load it into the Designer tool.
By default, Android Studio has provided a RelativeLayout component as the root element of the user interface layout so this will need to be deleted and replaced with a LinearLayout. Switch the Designer tool to Text mode using the Text tab along the bottom edge of the panel and modify the XML for the layout so that it matches the following listing:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent"
xmlns:android="http://schemas.android.com/apk/res/android">
</LinearLayout>
Return to Design mode, locate the Expert section of the Palette and drag and drop a GestureOverlayView object onto the layout canvas. Select the GestureOverlayView instance in the layout and use the Properties panel or Designer toolbar buttons to change the layout:width and layout:height properties to match_parent so that the view fills the available space.
Double click on the GestureOverlayView instance and use the popup property panel to change the ID to gOverlay. When completed, the activity_custom_gestures.xml file should read as follows:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent"
xmlns:android="http://schemas.android.com/apk/res/android">
<android.gesture.GestureOverlayView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/gOverlay"
android:layout_gravity="center_horizontal">
</android.gesture.GestureOverlayView>
</LinearLayout>
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 onCreate() method of the CustomGesturesActivity class located in the CustomGesturesActivity.java source file as follows:
package com.ebookfrenzy.customgestures;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.gesture.GestureLibraries;
import android.gesture.GestureLibrary;
import android.gesture.GestureOverlayView;
import android.gesture.GestureOverlayView.OnGesturePerformedListener;
public class CustomGesturesActivity extends AppCompatActivity
implements OnGesturePerformedListener {
private GestureLibrary gLibrary;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_custom_gestures);
gLibrary =
GestureLibraries.fromRawResource(this, R.raw.gestures);
if (!gLibrary.load()) {
finish();
}
}
.
.
.
}
<google>BUY_ANDROID_STUDIO6</google> In addition to some necessary import directives, the above code changes to the onCreate() method also create 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 onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_custom_gestures);
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. This is the method which will be called when a gesture is performed on the GestureOverlayView instance:
package com.ebookfrenzy.customgestures;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
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 AppCompatActivity implements OnGesturePerformedListener {
private GestureLibrary gLibrary;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_custom_gestures);
gLibrary =
GestureLibraries.fromRawResource(this, R.raw.gestures);
if (!gLibrary.load()) {
finish();
}
GestureOverlayView gOverlay =
(GestureOverlayView) findViewById(R.id.gOverlay);
gOverlay.addOnGesturePerformedListener(this);
}
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();
}
}
.
.
.
}
<google>BUY_ANDROID_STUDIO6</google> 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 together with 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 in the array) 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 while gestures about which the overlay is uncertain appear as a faded yellow line. While 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, the 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 activity_custom_gestures.xml file in the example project as follows:
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout
android:orientation="vertical"
android:layout_width="match_parent"
android:layout_height="match_parent"
xmlns:android="http://schemas.android.com/apk/res/android">
<android.gesture.GestureOverlayView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/gOverlay"
android:layout_gravity="center_horizontal"
android:gestureColor="#00000000"
android:uncertainGestureColor="#00000000"
android:gestureStrokeType="multiple" >
</android.gesture.GestureOverlayView>
</LinearLayout>
On re-running the application, gestures should now be invisible (since they are drawn in white on the white background of the LinearLayout view).
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 instance. 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. While 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 so far.
The simplest method for detecting pinch gestures is to use the Android ScaleGestureDetector class. In general terms, detecting pinch gestures involves the following three 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
Create a new project in Android Studio, entering PinchExample into the Application name field and ebookfrenzy.com as the Company Domain setting before clicking on the Next button.
On the form factors screen, enable the Phone and Tablet option and set the minimum SDK setting to API 8: Android 2.2 (Froyo). Continue to proceed through the screens, requesting the creation of an empty activity named PinchExampleActivity with a layout resource file named activity_pinch_example.
Within the activity_pinch_example.xml file, locate the TextView object and double click on it to change the ID to myTextView. <google>BUY_ANDROID_STUDIO6</google> Locate and load the PinchExampleActivity.java file into the Android Studio editor and modify the file as follows:
package com.ebookfrenzy.pinchexample;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.MotionEvent;
import android.view.ScaleGestureDetector;
import android.view.ScaleGestureDetector.SimpleOnScaleGestureListener;
import android.widget.TextView;
public class PinchExampleActivity extends AppCompatActivity {
TextView scaleText;
ScaleGestureDetector scaleGestureDetector;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_pinch_example);
scaleText = (TextView)findViewById(R.id.myTextView);
scaleGestureDetector =
new ScaleGestureDetector(this,
new MyOnScaleGestureListener());
}
@Override
public boolean onTouchEvent(MotionEvent event) {
scaleGestureDetector.onTouchEvent(event);
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 onTouchEvent() callback method is implemented for the activity, 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 Gesture 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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| Detecting Common Gestures using the Android 6 Gesture Detector Class | An Introduction to Android 6 Fragments |
