1 page.title= Responding to Touch Events 2 parent.title=Displaying Graphics with OpenGL ES 3 parent.link=index.html 4 5 trainingnavtop=true 6 previous.title=Adding Motion 7 previous.link=motion.html 8 9 @jd:body 10 11 <div id="tb-wrapper"> 12 <div id="tb"> 13 14 <h2>This lesson teaches you to</h2> 15 <ol> 16 <li><a href="#listener">Setup a Touch Listener</a></li> 17 <li><a href="#angle">Expose the Rotation Angle</a></li> 18 <li><a href="#rotate">Apply Rotation</a></li> 19 </ol> 20 21 <h2>You should also read</h2> 22 <ul> 23 <li><a href="{@docRoot}guide/topics/graphics/opengl.html">OpenGL</a></li> 24 </ul> 25 26 <div class="download-box"> 27 <a href="http://developer.android.com/shareables/training/OpenGLES.zip" 28 class="button">Download the sample</a> 29 <p class="filename">OpenGLES.zip</p> 30 </div> 31 32 </div> 33 </div> 34 35 <p>Making objects move according to a preset program like the rotating triangle is useful for 36 getting some attention, but what if you want to have users interact with your OpenGL ES graphics? 37 The key to making your OpenGL ES application touch interactive is expanding your implementation of 38 {@link android.opengl.GLSurfaceView} to override the {@link 39 android.opengl.GLSurfaceView#onTouchEvent onTouchEvent()} to listen for touch events.</p> 40 41 <p>This lesson shows you how to listen for touch events to let users rotate an OpenGL ES object.</p> 42 43 44 <h2 id="listener">Setup a Touch Listener</h2> 45 46 <p>In order to make your OpenGL ES application respond to touch events, you must implement the 47 {@link android.opengl.GLSurfaceView#onTouchEvent onTouchEvent()} method in your 48 {@link android.opengl.GLSurfaceView} class. The example implementation below shows how to listen for 49 {@link android.view.MotionEvent#ACTION_MOVE MotionEvent.ACTION_MOVE} events and translate them to 50 an angle of rotation for a shape.</p> 51 52 <pre> 53 @Override 54 public boolean onTouchEvent(MotionEvent e) { 55 // MotionEvent reports input details from the touch screen 56 // and other input controls. In this case, you are only 57 // interested in events where the touch position changed. 58 59 float x = e.getX(); 60 float y = e.getY(); 61 62 switch (e.getAction()) { 63 case MotionEvent.ACTION_MOVE: 64 65 float dx = x - mPreviousX; 66 float dy = y - mPreviousY; 67 68 // reverse direction of rotation above the mid-line 69 if (y > getHeight() / 2) { 70 dx = dx * -1 ; 71 } 72 73 // reverse direction of rotation to left of the mid-line 74 if (x < getWidth() / 2) { 75 dy = dy * -1 ; 76 } 77 78 mRenderer.setAngle( 79 mRenderer.getAngle() + 80 ((dx + dy) * TOUCH_SCALE_FACTOR); // = 180.0f / 320 81 requestRender(); 82 } 83 84 mPreviousX = x; 85 mPreviousY = y; 86 return true; 87 } 88 </pre> 89 90 <p>Notice that after calculating the rotation angle, this method calls {@link 91 android.opengl.GLSurfaceView#requestRender requestRender()} to tell the 92 renderer that it is time to render the frame. This approach is the most efficient in this example 93 because the frame does not need to be redrawn unless there is a change in the rotation. However, it 94 does not have any impact on efficiency unless you also request that the renderer only redraw when 95 the data changes using the {@link android.opengl.GLSurfaceView#setRenderMode setRenderMode()} 96 method, so make sure this line is uncommented in the renderer:</p> 97 98 <pre> 99 public MyGLSurfaceView(Context context) { 100 ... 101 // Render the view only when there is a change in the drawing data 102 <strong>setRenderMode(GLSurfaceView.RENDERMODE_WHEN_DIRTY);</strong> 103 } 104 </pre> 105 106 <h2 id="angle">Expose the Rotation Angle</h2> 107 108 <p>The example code above requires that you expose the rotation angle through your renderer by 109 adding a public member. Since the renderer code is running on a separate thread from the main user 110 interface thread of your application, you must declare this public variable as {@code volatile}. 111 Here is the code to do that:</p> 112 113 <pre> 114 public class MyGLRenderer implements GLSurfaceView.Renderer { 115 ... 116 public volatile float mAngle; 117 </pre> 118 119 120 <h2 id="rotate">Apply Rotation</h2> 121 122 <p>To apply the rotation generated by touch input, comment out the code that generates an angle and 123 add {@code mAngle}, which contains the touch input generated angle:</p> 124 125 <pre> 126 public void onDrawFrame(GL10 gl) { 127 ... 128 float[] scratch = new float[16]; 129 130 // Create a rotation for the triangle 131 // long time = SystemClock.uptimeMillis() % 4000L; 132 // float angle = 0.090f * ((int) time); 133 <strong>Matrix.setRotateM(mRotationMatrix, 0, mAngle, 0, 0, -1.0f);</strong> 134 135 // Combine the rotation matrix with the projection and camera view 136 // Note that the mMVPMatrix factor *must be first* in order 137 // for the matrix multiplication product to be correct. 138 Matrix.multiplyMM(scratch, 0, mMVPMatrix, 0, mRotationMatrix, 0); 139 140 // Draw triangle 141 mTriangle.draw(scratch); 142 } 143 </pre> 144 145 <p>When you have completed the steps described above, run the program and drag your finger over the 146 screen to rotate the triangle:</p> 147 148 <img src="{@docRoot}images/opengl/ogl-triangle-touch.png"> 149 <p class="img-caption"> 150 <strong>Figure 1.</strong> Triangle being rotated with touch input (circle shows touch 151 location).</p> 152
