使对象根据预设程序（例如旋转的三角形）移动有助于吸引用户的注意力，但如果您希望用户与您的 OpenGL ES 图形互动的话，该怎么办呢？让 OpenGL ES 应用触摸互动的关键是扩展您的 GLSurfaceView 实现以替换 onTouchEvent()，从而监听触摸事件。
本课程介绍了如何监听触摸事件，以使用户可以旋转 OpenGL ES 对象。
设置触摸监听器
为了使您的 OpenGL ES 应用响应触摸事件，您必须在 GLSurfaceView 类中实现 onTouchEvent() 方法。以下示例实现展示了如何监听 MotionEvent.ACTION_MOVE 事件，并将其平移到某个形状的旋转角度。
Kotlin
private const val TOUCH_SCALE_FACTOR: Float = 180.0f / 320f
...
private var previousX: Float = 0f
private var previousY: Float = 0f
override fun onTouchEvent(e: MotionEvent): Boolean {
// MotionEvent reports input details from the touch screen
// and other input controls. In this case, you are only
// interested in events where the touch position changed.
val x: Float = e.x
val y: Float = e.y
when (e.action) {
MotionEvent.ACTION_MOVE -> {
var dx: Float = x - previousX
var dy: Float = y - previousY
// reverse direction of rotation above the mid-line
if (y > height / 2) {
dx *= -1
}
// reverse direction of rotation to left of the mid-line
if (x < width / 2) {
dy *= -1
}
renderer.angle += (dx + dy) * TOUCH_SCALE_FACTOR
requestRender()
}
}
previousX = x
previousY = y
return true
}
Java
private final float TOUCH_SCALE_FACTOR = 180.0f / 320;
private float previousX;
private float previousY;
@Override
public boolean onTouchEvent(MotionEvent e) {
// MotionEvent reports input details from the touch screen
// and other input controls. In this case, you are only
// interested in events where the touch position changed.
float x = e.getX();
float y = e.getY();
switch (e.getAction()) {
case MotionEvent.ACTION_MOVE:
float dx = x - previousX;
float dy = y - previousY;
// reverse direction of rotation above the mid-line
if (y > getHeight() / 2) {
dx = dx * -1 ;
}
// reverse direction of rotation to left of the mid-line
if (x < getWidth() / 2) {
dy = dy * -1 ;
}
renderer.setAngle(
renderer.getAngle() +
((dx + dy) * TOUCH_SCALE_FACTOR));
requestRender();
}
previousX = x;
previousY = y;
return true;
}
请注意，计算过旋转角度之后，此方法会调用 requestRender() 以告知渲染程序该渲染帧了。此方法在本示例中效率最高，因为除非旋转发生变化，否则无需重新绘制帧。不过，除非您还要求渲染程序仅在数据发生更改时重新绘制（使用 setRenderMode() 方法），否则它对效率没有任何影响，因此请确保在渲染程序中对此行取消备注：
Kotlin
class MyGlSurfaceView(context: Context) : GLSurfaceView(context) {
init {
// Render the view only when there is a change in the drawing data
renderMode = GLSurfaceView.RENDERMODE_WHEN_DIRTY
}
}
Java
public MyGLSurfaceView(Context context) {
...
// Render the view only when there is a change in the drawing data
setRenderMode(GLSurfaceView.RENDERMODE_WHEN_DIRTY);
}
公开旋转角度
上述示例代码要求您添加公开成员，以此来通过渲染程序公开旋转角度。由于渲染程序代码在独立于应用的主界面线程的线程上运行，因此必须将此公开变量声明为 volatile。以下是用于声明变量和公开 getter 和 setter 对的代码：
Kotlin
class MyGLRenderer4 : GLSurfaceView.Renderer {
@Volatile
var angle: Float = 0f
}
Java
public class MyGLRenderer implements GLSurfaceView.Renderer {
...
public volatile float mAngle;
public float getAngle() {
return mAngle;
}
public void setAngle(float angle) {
mAngle = angle;
}
}
应用旋转
要应用触摸输入生成的旋转，请为生成角度的代码取消备注并添加包含触摸输入生成的角度的变量：
Kotlin
override fun onDrawFrame(gl: GL10) {
...
val scratch = FloatArray(16)
// Create a rotation for the triangle
// long time = SystemClock.uptimeMillis() % 4000L;
// float angle = 0.090f * ((int) time);
Matrix.setRotateM(rotationMatrix, 0, angle, 0f, 0f, -1.0f)
// Combine the rotation matrix with the projection and camera view
// Note that the mvpMatrix factor *must be first* in order
// for the matrix multiplication product to be correct.
Matrix.multiplyMM(scratch, 0, mvpMatrix, 0, rotationMatrix, 0)
// Draw triangle
triangle.draw(scratch)
}
Java
public void onDrawFrame(GL10 gl) {
...
float[] scratch = new float[16];
// Create a rotation for the triangle
// long time = SystemClock.uptimeMillis() % 4000L;
// float angle = 0.090f * ((int) time);
Matrix.setRotateM(rotationMatrix, 0, mAngle, 0, 0, -1.0f);
// Combine the rotation matrix with the projection and camera view
// Note that the vPMatrix factor *must be first* in order
// for the matrix multiplication product to be correct.
Matrix.multiplyMM(scratch, 0, vPMatrix, 0, rotationMatrix, 0);
// Draw triangle
mTriangle.draw(scratch);
}
完成上述步骤后，运行程序并在屏幕上拖动手指以旋转该三角形：
图 1.使用触摸输入进行旋转的三角形（圆圈表示触摸位置）。