响应触摸事件

根据预设程序（如旋转三角形）移动物体对于吸引一些注意力很有用，但如果要让用户与 OpenGL ES 图形交互呢？让 OpenGL ES 应用触控交互的关键是扩展 GLSurfaceView 的实现，以覆盖 onTouchEvent() 以监听触控事件。

本课将展示如何监听触控事件，让用户旋转 OpenGL ES 物体。

设置触控监听器

为了让 OpenGL ES 应用响应触控事件，您必须在 GLSurfaceView 类中实现 onTouchEvent() 方法。下面的示例实现展示了如何监听 MotionEvent.ACTION_MOVE 事件并将其转换为形状的旋转角度。

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
}

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;
}

注意，在计算旋转角度后，此方法调用 requestRender() 来告诉渲染器现在是渲染帧的时候了。这种方法在本例中最有效，因为只有在旋转发生变化时才需要重新绘制帧。但是，除非您还请求渲染器仅在数据发生变化时才重新绘制，使用 setRenderMode() 方法，否则它对效率没有影响，因此请确保渲染器中的此行代码已取消注释

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
    }
}

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

上面的示例代码要求您通过渲染器公开旋转角度，方法是添加一个公共成员。由于渲染器代码在与应用的主用户界面线程不同的线程上运行，因此必须将此公共变量声明为 volatile。以下是声明变量并公开 getter 和 setter 对的代码

class MyGLRenderer4 : GLSurfaceView.Renderer {

    @Volatile
    var angle: Float = 0f
}

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

要应用由触控输入生成的旋转，请注释掉生成角度的代码，并添加一个包含触控输入生成的角的变量

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)
}

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，然后用手指在屏幕上拖动以旋转三角形

图 1. 用触控输入旋转三角形（圆圈显示触控位置）。