响应触摸事件

让对象根据预设程序(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,该怎么办? 使 OpenGL ES 应用触摸交互的关键是扩展 GLSurfaceView 实现以替换 onTouchEvent() 以监听触摸事件。

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,计算旋转角度后,此方法会调用 requestRender() 以告知渲染程序该渲染帧了。此方法在本示例中最高效,因为除非旋转发生变化,否则无需重新绘制帧。不过,这不会对效率产生任何影响,除非您还使用 setRenderMode() 方法要求渲染程序仅在数据发生更改时重新绘制,因此请确保在渲染程序中取消注释此行:

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通过添加公开成员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,因此您必须将此公共变量声明为 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

如需应用触控输入生成的旋转,请注释掉用于生成角度的代码,并添加包含触控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三角形:

图 1. 通过触控输入旋转的三角形(圆圈表示触摸位置)。