（六）响应点击事件（Responding to Touch Ev

原文：https://developer.android.com/training/graphics/opengl/touch.html#angle
示例代码

Making objects move according to a preset program like the rotating triangle is useful for getting some attention, but what if you want to have users interact with your OpenGL ES graphics? The key to making your OpenGL ES application touch interactive is expanding your implementation of GLSurfaceView to override the onTouchEvent() to listen for touch events.
让一个三角形转动看起来很屌，但是怎么才能让用户和OpenGL ES图形进行交互？方法就是重写GLSurfaceView的onTouchEvent() 方法来处理用户点击事件。

This lesson shows you how to listen for touch events to let users rotate an OpenGL ES object.
这一节讲解如何根据点击事件来旋转OpenGL ES图形。

Setup a Touch Listener

设置点击监听器

In order to make your OpenGL ES application respond to touch events, you must implement the onTouchEvent() method in your GLSurfaceView class. The example implementation below shows how to listen for MotionEvent.ACTION_MOVE events and translate them to an angle of rotation for a shape.
为了让OpenGL ES应用能够响应你的点击事件，你必须实现GLSurfaceView的onTouchEvent()方法。下面的例子展示如何把MotionEvent.ACTION_MOVE事件转化为图形旋转的角度。

private final float TOUCH_SCALE_FACTOR = 180.0f / 320;
private float mPreviousX;
private float mPreviousY;

@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 - mPreviousX;
            float dy = y - mPreviousY;

            // 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 ;
            }

            mRenderer.setAngle(
                    mRenderer.getAngle() +
                    ((dx + dy) * TOUCH_SCALE_FACTOR));
            requestRender();
    }

    mPreviousX = x;
    mPreviousY = y;
    return true;
}

Notice that after calculating the rotation angle, this method calls requestRender() to tell the renderer that it is time to render the frame. This approach is the most efficient in this example because the frame does not need to be redrawn unless there is a change in the rotation. However, it does not have any impact on efficiency unless you also request that the renderer only redraw when the data changes using the setRenderMode() method, so make sure this line is uncommented in the renderer:
注意，每次计算好角度之后都要调用一次requestRender() 方法告诉renderer该重绘了。这是最高效的方法，因为没有改变角度就没有必要重绘。（下面这句不翻了，看不懂😱）确保setRenderMode(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);
}

Expose the Rotation Angle

传递旋转角度

The example code above requires that you expose the rotation angle through your renderer by adding a public member. Since the renderer code is running on a separate thread from the main user interface thread of your application, you must declare this public variable as volatile. Here is the code to declare the variable and expose the getter and setter pair:
上面的代码还需要你在renderer中建立一个public变量，用来传递旋转角度。因为renderer的代码是在另一个线程中运行，而不是UI线程，所以这个变量必须设为volatile(UI线程中获取了旋转角度，绘制线程中调用这个角度）。如下：

public class MyGLRenderer implements GLSurfaceView.Renderer {
    ...

    public volatile float mAngle;

    public float getAngle() {
        return mAngle;
    }

    public void setAngle(float angle) {
        mAngle = angle;
    }
}

Apply Rotation

旋转

To apply the rotation generated by touch input, comment out the code that generates an angle and add mAngle, which contains the touch input generated angle:
加上以下代码，将点击产生的角度加到矩阵中。

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(mRotationMatrix, 0, mAngle, 0, 0, -1.0f);

    // Combine the rotation matrix with the projection and camera view
    // Note that the mMVPMatrix factor *must be first* in order
    // for the matrix multiplication product to be correct.
    Matrix.multiplyMM(scratch, 0, mMVPMatrix, 0, mRotationMatrix, 0);

    // Draw triangle
    mTriangle.draw(scratch);
}

When you have completed the steps described above, run the program and drag your finger over the screen to rotate the triangle:
完成之后，运行程序，用手指转动三角形。

Figure 1. Triangle being rotated with touch input (circle shows touch location).