Question

我正在尝试使用加速度计在圆圈内移动图像。我有一个问题，当图像碰到圆的边缘时，它只是移动圆的另一边。我的代码如下：

- (void)accelerometer:(UIAccelerometer *)accelerometer didAccelerate:(UIAcceleration *)acceleration {
//NSLog(@"x : %g", acceleration.x);
//NSLog(@"y : %g", acceleration.y);
//NSLog(@"z : %g", acceleration.z);

delta.x = acceleration.x * 10;
delta.y = acceleration.y * 10;

joypadCap.center = CGPointMake(joypadCap.center.x + delta.x, joypadCap.center.y - delta.y);

distance = sqrtf(((joypadCap.center.x - 160) * (joypadCap.center.x - 160)) +
                 ((joypadCap.center.y -206) * (joypadCap.center.y - 206)));
//NSLog(@"Distance : %f", distance);


touchAngle = atan2(joypadCap.center.y, joypadCap.center.x);
NSLog(@"Angle : %f", touchAngle);


if (distance > 50) {
    joypadCap.center = CGPointMake(160 - cosf(touchAngle) * 50, 206 - sinf(touchAngle) * 50);
}

Answer 1

尝试使用CMDeviceMotion实现循环水平仪时遇到了同样的问题。我发现这是传递给atan2(y,x)的坐标的问题。此功能需要笛卡尔坐标，视图中心有（0,0）。但是，屏幕坐标左上角有（0,0）。我创建了在两个坐标系之间转换点的方法，现在它运行良好。

我在github上设置了一个示例项目here，但这是最重要的部分：

float distance = sqrtf(((point.x - halfOfWidth) * (point.x - halfOfWidth)) +
                       ((point.y - halfOfWidth) * (point.y - halfOfWidth)));

if (distance > maxDistance)
{
    // Convert point from screen coordinate system to cartesian coordinate system,
    // with (0,0) located in the centre of the view
    CGPoint pointInCartesianCoordSystem = [self convertScreenPointToCartesianCoordSystem:point
                                                                                 inFrame:self.view.frame];

    // Calculate angle of point in radians from centre of the view
    CGFloat angle = atan2(pointInCartesianCoordSystem.y, pointInCartesianCoordSystem.x);

    // Get new point on the edge of the circle
    point = CGPointMake(cos(angle) * maxDistance, sinf(angle) * maxDistance);

    // Convert back to screen coordinate system
    point = [self convertCartesianPointToScreenCoordSystem:point inFrame:self.view.frame];
}

并且：

- (CGPoint)convertScreenPointToCartesianCoordSystem:(CGPoint)point
                                            inFrame:(CGRect)frame
{
    float x = point.x - (frame.size.width / 2.0f);
    float y = (point.y - (frame.size.height / 2.0f)) * -1.0f;

    return CGPointMake(x, y);
}

- (CGPoint)convertCartesianPointToScreenCoordSystem:(CGPoint)point
                                            inFrame:(CGRect)frame
{
    float x = point.x + (frame.size.width / 2.0f);
    float y = (point.y * -1.0f) + (frame.size.height / 2.0f);

    return CGPointMake(x, y);
}

iOS使用加速度计在圆圈内移动对象

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