说明：
这是一个UI界面中操作方向盘的演示，源于一个游戏项目，仅供学习参考，其中：
1、方向盘可复原
2、Android测试能够正常操作，没有出现轴心偏离现象（测试机型Galaxy S8-分辨率：2960x1440）
注意：
如果你要使用这个脚本，应将Rect Transfrom设置成如下属性，当然，某些属性不是必须要这么设定，但至少这样做可以正常工作，作为一个案例，供参考！
1、Rect Transfrom相关属性设定
2、其画布下的SteerWheel是Image类型的UI元素，图像设定为方向盘图片
3、Game
4、Inspector
5、演示结果
代码：
using UnityEngine;
using UnityEngine.UI;
using UnityEngine.EventSystems;
using UnityEngine.Events;
using System.Collections;
/// <summary>
/// Steering wheel.
/// Defines the steerign wheel behaviour.
/// Original source:
/// http://forum.unity3d.com/threads/touchscreen-steering-wheel-rotation-example-mouse-supported.196741/
/// </summary>
public class SteeringWheel2D : MonoBehaviour
{
[Tooltip("UI_Element通常它是一个Image类型的图像")]
[Header("方向盘UI本体")]
public Graphic UI_Element;
[Header("方向盘最大旋转角度")]
public float maximumSteeringAngle = 520f;
[Header("方向盘旋转复原速度")]
public float wheelReleasedSpeed = 700f;
[Header("方向盘当前旋转角度")]
public float wheelAngle = 0f;
float wheelPrevAngle = 0f;
bool wheelBeingHeld = false;
RectTransform rectT;
[Tooltip("centerPoint是方向盘旋转的中心，其中你应将设定UI_Element的Img图片")]
[Header("方向盘旋转中心点")]
public Vector2 centerPoint;
/// <summary>
/// Gets the clamped value.
/// Returns a value in range [-1,1] similar to GetAxis("Horizontal")
/// 返回旋转进度比
/// </summary>
/// <returns>The clamped value.</returns>
public float GetClampedValue()
{
return wheelAngle / maximumSteeringAngle;
}
/// <summary>
/// Gets the angle.
/// returns the wheel angle itself without clamp operation
/// 获取实际旋转角度（-520°~520°）
/// </summary>
/// <returns>The angle.</returns>
public float GetAngle()
{
return wheelAngle;
}
/// <summary>
/// Use this for initialization.
/// </summary>
void Start()
{
rectT = UI_Element.rectTransform;
InitEventsSystem();
UpdateRect();
}
/// <summary>
/// Initializes the events system to catch
/// the user movements.i.e. MouseDown/MouseDrag/MouseUp.
/// </summary>
void InitEventsSystem()
{
EventTrigger events = UI_Element.gameObject.GetComponent<EventTrigger>();
if (events == null)
events = UI_Element.gameObject.AddComponent<EventTrigger>();
if (events.triggers == null)
events.triggers = new System.Collections.Generic.List<EventTrigger.Entry>();
EventTrigger.Entry entry = new EventTrigger.Entry();
EventTrigger.TriggerEvent callback = new EventTrigger.TriggerEvent();
UnityAction<BaseEventData> functionCall = new UnityAction<BaseEventData>(PressEvent);
callback.AddListener(functionCall);
entry.eventID = EventTriggerType.PointerDown;
entry.callback = callback;
events.triggers.Add(entry);
entry = new EventTrigger.Entry();
callback = new EventTrigger.TriggerEvent();
functionCall = new UnityAction<BaseEventData>(DragEvent);
callback.AddListener(functionCall);
entry.eventID = EventTriggerType.Drag;
entry.callback = callback;
events.triggers.Add(entry);
entry = new EventTrigger.Entry();
callback = new EventTrigger.TriggerEvent();
functionCall = new UnityAction<BaseEventData>(ReleaseEvent);
callback.AddListener(functionCall);
entry.eventID = EventTriggerType.PointerUp;
entry.callback = callback;
events.triggers.Add(entry);
}
/// <summary>
/// Initializes the center point of seetring wheel transform
/// rectangle.
/// 初始化方向盘变换矩形的旋转中心点。
/// </summary>
void UpdateRect()
{
Vector3[] corners = new Vector3[4];
rectT.GetWorldCorners(corners);
for (int i = 0; i < 4; i++)
{
corners[i] = RectTransformUtility.WorldToScreenPoint(null, corners[i]);
}
Vector3 bottomLeft = corners[0];
Vector3 topRight = corners[2];
float width = topRight.x - bottomLeft.x;
float height = topRight.y - bottomLeft.y;
Rect _rect = new Rect(bottomLeft.x, topRight.y, width, height);
centerPoint = new Vector2(_rect.x + _rect.width * 0.5f, _rect.y - _rect.height * 0.5f);
}
/// <summary>
/// Update is called once per frame.
/// Update the wheel status. If the
/// wheel is released, reset the rotation
/// to initial (zero) rotation by
/// wheelReleasedSpeed degrees per second
/// </summary>
void Update()
{
if (!wheelBeingHeld && !Mathf.Approximately(0f, wheelAngle))
{
float deltaAngle = wheelReleasedSpeed * Time.deltaTime;
if (Mathf.Abs(deltaAngle) > Mathf.Abs(wheelAngle))
wheelAngle = 0f;
else if (wheelAngle > 0f)
wheelAngle -= deltaAngle;
else
wheelAngle += deltaAngle;
}
rectT.localEulerAngles = Vector3.back * wheelAngle;
}
/// <summary>
/// Presses the event.
/// Executed when mouse/finger starts touching the steering wheel.
/// </summary>
/// <param name="eventData">Event data.</param>
public void PressEvent(BaseEventData eventData)
{
Vector2 pointerPos = ((PointerEventData)eventData).position;
wheelBeingHeld = true;
wheelPrevAngle = Vector2.Angle(Vector2.up, pointerPos - centerPoint);
}
/// <summary>
/// Drags the event.
/// Executed when mouse/finger is dragged over the steering wheel.
/// Do nothing if the pointer is too close to the center of the wheel.
/// Make sure wheel angle never exceeds maximumSteeringAngle.
/// </summary>
/// <param name="eventData">Event data.</param>
public void DragEvent(BaseEventData eventData)
{
Vector2 pointerPos = ((PointerEventData)eventData).position;
float wheelNewAngle = Vector2.Angle(Vector2.up, pointerPos - centerPoint);
if (Vector2.Distance(pointerPos, centerPoint) > 20f)
{
if (pointerPos.x > centerPoint.x)
wheelAngle += wheelNewAngle - wheelPrevAngle;
else
wheelAngle -= wheelNewAngle - wheelPrevAngle;
}
wheelAngle = Mathf.Clamp(wheelAngle, -maximumSteeringAngle, maximumSteeringAngle);
wheelPrevAngle = wheelNewAngle;
}
/// <summary>
/// Executed when mouse/finger stops touching the steering wheel
/// Performs one last DragEvent, just in case
/// </summary>
/// <param name="eventData">Event data.</param>
public void ReleaseEvent(BaseEventData eventData)
{
DragEvent(eventData);
wheelBeingHeld = false;
}
}
—END—