Unity实现Image透明度渐变（Graphic）

• • • 实现原理
    • 代码实现
    • 使用效果
    • 赞赏与支持

通过Graphic中OnPopulateMesh函数修改UI传入的顶点数据（color属性中的Alpha值）

UI传入的四个顶点如下：

OnPopulateMesh函数

当一个UI元素生成顶点数据时会调用OnPopulateMesh(VertexHelper vh)函数，我们可以在这个函数中修改顶点的数据或者获取顶点的数据。

FadeImage ：

using UnityEngine;
using UnityEngine.UI;

public enum FadeMode
{
	LeftTORight,
	UpToDown,
	LeftUpTORightDown,
	LeftDownTORightUp,
}

public class FadeImage : Image
{
	public FadeMode FadeMode;
	public byte FadeStartAlpha = 0;
	public byte FadeEndAlpha = 255;

	protected override void OnPopulateMesh(VertexHelper toFill)
	{
		base.OnPopulateMesh(toFill);
		UIVertex vertex = new UIVertex();
		for (int i = 0; i < toFill.currentVertCount; i++)
		{
			toFill.PopulateUIVertex(ref vertex, i);

			if (FadeMode == FadeMode.UpToDown)
			{
				if (i == 0 || i == 3)
				{
					vertex.color = new Color32(vertex.color.r, vertex.color.g, vertex.color.b, FadeStartAlpha);
					toFill.SetUIVertex(vertex, i);
				}
				else
				{
					vertex.color = new Color32(vertex.color.r, vertex.color.g, vertex.color.b, FadeEndAlpha);
					toFill.SetUIVertex(vertex, i);
				}
			}
			else if (FadeMode == FadeMode.LeftTORight)
			{
				if (i == 0 || i == 1)
				{
					vertex.color = new Color32(vertex.color.r, vertex.color.g, vertex.color.b, FadeStartAlpha);
					toFill.SetUIVertex(vertex, i);
				}
				else
				{
					vertex.color = new Color32(vertex.color.r, vertex.color.g, vertex.color.b, FadeEndAlpha);
					toFill.SetUIVertex(vertex, i);
				}
			}
			else if (FadeMode == FadeMode.LeftDownTORightUp)
			{
				if (i == 0)
				{
					vertex.color = new Color32(vertex.color.r, vertex.color.g, vertex.color.b, FadeStartAlpha);
					toFill.SetUIVertex(vertex, i);
				}
				else if (i == 1 || i == 3)
				{
					vertex.color = new Color32(vertex.color.r, vertex.color.g, vertex.color.b, (byte)((FadeEndAlpha + FadeStartAlpha) / 2));
					toFill.SetUIVertex(vertex, i);
				}
				else
				{
					vertex.color = new Color32(vertex.color.r, vertex.color.g, vertex.color.b, FadeEndAlpha);
					toFill.SetUIVertex(vertex, i);
				}
			}
			else if (FadeMode == FadeMode.LeftUpTORightDown)
			{
				if (i == 1)
				{
					vertex.color = new Color32(vertex.color.r, vertex.color.g, vertex.color.b, FadeStartAlpha);
					toFill.SetUIVertex(vertex, i);
				}
				else if (i == 0 || i == 2)
				{
					vertex.color = new Color32(vertex.color.r, vertex.color.g, vertex.color.b, (byte)((FadeEndAlpha + FadeStartAlpha) / 2));
					toFill.SetUIVertex(vertex, i);
				}
				else
				{
					vertex.color = new Color32(vertex.color.r, vertex.color.g, vertex.color.b, FadeEndAlpha);
					toFill.SetUIVertex(vertex, i);
				}
			}
		}
	}
}

FadeImageEditor：

using UnityEditor;
using UnityEditor.UI;

[CustomEditor(typeof(FadeImage))]
public class FadeImageEditor : ImageEditor
{
    protected SerializedProperty FadeMode;
    protected SerializedProperty FadeStartAlpha;
    protected SerializedProperty FadeEndAlpha;

    protected override void OnEnable()
    {
        base.OnEnable();
        FadeMode = serializedObject.FindProperty("FadeMode");
        FadeStartAlpha = serializedObject.FindProperty("FadeStartAlpha");
        FadeEndAlpha = serializedObject.FindProperty("FadeEndAlpha");
    }

    public override void OnInspectorGUI()
    {
        base.OnInspectorGUI();
        serializedObject.Update();
        EditorGUILayout.PropertyField(FadeMode);
        EditorGUILayout.PropertyField(FadeStartAlpha);
        EditorGUILayout.PropertyField(FadeEndAlpha);
        serializedObject.ApplyModifiedProperties();
    }
}

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