UGUI源码分析-编辑器右键创建Image的过程

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在一个空的scene，UGUI是如何创建一个Image组件的?

简单的介绍

  1. Canvas 画布，所有的UI显示元素必须在canvas中,Canvas 是UGUI组织界面的根节点。
  2. Image 创建的图片。
  3. EventSystem 事件系统，所有的点击，拖拽，按钮都是在这里处理完转发给UI组件的。

初始化的这部分代码都在MenuOptions.cs中，任何右键相关的创建命令都在这里编写，我们这里就看下Image的创建过程，其他都是类似的。

MenuOptions.cs
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[MenuItem("GameObject/UI/Image", false, 2001)]
static public void AddImage(MenuCommand menuCommand)
{
    GameObject go = DefaultControls.CreateImage(GetStandardResources());
    PlaceUIElementRoot(go, menuCommand);
}

private static void PlaceUIElementRoot(GameObject element, MenuCommand menuCommand)
{
    GameObject parent = menuCommand.context as GameObject;
    bool explicitParentChoice = true;
    if (parent == null)
    {
        parent = GetOrCreateCanvasGameObject();
        explicitParentChoice = false;

        // If in Prefab Mode, Canvas has to be part of Prefab contents,
        // otherwise use Prefab root instead.
        PrefabStage prefabStage = PrefabStageUtility.GetCurrentPrefabStage();
        if (prefabStage != null && !prefabStage.IsPartOfPrefabContents(parent))
            parent = prefabStage.prefabContentsRoot;
    }
    if (parent.GetComponentsInParent<Canvas>(true).Length == 0)
    {
        // Create canvas under context GameObject,
        // and make that be the parent which UI element is added under.
        GameObject canvas = MenuOptions.CreateNewUI();
        canvas.transform.SetParent(parent.transform, false);
        parent = canvas;
    }

    // Setting the element to be a child of an element already in the scene should
    // be sufficient to also move the element to that scene.
    // However, it seems the element needs to be already in its destination scene when the
    // RegisterCreatedObjectUndo is performed; otherwise the scene it was created in is dirtied.
    SceneManager.MoveGameObjectToScene(element, parent.scene);

    Undo.RegisterCreatedObjectUndo(element, "Create " + element.name);

    if (element.transform.parent == null)
    {
        Undo.SetTransformParent(element.transform, parent.transform, "Parent " + element.name);
    }

    GameObjectUtility.EnsureUniqueNameForSibling(element);

    // We have to fix up the undo name since the name of the object was only known after reparenting it.
    Undo.SetCurrentGroupName("Create " + element.name);

    GameObjectUtility.SetParentAndAlign(element, parent);
    if (!explicitParentChoice) // not a context click, so center in sceneview
        SetPositionVisibleinSceneView(parent.GetComponent<RectTransform>(), element.GetComponent<RectTransform>());

    Selection.activeGameObject = element;
}

//创建canvas
static public GameObject CreateNewUI()
{
    // Root for the UI
    var root = new GameObject("Canvas");
    root.layer = LayerMask.NameToLayer(kUILayerName);
    Canvas canvas = root.AddComponent<Canvas>();
    canvas.renderMode = RenderMode.ScreenSpaceOverlay;
    root.AddComponent<CanvasScaler>();
    root.AddComponent<GraphicRaycaster>();

    // Works for all stages.
    StageUtility.PlaceGameObjectInCurrentStage(root);
    bool customScene = false;
    PrefabStage prefabStage = PrefabStageUtility.GetCurrentPrefabStage();
    if (prefabStage != null)
    {
        root.transform.SetParent(prefabStage.prefabContentsRoot.transform, false);
        customScene = true;
    }

    Undo.RegisterCreatedObjectUndo(root, "Create " + root.name);

    // If there is no event system add one...
    // No need to place event system in custom scene as these are temporary anyway.
    // It can be argued for or against placing it in the user scenes,
    // but let's not modify scene user is not currently looking at.
    if (!customScene)
        CreateEventSystem(false);
    return root;
}


private static void CreateEventSystem(bool select)
{
    CreateEventSystem(select, null);
}

//创建事件系统
private static void CreateEventSystem(bool select, GameObject parent)
{
    StageHandle stage = parent == null ? StageUtility.GetCurrentStageHandle() : StageUtility.GetStageHandle(parent);
    var esys = stage.FindComponentOfType<EventSystem>();
    if (esys == null)
    {
        var eventSystem = new GameObject("EventSystem");
        if (parent == null)
            StageUtility.PlaceGameObjectInCurrentStage(eventSystem);
        else
            GameObjectUtility.SetParentAndAlign(eventSystem, parent);
        esys = eventSystem.AddComponent<EventSystem>();
        eventSystem.AddComponent<StandaloneInputModule>();

        Undo.RegisterCreatedObjectUndo(eventSystem, "Create " + eventSystem.name);
    }

    if (select && esys != null)
    {
        Selection.activeGameObject = esys.gameObject;
    }
}