Day 1

1. SlateCore\Public\Widgets\SWidget.h
   所有Widget的基类，定义如下几类接口：

• keyboard事件接口
• mouse事件接口
• Drag & Drop事件接口
• Paint接口
• 计算widget大小和排列子窗口接口

/**
 * Abstract base class for Slate widgets.
 *
 * STOP. DO NOT INHERIT DIRECTLY FROM WIDGET!
 *
 * Inheritance:
 *   Widget is not meant to be directly inherited. Instead consider inheriting from LeafWidget or Panel,
 *   which represent intended use cases and provide a succinct set of methods which to override.
 *
 *   SWidget is the base class for all interactive Slate entities. SWidget's public interface describes
 *   everything that a Widget can do and is fairly complex as a result.
 * 
 * Events:
 *   Events in Slate are implemented as virtual functions that the Slate system will call
 *   on a Widget in order to notify the Widget about an important occurrence (e.g. a key press)
 *   or querying the Widget regarding some information (e.g. what mouse cursor should be displayed).
 *
 *   Widget provides a default implementation for most events; the default implementation does nothing
 *   and does not handle the event.
 *
 *   Some events are able to reply to the system by returning an FReply, FCursorReply, or similar
 *   object. 
 */
class SLATECORE_API SWidget
    : public FSlateControlledConstruction,
      public TSharedFromThis<SWidget>       // Enables 'this->AsShared()'
;

问题列表:
a. FGeometry做什么用？

2. SlateCore\Public\Widgets\SPanel.h

/**
 * A Panel arranges its child widgets on the screen. 
 *
 * Each child widget should be stored in a Slot. The Slot describes how the individual child should be arranged with
 * respect to its parent (i.e. the Panel) and its peers Widgets (i.e. the Panel's other children.)
 * For a simple example see StackPanel.
 */
class SLATECORE_API SPanel
    : public SWidget
{
public: 

    /**
     * Panels arrange their children in a space described by the AllottedGeometry parameter. The results of the arrangement
     * should be returned by appending a FArrangedWidget pair for every child widget. See StackPanel for an example
     *
     * @param AllottedGeometry    The geometry allotted for this widget by its parent.
     * @param ArrangedChildren    The array to which to add the WidgetGeometries that represent the arranged children.
     */
    virtual void OnArrangeChildren( const FGeometry& AllottedGeometry, FArrangedChildren& ArrangedChildren ) const override = 0;

    /**
     * A Panel's desired size in the space required to arrange of its children on the screen while respecting all of
     * the children's desired sizes and any layout-related options specified by the user. See StackPanel for an example.
     *
     * @return The desired size.
     */
    virtual FVector2D ComputeDesiredSize(float) const override = 0;

    /**
     * All widgets must provide a way to access their children in a layout-agnostic way.
     * Panels store their children in Slots, which creates a dilemma. Most panels
     * can store their children in a TPanelChildren<Slot>, where the Slot class
     * provides layout information about the child it stores. In that case
     * GetChildren should simply return the TPanelChildren<Slot>. See StackPanel for an example.
     */
    virtual FChildren* GetChildren() override = 0;

public:

    /**
     * Most panels do not create widgets as part of their implementation, so
     * they do not need to implement a Construct()
     */
    void Construct() { }

public:

    // SWidget overrides

    virtual int32 OnPaint( const FPaintArgs& Args, const FGeometry& AllottedGeometry, const FSlateRect& MyCullingRect, FSlateWindowElementList& OutDrawElements, int32 LayerId, const FWidgetStyle& InWidgetStyle, bool bParentEnabled ) const override;

protected:

    /**
     * Just like OnPaint, but takes already arranged children. Can be handy for writing custom SPanels.
     */
    int32 PaintArrangedChildren( const FPaintArgs& Args, const FArrangedChildren& ArrangedChildren, const FGeometry& AllottedGeometry, const FSlateRect& MyCullingRect, FSlateWindowElementList& OutDrawElements, int32 LayerId, const FWidgetStyle& InWidgetStyle, bool bParentEnabled  ) const;
    
protected:

    /** Hidden default constructor. */
    SPanel( ) { }

public:
    virtual void SetVisibility( TAttribute<EVisibility> InVisibility ) override final;
};

Panel接口类, 用于排版子widgets.