void Construct( const FArguments& Args )
    {
        SetCanTick(false);

        this->ChildSlot
            [
                SNew(SVerticalBox)
                + SVerticalBox::Slot()
                .Padding( FMargin( 0.0f, 0.0f, 2.0f, 0.0f ) )
                [
                    FSlateApplicationBase::Get().MakeImage(
                        FSlateApplicationBase::Get().GetAppIcon(),
                        Args._IconColorAndOpacity,
                        EVisibility::HitTestInvisible
                    )
                ]
            ];
    }

slate代码这样写的，一开始学感觉很奇怪

其实这是为了减少代码而设计的.

首先这个widget继承自SCompoundWidget，这个可以有子控件，SCompoundWidget继承自SWidget，这个SWidget是基础的控件，它不可以有子控件

SCompoundWidget里面有成员 ChildSlot ，是这样定义的 FSimpleSlot ChildSlot;  这个是子控件容器

/** A slot that support alignment of content and padding */
class SLATECORE_API FSimpleSlot : public TSupportsOneChildMixin<FSimpleSlot>, public TSupportsContentAlignmentMixin<FSimpleSlot>, public TSupportsContentPaddingMixin<FSimpleSlot>
{
public:
    FSimpleSlot(SWidget* InParent)
    : TSupportsOneChildMixin<FSimpleSlot>(InParent)
    , TSupportsContentAlignmentMixin<FSimpleSlot>(HAlign_Fill, VAlign_Fill)
    {
    }
};

下面是TSupportsOneChildMixin定义

/**
 * Widgets that will only have one child can return an instance of FOneChild.
 */
template <typename MixedIntoType>
class TSupportsOneChildMixin : public FChildren, public TSlotBase<MixedIntoType>
{
public:
    TSupportsOneChildMixin(SWidget* InOwner)
        : FChildren(InOwner)
        , TSlotBase<MixedIntoType>()
    {
        this->RawParentPtr = InOwner;
    }

    virtual int32 Num() const override { return 1; }

    virtual TSharedRef<SWidget> GetChildAt( int32 ChildIndex ) override
    {
        check(ChildIndex == 0);
        return FSlotBase::GetWidget();
    }

    virtual TSharedRef<const SWidget> GetChildAt( int32 ChildIndex ) const override
    {
        check(ChildIndex == 0);
        return FSlotBase::GetWidget();
    }

private:
    virtual const FSlotBase& GetSlotAt(int32 ChildIndex) const override { check(ChildIndex == 0); return *this; }
};

而TSlotBase有重写[]  方法

template<typename SlotType>
class TSlotBase : public FSlotBase
{
public:

    TSlotBase()
    : FSlotBase()
    {}

    TSlotBase( const TSharedRef<SWidget>& InWidget )
    : FSlotBase( InWidget )
    {}

    SlotType& operator[]( const TSharedRef<SWidget>& InChildWidget )
    {
        this->AttachWidget(InChildWidget);
        return (SlotType&)(*this);
    }

    SlotType& Expose( SlotType*& OutVarToInit )
    {
        OutVarToInit = (SlotType*)this;
        return (SlotType&)(*this);
    }
};

所以有上面那个方括号里面写代码的内容，意思就是方括号里面的widget 加到子控件数组里