UE富文本(RichText)和自定义装饰器(Custom Decorator)

参考文档：

Epic提供了一个ImageDecorator的实现，我们可以模仿它来实现自定义Decorator。

剖析RichTextBlockImageDecorator

RichTextBlockImageDecorator.h

// Copyright Epic Games, Inc. All Rights Reserved.

#pragma once

#include "CoreMinimal.h"
#include "UObject/ObjectMacros.h"
#include "UObject/Object.h"
#include "Fonts/SlateFontInfo.h"
#include "Styling/SlateTypes.h"
#include "Framework/Text/TextLayout.h"
#include "Framework/Text/ISlateRun.h"
#include "Framework/Text/ITextDecorator.h"
#include "Components/RichTextBlockDecorator.h"
#include "Engine/DataTable.h"
#include "RichTextBlockImageDecorator.generated.h"

class ISlateStyle;

/** Simple struct for rich text styles */
/** Data的数据行定义，这里只含有一个FSlateBrush*/
USTRUCT(Blueprintable, BlueprintType)
struct UMG_API FRichImageRow : public FTableRowBase
{
    GENERATED_USTRUCT_BODY()

public:

    UPROPERTY(EditAnywhere, Category = Appearance)
    FSlateBrush Brush;
};

/**
 * Allows you to setup an image decorator that can be configured
 * to map certain keys to certain images.  We recommend you subclass this
 * as a blueprint to configure the instance.
 *
 * Understands the format <img id="NameOfBrushInTable"></>
 */
/**  图片装饰器类，可以在RichTextBlock的Decorator详情里看到UPROPERTY */
UCLASS(Abstract, Blueprintable) 
class UMG_API URichTextBlockImageDecorator : public URichTextBlockDecorator
{
    GENERATED_BODY()

public:
    URichTextBlockImageDecorator(const FObjectInitializer& ObjectInitializer);

        // 必须实现这个重写
    virtual TSharedPtr<ITextDecorator> CreateDecorator(URichTextBlock* InOwner) override;

       //找到数据表中相应的数据，即我们设置的FSlateBrush
    virtual const FSlateBrush* FindImageBrush(FName TagOrId, bool bWarnIfMissing);

protected:
       // 找到数据表中对应的行
    FRichImageRow* FindImageRow(FName TagOrId, bool bWarnIfMissing);

      // 数据表引用
    UPROPERTY(EditAnywhere, Category=Appearance, meta=(RowType="RichImageRow"))
    class UDataTable* ImageSet;
};

RichTextBlockImageDecorator.cpp
除了实现RichTextBlockImageDecorator之外，还定义并实现了两个类，分别是SRichInlineImage和FRichInlineImage。其中，SRichInlineImage为Slate，负责标记文本解析后的UI渲染；FRichInlineImage负责对标记进行实际的解析/替换。通过阅读后面的代码我们可以知道ImageDecorator支持的标签为：<img id="",width= ,height= ,strecth= />

// Copyright Epic Games, Inc. All Rights Reserved.

#include "Components/RichTextBlockImageDecorator.h"
#include "UObject/SoftObjectPtr.h"
#include "Rendering/DrawElements.h"
#include "Framework/Text/SlateTextRun.h"
#include "Framework/Text/SlateTextLayout.h"
#include "Slate/SlateGameResources.h"
#include "Widgets/SCompoundWidget.h"
#include "Widgets/DeclarativeSyntaxSupport.h"
#include "Framework/Application/SlateApplication.h"
#include "Fonts/FontMeasure.h"
#include "Math/UnrealMathUtility.h"
#include "Widgets/Images/SImage.h"
#include "Widgets/Layout/SScaleBox.h"
#include "Widgets/Layout/SBox.h"
#include "Misc/DefaultValueHelper.h"
#include "UObject/UObjectGlobals.h"
#include "UObject/Package.h"

#define LOCTEXT_NAMESPACE "UMG"

/**  Slate ！！不熟啊 ~~~~*/
class SRichInlineImage : public SCompoundWidget
{
public:
    SLATE_BEGIN_ARGS(SRichInlineImage)
    {}
    SLATE_END_ARGS()

public:
    void Construct(const FArguments& InArgs, const FSlateBrush* Brush, const FTextBlockStyle& TextStyle, TOptional<int32> Width, TOptional<int32> Height, EStretch::Type Stretch)
    {
        if (ensure(Brush))
        {
            //获取FontMeasureService实例
            const TSharedRef<FSlateFontMeasure> FontMeasure = FSlateApplication::Get().GetRenderer()->GetFontMeasureService();
          // 富文本图片高度：取字体高度、Brush中ImageSize的Y值二者中更小的值，即图片大小受两个参数影响
            float IconHeight = FMath::Min((float)FontMeasure->GetMaxCharacterHeight(TextStyle.Font, 1.0f), Brush->ImageSize.Y);
            float IconWidth = IconHeight;

          // 还可以直接传入宽高值
            if (Width.IsSet())
            {
                IconWidth = Width.GetValue();
            }

            if (Height.IsSet())
            {
                IconHeight = Height.GetValue();
            }
          // 开始绘制
            ChildSlot
            [
                SNew(SBox)
                .HeightOverride(IconHeight)
                .WidthOverride(IconWidth)
                [
                    SNew(SScaleBox)
                    .Stretch(Stretch)
                    .StretchDirection(EStretchDirection::DownOnly)
                    .VAlign(VAlign_Center)
                    [
                        SNew(SImage)
                        .Image(Brush)
                    ]
                ]
            ];
        }
    }
};

class FRichInlineImage : public FRichTextDecorator
{
public:
//构造函数，先调用父类构造函数，然后传入Decorator指针
    FRichInlineImage(URichTextBlock* InOwner, URichTextBlockImageDecorator* InDecorator)
        : FRichTextDecorator(InOwner)
        , Decorator(InDecorator)
    {
    }
/**解析文本，如果返回true则表明支持这个标签文本，false则当作普通文字
 我们使用ImageDecorator的标签文本为： <img id="RowName"/> ，所以我们可以大致了解Parser是如何工作的，当然如果你想的话，你还可以自定义Parser (⊙﹏⊙)
*/
    virtual bool Supports(const FTextRunParseResults& RunParseResult, const FString& Text) const override
    {
        if (RunParseResult.Name == TEXT("img") && RunParseResult.MetaData.Contains(TEXT("id")))
        {
            const FTextRange& IdRange = RunParseResult.MetaData[TEXT("id")];
              //Text.Mid：取子串。这里取出Id值
            const FString TagId = Text.Mid(IdRange.BeginIndex, IdRange.EndIndex - IdRange.BeginIndex);

            const bool bWarnIfMissing = false;
            return Decorator->FindImageBrush(*TagId, bWarnIfMissing) != nullptr;
        }

        return false;
    }

protected:
/**   显然，只有当Support方法返回true之后，才会执行。找到我们在DataTable中配置的数据，用以生成Slate
*/
    virtual TSharedPtr<SWidget> CreateDecoratorWidget(const FTextRunInfo& RunInfo, const FTextBlockStyle& TextStyle) const override
    {
        const bool bWarnIfMissing = true;
        const FSlateBrush* Brush = Decorator->FindImageBrush(*RunInfo.MetaData[TEXT("id")], bWarnIfMissing);

            // 除了id之外，我们还可以在标签中写 width= ，height=，strech=
        TOptional<int32> Width;
        if (const FString* WidthString = RunInfo.MetaData.Find(TEXT("width")))
        {
            int32 WidthTemp;
            Width = FDefaultValueHelper::ParseInt(*WidthString, WidthTemp) ? WidthTemp : TOptional<int32>();
        }

        TOptional<int32> Height;
        if (const FString* HeightString = RunInfo.MetaData.Find(TEXT("height")))
        {
            int32 HeightTemp;
            Height = FDefaultValueHelper::ParseInt(*HeightString, HeightTemp) ? HeightTemp : TOptional<int32>();
        }

        EStretch::Type Stretch = EStretch::ScaleToFit;
        if (const FString* SstretchString = RunInfo.MetaData.Find(TEXT("stretch")))
        {
            static const UEnum* StretchEnum = StaticEnum<EStretch::Type>();
            int64 StretchValue = StretchEnum->GetValueByNameString(*SstretchString);
            if (StretchValue != INDEX_NONE)
            {
                Stretch = static_cast<EStretch::Type>(StretchValue);
            }
        }
            // 使用解析的数据构造Slate
        return SNew(SRichInlineImage, Brush, TextStyle, Width, Height, Stretch);
    }

private:
    URichTextBlockImageDecorator* Decorator;
};

/////////////////////////////////////////////////////
// URichTextBlockImageDecorator

URichTextBlockImageDecorator::URichTextBlockImageDecorator(const FObjectInitializer& ObjectInitializer)
    : Super(ObjectInitializer)
{
}

const FSlateBrush* URichTextBlockImageDecorator::FindImageBrush(FName TagOrId, bool bWarnIfMissing)
{
    const FRichImageRow* ImageRow = FindImageRow(TagOrId, bWarnIfMissing);
    if (ImageRow)
    {
        return &ImageRow->Brush;
    }

    return nullptr;
}

FRichImageRow* URichTextBlockImageDecorator::FindImageRow(FName TagOrId, bool bWarnIfMissing)
{
    if (ImageSet)
    {
        FString ContextString;
        return ImageSet->FindRow<FRichImageRow>(TagOrId, ContextString, bWarnIfMissing);
    }
    
    return nullptr;
}

// 必须实现，创建FRichInlineImage用来解析文本
TSharedPtr<ITextDecorator> URichTextBlockImageDecorator::CreateDecorator(URichTextBlock* InOwner)
{
    return MakeShareable(new FRichInlineImage(InOwner, this));
}

/////////////////////////////////////////////////////

#undef LOCTEXT_NAMESPACE

超链接Decorator：

上面的参考文章中有详细的代码可供查阅。引擎提供了SRichTextHyperlink和FHyperlinkStyle供使用，如果不能满足你的需求，那么你必须先好好学习Slate。