参数均按顺序给出

1. GEngine->AddOnScreenDebugMessage(-1, 5.0f, FColor::Red, TEXT(" Hello "));

函数申明：

/**
*   This function will add a debug message to the onscreen message list.
*   It will be displayed for FrameCount frames.
*   @param  Key  A unique key to prevent the same message from being added multiple times.
*   @param  TimeToDisplay   How long to display the message, in seconds.
*   @param  DisplayColor    The color to display the text in.
*   @param  DebugMessage    The message to display.
*/
void AddOnScreenDebugMessage(int32 Key, float TimeToDisplay, FColor DisplayColor, const FString& DebugMessage, bool bNewerOnTop = true, const FVector2D& TextScale = FVector2D::UnitVector);

此外，两个默认参数

bNewerOnTop(true)：是否顶层显示，
TextScale(FVector2D::UnitVector)：字体比例。
此外，RemoveOnScreenDebugMessage(Key) 可以根据具体的Key去除正在显示的消息。

此外，可以使用宏定义简化操作

#define PRINT(String) {if (GEngine){GEngine->AddOnScreenDebugMessage(-1,10.0f,FColor::Red,*(String));}}

带参数的写法：FString::Printf(TEXT("%d, %d, %f"), A, B, C)

GEngine->AddOnScreenDebugMessage(-1, 10.f, FColor::White, FString::Printf(TEXT("-- TestFuncThreeParams,%d, %d,%f"), bVal, iVal, fVal));

2. UKismetSystemLibrary::PrintString(this,TEXT(" Hello "));

函数申明：

/**
 * Prints a string to the log, and optionally, to the screen
 * If Print To Log is true, it will be visible in the Output Log window.  Otherwise it will be logged only as 'Verbose', so it generally won't show up.
 *
 * @param   InString        The string to log out
 * @param   bPrintToScreen  Whether or not to print the output to the screen
 * @param   bPrintToLog     Whether or not to print the output to the log
 * @param   bPrintToConsole Whether or not to print the output to the console
 * @param   TextColor       Whether or not to print the output to the console
 * @param   Duration        The display duration (if Print to Screen is True). Using negative number will result in loading the duration time from the config.
 */
UFUNCTION(BlueprintCallable, meta=(WorldContext="WorldContextObject", CallableWithoutWorldContext, Keywords = "log print", AdvancedDisplay = "2", DevelopmentOnly), Category="Utilities|String")
static void PrintString(const UObject* WorldContextObject, const FString& InString = FString(TEXT("Hello")), bool bPrintToScreen = true, bool bPrintToLog = true, FLinearColor TextColor = FLinearColor(0.0, 0.66, 1.0), float Duration = 2.f);

原注释不祥，给中文注释

const UObject* WorldContextObject：用于得到当前世界的上下文
const FString& InString：文本内容
bool bPrintToScreen：是否打印到屏幕
bool bPrintToLog：是否打印到Log
FLinearColor TextColor：文本颜色
float Duration：显示持续时间
此外，注意 #include "Kismet/KismetSystemLibrary.h"

此外，还有 PrintText() 函数用于打印 FText 类型的变量。

此外，可以看出，源代码的命名都十分规范，其含义一目了然。

3. UE_LOG(LogTemp, Log, TEXT(" Hello "));

宏定义 ：

/** 
 * A  macro that outputs a formatted message to log if a given logging category is active at a given verbosity level
 * @param CategoryName name of the logging category
 * @param Verbosity verbosity level to test against
 * @param Format format text
 **/
#define UE_LOG(CategoryName, Verbosity, Format, ...) \
    { \
        CA_CONSTANT_IF((ELogVerbosity::Verbosity & ELogVerbosity::VerbosityMask) <= ELogVerbosity::COMPILED_IN_MINIMUM_VERBOSITY && (ELogVerbosity::Warning & ELogVerbosity::VerbosityMask) <= FLogCategory##CategoryName::CompileTimeVerbosity) \
        { \
            UE_INTERNAL_LOG_IMPL(CategoryName, Verbosity, Format, ##__VA_ARGS__); \
        } \
    }

打印消息到Output Log ,在编辑器的 Windows/Log/OutputLog 或者Rider的 Unreal模板 可见，内容保存在 \项目文件夹\Saved\Logs 。

4. UE_LOG(LogDiy, Error, TEXT(" Hello "));

宏定义：

/**
 * @param CategoryName 决定Log的名称
 * @param DefaultVerbosity 决定Log的颜色
 * @param CompileTimeVerbosity 决定Log的最大级别
**/
#define DECLARE_LOG_CATEGORY_EXTERN(CategoryName, DefaultVerbosity, CompileTimeVerbosity) \
        extern struct FLogCategory##CategoryName : public FLogCategory<ELogVerbosity::DefaultVerbosity, ELogVerbosity::CompileTimeVerbosity> \
        { \
            FORCEINLINE FLogCategory##CategoryName() : FLogCategory(TEXT(#CategoryName)) {} \
        } CategoryName;
/** 
 * @param **CategoryName** 定义的Log名称
 **/
#define DEFINE_LOG_CATEGORY(CategoryName) FLogCategory##CategoryName CategoryName;

在.h文件添加 DECLARE_LOG_CATEGORY_EXTERN(CategoryName, DefaultVerbosity, CompileTimeVerbosity);

在.cpp文件添加 DEFINE_LOG_CATEGORY(CategoryName);

在.cpp文件使用 UE_LOG(CategoryName, Log, TEXT(“Hello”))；

代码测试(含多参数)

TestLog.h

// Fill out your copyright notice in the Description page of Project Settings.
#pragma once

#include "CoreMinimal.h"
#include "GameFramework/Actor.h"
#include "TestLog.generated.h"
/**
 * @param CategoryName 决定Log的名称
 * @param DefaultVerbosity 决定Log的颜色
 * @param CompileTimeVerbosity 决定Log的最大级别
 * 在.h文件使用DECLARE_LOG_CATEGORY_EXTERN(CategoryName, DefaultVerbosity, CompileTimeVerbosity);
 * 
 * @param CategoryName 定义的Log名称
 * 在.cpp文件使用DEFINE_LOG_CATEGORY(CategoryName);
**/
DECLARE_LOG_CATEGORY_EXTERN(LogForInit, Log, All);

/**
 * @brief Log测试
 */
UCLASS()
class MICROWORLD_API ATestLog : public AActor{
    GENERATED_BODY()
protected:
    /**
     * @brief 打印所调用的函数
     * @param Condition 打印的方式
     */
    UFUNCTION(BlueprintCallable)
    bool SwitchLog(int32 Condition);
};

TestLog.cpp

// Fill out your copyright notice in the Description page of Project Settings.


#include "TestLog.h"

#include "Kismet/KismetSystemLibrary.h"

DEFINE_LOG_CATEGORY(LogForInit);

bool ATestLog::SwitchLog(int32 Condition){
    bool bResult = true;
    switch (Condition){
        case 1: {
            //使用AddOnScreenDebugMessage,打印到屏幕
            if (GEngine != nullptr){
                GEngine->AddOnScreenDebugMessage(-1, 5.0f, FColor::Red,
                                                 TEXT("Log by GEngine.AddOnScreenDebugMessage"));
            }
            break;
        }
        case 2: {
            //使用PrintString,打印到屏幕
            UKismetSystemLibrary::PrintString(this,
                                              TEXT("Log by UKismetSystemLibrary.PrintString"));
            break;
        }
        case 3: {
            //使用Log
            UE_LOG(LogTemp, Log, TEXT("Log by UE_Log in Different Level"));
            break;
        }
        case 4: {
            //使用自定义Log
            UE_LOG(LogForInit, Error, TEXT("Log by UE_Log LogForInit"));
            break;
        }
        case 5: {
            //带参数的打印
            //与c语言的Printf函数类似,格式符一致
            //主要是对TEXT字符串做处理,若不是基础类型或者FString类型，先转换为FString类型在打印
            //注意浮点数默认打印6位小数
            const bool ParamBool = true;
            const int32 ParamInt = 1;
            const float ParamFloat = 1233.1415f;
            const double ParamDouble = 3.1415926;

            //更为简单的字符串插值
            //TArray<FStringFormatArg> Args;
            //Args.Add(FStringFormatArg(FString::SanitizeFloat(ParamDouble,10)));
            //const FString Message =FString::Format(TEXT("{0}"),Args);

            const FString Message = FString::Printf(TEXT("Log by Format Parameter:\
                \nParamBool:%s,\nParamInt:%d,\nParamDouble:%-20.3f,\nParamDouble:%.5lf")
                                                    , (ParamBool ? TEXT("true") : TEXT("false")), ParamInt, ParamFloat,
                                                    ParamDouble);

            if (GEngine != nullptr){
                GEngine->AddOnScreenDebugMessage(-1, 20.0f, FColor::Red,
                                                 *Message);
            }
        }
        default: {
            bResult = false;
            break;
        }
    }
    return bResult;
}

Log级别

enum Type : uint8
    {
        /** Not used */
        NoLogging       = 0,

        /** Always prints a fatal error to console (and log file) and crashes (even if logging is disabled) */
        Fatal,

        /** 
         * Prints an error to console (and log file). 
         * Commandlets and the editor collect and report errors. Error messages result in commandlet failure.
         */
        Error,

        /** 
         * Prints a warning to console (and log file).
         * Commandlets and the editor collect and report warnings. Warnings can be treated as an error.
         */
        Warning,

        /** Prints a message to console (and log file) */
        Display,

        /** Prints a message to a log file (does not print to console) */
        Log,

        /** 
         * Prints a verbose message to a log file (if Verbose logging is enabled for the given category, 
         * usually used for detailed logging) 
         */
        Verbose,

        /** 
         * Prints a verbose message to a log file (if VeryVerbose logging is enabled, 
         * usually used for detailed logging that would otherwise spam output) 
         */
        VeryVerbose,

        // Log masks and special Enum values

        All             = VeryVerbose,
        NumVerbosity,
        VerbosityMask   = 0xf,
        SetColor        = 0x40, // not actually a verbosity, used to set the color of an output device 
        BreakOnLog      = 0x80
    };