背景

先前一段時間用RichTextBox實現了Microsoft.Extension.Logger的日志顯示。雖然是用RichTextBox總感覺哪里不對勁，想要添加過濾顯得非常復雜。最近了解并學習了ObservableCollection這個庫（有點火星救援了啊），遂想到了一個更好的實現方式。

引入ObservableCollections庫

1. 在包管理中引入ObservableCollections庫

可觀察的日志

1. 首先定義一個Log實體LogMessage。為了后面更好實現Filiter功能，添加LogLevel，EventId等屬性。
2. 在應用程序運行期間，需要有個存儲LogMessage的地方，這里定義接口ILogMessageHolder, 使用ObservableFixedSizeRingBuffer做容器（環形數組，可以使用指定大小的size），整個Observable Logs就是這個ObservableCollections庫里的容器。

public struct LogMessage
{
    public LogLevel LogLevel { get; set;}
    public EventId EventId { get; set;}
    public string Category { get; set;}
    public DateTime Time { get; set;}
    public string Message { get; set;}
}

public interface ILoggerMessageHolder
{
    public ObservableFixedSizeRingBuffer<LogMessage> LogMessages {get;}
}

實現Logger等其他東西

1. 實現LogMessageProcessor, 這里依然參照ConsoleLoggerProcessor的實現。（有個疑問，直接往LogMessage容器里添加新項，性能能開銷應該不大，這里還需要使用工作線程來執行Enqueue操作嗎?）

internal class LogMessageProcessor
{
    //... 省略字段和其余方法實現

    public LogMessageProcessor(ILogMessageHolder logMessageHolder, LoggerQueueFullMode fullMode, int maxQueueLength)
    {
        _logMessageHolder = logMessageHolder;
        _messageQueue = new();
        FullMode = fullMode;
        MaxQueueLength = maxQueueLength;
        _outputThread = new Thread(ProcessMessageQueue)
        {
            IsBackground = true,
            Name = "LogMessage queue processing thread"
        };
        _outputThread.Start();
    }

    //將 WriteMessage 中寫入Console的部分修改為往LogMessage容器里添加LogMessage
    internal void WriteMessage(LogMessage message)
    {
        try
        {
            _logMessageHolder.LogMessages.AddLast(message);
        }
        catch
        {
            CompleteAdding();
        }
    }
}

2. 實現Logger。其實這里的LogFormatter屬性可要可不要，因為這里的Formatter只對TState做格式化，可以直接做成一個內部的格式化器。當然，這里使用LogFormatter方便后期直接在配置中直接替換實現

internal class Logger : ILogger
{
    private readonly string _category;
    private readonly LogMessageProcessor _processor;
    private StringWriter? t_stringWriter;
    internal IExternalScopeProvider ScopeProvider { get; set; }
    public Logger(string category, LogMessageProcessor processor, LogFormatter formatter,IExternalScopeProvider scopeProvider)
    {
        _category = category;
        _processor = processor;
        Formatter = formatter;
        ScopeProvider = scopeProvider;
    }

    public LogFormatter Formatter { get; set; }

    public IDisposable? BeginScope<TState>(TState state) where TState : notnull
    {
        return ScopeProvider.Push(state) ?? NullScope.Instance;
    }

    public bool IsEnabled(LogLevel logLevel)
    {
        return true;
    }

    public void Log<TState>(LogLevel logLevel, EventId eventId, TState state, Exception? exception, Func<TState, Exception?, string> formatter)
    {
        t_stringWriter ??= new StringWriter();
        var entry = new LogEntry<TState>(logLevel, _category, eventId, state, exception, formatter);

        Formatter.Write(entry, t_stringWriter);

        var sb = t_stringWriter.GetStringBuilder();
        var computedString = sb.ToString();
        sb.Clear();
        _processor.WriteMessage(new LogMessage()
        {
            Time = DateTime.Now,
            Id = eventId,
            Level = logLevel,
            Category = _category,
            Message = computedString,
        });
    }
}

3. 實現LoggerProvider和LoggingBuilderExtension

internal class LoggerProvider : ILoggerProvider
{
    private readonly LogFormatter _formatter;
    private readonly ConcurrentDictionary<string, Logger> _loggers = [];
    private readonly LogMessageProcessor _processor;

    public LoggerProvider(ILogMessageHolder holder, LogFormatter formatter)
    {
        _formatter = formatter;
        _processor = new LogMessageProcessor(holder, LoggerQueueFullMode.Wait, 2500);
    }

    public ILogger CreateLogger(string categoryName)
    {
        return _loggers.GetOrAdd(categoryName, new Logger(categoryName, _processor, _formatter));
    }

    public void Dispose()
    {
        _processor.Dispose();
    }
}

public static class LoggingBuilderExtension
{
    public static ILoggingBuilder AddObservableLogs(this ILoggingBuilder builder)
    {
        builder.Services.AddSingleton<ILoggerProvider, LoggerProvider>();
        builder.Services.AddSingleton<IMfgLoggerProvider, LoggerProvider>();
        builder.Services.AddTransient<LogFormatter, SimpleLogFormatter>();
        builder.Services.AddSingleton<ILogMessageHolder, LogMessageHolder>();
        return builder;
    }
}

• 至此，Logger的核心已經完成，接下來是在View中顯示Log

簡單實現LogViewer

1. 創建LogWindow,使用ItemsControl顯示LogMessage。
   實際上，LogMessage的Formatter，是下文的LogMessageConverter。

public class LogMessageConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
    if (value is not LogMessage message)
    {
        return "";
    }
    return $"{message.Time:yyyy-MM-dd HH:mm:ss,fff} {GetLogLevelString(message.LogLevel)}: {message.Category} [{message.EventId}]\r\n{message.Message}";
}

public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
    throw new NotImplementedException();
}

private static string GetLogLevelString(LogLevel logLevel) => logLevel switch
{
    LogLevel.Trace => "trce",
    LogLevel.Debug => "dbug",
    LogLevel.Information => "info",
    LogLevel.Warning => "warn",
    LogLevel.Error => "fail",
    LogLevel.Critical => "crit",
    _ => throw new ArgumentOutOfRangeException(nameof(logLevel))
};
}

<!--省略Window的命名空間和其余屬性-->
<ItemsControl ItemsSource="{Binding LogMessages}">
    <ItemsControl.ItemTemplate>
        <DataTemplate>
            <!--如果想要顯示日志等級的顏色，需要其他TextBlock和添加trigger-->
            <TextBlock Text="{Binding ., Converter={StaticResource LogMessageConverter}}"></TextBlock>
        </DataTemplate>
    </ItemsControl.ItemTemplate>
</ItemsControl>

2. 創建LogWindow的DataContext
   這里就涉及到ObservableCollections庫的知識了，最后綁定到View上的是LogMessages屬性。如果想對LogMessages進行過濾，比如選取LogLevel.Information等，使用_viewList的AttachFiliter就好啦。

public class LogWindowViewModel
{
    private readonly ISynchronizedView<LogMessage, LogMessage> _viewList;
    private readonly ILogMessageHolder _logMessageHolder;
      
    public LogWindowViewModel(ILogMessageHolder logMessageHolder)
    {
        _logMessageHolder = logMessageHolder;
        _viewList = logMessageHolder.LogMessages.CreateView(x => x);
        LogMessages = _viewList.ToNotifyCollectionChanged();
    }

    public INotifyCollectionChangedSynchronizedViewList<LogMessage> LogMessages { get; }
}

結尾

使用ObservableCollections可以非常簡單的將LogMessage顯示到某個UI上，并且LogMessages的生命周期是跟隨應用程序，隨時可以打開查看應用程序的日志。由于ObservableCollections是純C#實現，理論上是可以在Avalonia中使用的。