CinemachineBrain是Unity Cinemachine的核心组件，它和Camera组件挂载在一起，监控场景中所有的virtual camera。CinemachineBrain在inspector中暴露的属性如下：

Live Camera和Live Blend分别表示当前active的virtual camera以及blend的进度（如果有的话）。相关的代码可以在CinemachineBrainEditor找到：

[CustomEditor(typeof(CinemachineBrain))]
[CanEditMultipleObjects]
class CinemachineBrainEditor : UnityEditor.Editor
{
    CinemachineBrain Target => target as CinemachineBrain;

    public override VisualElement CreateInspectorGUI()
    {
        var ux = new VisualElement();

        ux.ContinuousUpdate(() =>
        {
            if (target == null || liveCamera == null)
                return;
            liveCamera.value = Target.ActiveVirtualCamera as CinemachineVirtualCameraBase;
            liveBlend.value = Target.ActiveBlend != null ? Target.ActiveBlend.Description : string.Empty;
        });

        return ux;
    }
}

可以看到引用的是CinemachineBrain的ActiveVirtualCamera和ActiveBlend字段。

public class CinemachineBrain : MonoBehaviour, ICameraOverrideStack, ICinemachineMixer
{
    /// <summary>
    /// Get the current active virtual camera.
    /// </summary>
    public ICinemachineCamera ActiveVirtualCamera 
        => CinemachineCore.SoloCamera ?? m_BlendManager.ActiveVirtualCamera;

    /// <summary>
    /// Get the current blend in progress.  Returns null if none.
    /// It is also possible to set the current blend, but this is not a recommended usage
    /// unless it is to set the active blend to null, which will force completion of the blend.
    /// </summary>
    public CinemachineBlend ActiveBlend 
    {
        get => m_BlendManager.ActiveBlend;
        set => m_BlendManager.ActiveBlend = value;
    }
}

ActiveBlend.Description描述了从当前virtual camera切换到下一个virtual camera的进度。

Show Debug Text字段，如果勾上，就会在Game窗口中显示当前live的virtual camera信息。

Show Camera Frustum字段，如果勾上，就会在Scene窗口显示当前live的virtual camera视锥体，不用选中任何GameObject。

Ignore Time Scale字段，勾上的话会使Cinemachine实时响应用户输入和阻尼，说白了就是不受time scale的影响。这里同样可以查看相关代码:

float GetEffectiveDeltaTime(bool fixedDelta)
{
    if (CinemachineCore.UniformDeltaTimeOverride >= 0)
        return CinemachineCore.UniformDeltaTimeOverride;

    if (CinemachineCore.SoloCamera != null)
        return Time.unscaledDeltaTime;

    if (!Application.isPlaying)
        return m_BlendManager.GetDeltaTimeOverride();

    if (IgnoreTimeScale)
        return fixedDelta ? Time.fixedDeltaTime : Time.unscaledDeltaTime;

    return fixedDelta ? Time.fixedDeltaTime : Time.deltaTime;
}

fixedDelta参数表示是否要使用Time.fixedDeltaTime，如果为false，在勾选的情况下，就会选择Time.unscaledDeltaTime，而不是Time.deltaTime。

World Up Override字段，指定的transform的Y轴定义了Cinemachine在世界空间的up向量。如果为空，则默认为(0,1,0)。这点在代码中同样有体现：

/// <summary>Get the default world up for the virtual cameras.</summary>
public Vector3 DefaultWorldUp => (WorldUpOverride != null) ? WorldUpOverride.transform.up : Vector3.up;

Channel Mask字段主要用于分屏效果的实现。这种情况下，场景中会有多个camera，也就会有多个CinemachineBrain组件的存在。为了确定某一个virtual camera是属于哪个CinemachineBrain管理，就需要用到Channel Mask字段。相关的判断代码如下：

/// <summary>Returns true if camera is on a channel that is handles by this Brain.</summary>
/// <param name="vcam">The camera to check</param>
/// <returns></returns>
public bool IsValidChannel(CinemachineVirtualCameraBase vcam) 
    => vcam != null && ((uint)vcam.OutputChannel & (uint)ChannelMask) != 0;

不过在大部分情况下，只需要一个屏幕，场景中也就只有一个CinemachineBrain，此时Channel Mask字段保持默认值即可，默认值为-1，这样转为uint就是0xffffffff了。

/// <summary>The CinemachineBrain will find the highest-priority CinemachineCamera that outputs 
/// to any of the channels selected.  CinemachineCameras that do not output to one of these 
/// channels will be ignored.  Use this in situations where multiple CinemachineBrains are 
/// needed (for example, Split-screen).</summary>
[Tooltip("The CinemachineBrain will find the highest-priority CinemachineCamera that outputs to "
    + "any of the channels selected.  CinemachineCameras that do not output to one of these "
    + "channels will be ignored.  Use this in situations where multiple CinemachineBrains are "
    + "needed (for example, Split-screen).")]
public OutputChannels ChannelMask = (OutputChannels)(-1);  // default is Everything

Update Method字段表示virtual camera更新position和rotation的时机，有以下几种。

Update Method
Fixed Update	和物理模块保持同步，在FixedUpdate时更新
Late Update	在MonoBehaviour的LateUpdate时更新
Smart Update	根据virtual camera当前的更新情况更新，推荐设置
Manual Update	virtual camera不会自动更新，需要手动调用brain.ManualUpdate()

Smart Update具体是如何实现的呢？通过搜索UpdateMethods.SmartUpdate，可以查到相关的代码集中在ManualUpdate和DoFixedUpdate这两个函数上。容易猜到，触发这两个函数的时机，一个是在LateUpdate，一个是在FixedUpdate期间：

void LateUpdate()
{
    if (UpdateMethod != UpdateMethods.ManualUpdate)
        ManualUpdate();
}

// Instead of FixedUpdate() we have this, to ensure that it happens
// after all physics updates have taken place
IEnumerator AfterPhysics()
{
    while (true)
    {
        // FixedUpdate can be called multiple times per frame
        yield return m_WaitForFixedUpdate;
        DoFixedUpdate();
    }
}

由Unity的脚本执行顺序[3]可知，DoFixedUpdate会在Unity所有的FixedUpdate执行之后立刻执行。

决定在哪个Update阶段进行更新的逻辑，位于UpdateTracker.OnUpdate这个函数：

public void OnUpdate(int currentFrame, UpdateClock currentClock, Matrix4x4 pos)
{
    if (lastPos == pos)
        return;

    if (currentClock == UpdateClock.Late)
        ++numWindowLateUpdateMoves;
    else if (lastFrameUpdated != currentFrame) // only count 1 per rendered frame
        ++numWindowFixedUpdateMoves;
    lastPos = pos;

    UpdateClock choice;
    if (numWindowFixedUpdateMoves > 3 && numWindowLateUpdateMoves < numWindowFixedUpdateMoves / 3)
        choice = UpdateClock.Fixed;
    else
        choice =  UpdateClock.Late;
    if (numWindows == 0)
        PreferredUpdate = choice;

    if (windowStart + kWindowSize <= currentFrame)
    {
#if DEBUG_LOG_NAME
        Debug.Log(name + ": Window " + numWindows + ": Late=" + numWindowLateUpdateMoves + ", Fixed=" + numWindowFixedUpdateMoves);
#endif
        PreferredUpdate = choice;
        ++numWindows;
        windowStart = currentFrame;
        numWindowLateUpdateMoves = (PreferredUpdate == UpdateClock.Late) ? 1 : 0;
        numWindowFixedUpdateMoves = (PreferredUpdate == UpdateClock.Fixed) ? 1 : 0;
    }
}

其主要逻辑，就是采样前一段时间kWindowSize = 30帧内，virtual camera的target position在LateUpdate和FixedUpdate时发生变化的次数，如果FixedUpdate次数是LateUpdate次数的三倍以上，那就选择在FixedUpdate更新virtual camera，否则选择LateUpdate。

Blend Update Method表示混合并更新主相机的时机，推荐使用Late Update。

Lens Mode Override表示是否允许virtual camera修改主相机的模式（透视，正交，物理）。如果不勾上，那么在virtual camera的设置里修改是不生效的：

Default Blend表示两个virtual camera混合的方式，Unity默认提供了若干种，当然也可以自定义。

btw，在旧版本中CinemachineBrain监听的事件也在CinemachineBrain组件中，而新版本（3.1.0）这部分已经剥离出来，单独作为Cinemachine Brain Event组件存在了。现在支持六种事件：

Reference

[1] Cinemachine Brain component

[2] Cinemachine（一）VirtualCamera和Brain的简单介绍

[3] Order of execution for event functions