挖一下 SuspendLayout, ResumeLayout 的原理

最近看到一些 winform 控件的写法里面，在设定 DockStyle 或者增减子控件的时候，往往先调用 SuspendLayout 方法，操作完毕之后调用一下 ResumeLayout. 不太明白其中的道理。所以用 Reflector 来看一下。
代码在 System.Windows.Forms.Control 中。
做一个简单的记录如下：

public void SuspendLayout()
{
      this.layoutSuspendCount = (byte) (this.layoutSuspendCount + 1);
}

public void ResumeLayout()
{
      this.ResumeLayout(true);
} 

public void ResumeLayout(bool performLayout)
{
      if (this.layoutSuspendCount > 0)
      {
            this.layoutSuspendCount = (byte) (this.layoutSuspendCount - 1);
            if (((this.layoutSuspendCount == 0) && ((this.state & 0x200) != 0)) && performLayout)
            {
                  this.PerformLayout();
            }
      }
      if (!performLayout)
      {
            Control.ControlCollection collection1 = (Control.ControlCollection) this.Properties.GetObject(Control.PropControlsCollection);
            if (collection1 != null)
            {
                  for (int num1 = 0; num1 < collection1.Count; num1++)
                  {
                        Control.LayoutManager.UpdateAnchorInfo(collection1[num1]);
                  }
            }
      }
}
 
[EditorBrowsable(EditorBrowsableState.Advanced)]
public void PerformLayout()
{
      this.PerformLayout(null, null);
}
 
[EditorBrowsable(EditorBrowsableState.Advanced)]
public void PerformLayout(Control affectedControl, string affectedProperty)
{
      if (!this.GetAnyDisposingInHierarchy())
      {
            if (this.layoutSuspendCount > 0)
            {
                  this.state |= 0x200;
            }
            else
            {
                  this.layoutSuspendCount = 1;
                  try
                  {
                        this.OnLayout(new LayoutEventArgs(affectedControl, affectedProperty));
                  }
                  finally
                  {
                        this.state &= -513;
                        this.layoutSuspendCount = 0;
                  }
            }
      }
}