C语言解释器-12 控制结构之break、continue和return

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这三种控制结构都必须结合上下文环境共同作用。看起来它们更像是一种标记。因此上，它们的实现也相当简单：

public class Break : ControlFlow    {        public override void Print(int tabs = 0)        {            Debug.WriteLine(new string('\t', tabs) + "break;");                    }    }

public class Continue : ControlFlow    {        public override void Print(int tabs = 0)        {            Debug.WriteLine(new string('\t', tabs) + "continue;");        }    }

return多了一个返回值的表达式：

public class Return : ControlFlow    {        public Expression.ExpressionNode Expression;        public override void Print(int tabs = 0)        {            if (Expression == null)                Debug.WriteLine(new string('\t', tabs) + "return ;");            else            {                Debug.WriteLine(new string('\t', tabs) + "return " + Expression.ToString());            }        }    }

一切需要回溯到Block，在Block运行时，会检测这三种控制结构：

public override void Run(Context ctx)        {            if (IsFirstRunning)            {                Block parentBlock = this.ParentBlock;                if (parentBlock != null)                {                    this.OnReturn += parentBlock.OnReturn;                    this.OnBreak += parentBlock.OnBreak;                    this.OnContinue += parentBlock.OnContinue;                }                IsFirstRunning = false;            }            foreach (Context stx in Children)            {                if (stx is ControlFlow.ControlFlow)                {                    if (!ExecuteControlFlow(stx))                        break;

再看ExecuteControlFlow()：

private bool ExecuteControlFlow(Context stx)        {            if (stx is ControlFlow.Return)            {                if (OnReturn != null)                    OnReturn(this, stx as ControlFlow.Return);                return false;            }            else            {                if (stx is ControlFlow.Break)                {                    if (OnBreak != null)                        OnBreak(this);                    return false;                }                else                    if (stx is ControlFlow.Continue)                    {                        if (OnContinue != null)                            OnContinue(this);                        return false;                    }                    else                        stx.Run(this);            }            return true;        }

通过返回false来立即中止上层Block的运行循环。