在我的应用程序中，我使用基于线程的任务。它们工作正常，但有时会挂起该应用程序。在下面的代码中，procedure Stop有时会卡在WaitFor过程中。这是因为FStopEvent.SetEvent并不总是起作用。
在正常执行期间，线程进入Execute过程，执行OnWork过程，直到Stop被调用(这将设置Terminated)，然后进行一些后处理然后退出。这是WaitFor退出的信号，每个人都很高兴。在我的用法中，这是因为任务已销毁。在这种情况下，将调用基类的析构函数，该析构函数将调用Stop。
在某些情况下，这不起作用。正确输入了Execute，可以正常执行OnWork过程调用，但对FStopEvent.SetEvent没有任何 react 。没有崩溃(只是不执行except处的语句)。该程序挂起，因为WaitFor不返回。使用调试DCU，我可以将其追溯到WaitFor单元中的Classes，其中程序卡在WaitForSingleObject(H[0], INFINITE);上。 OnWork回调是相同的。
OnBeforeWork和OnAfterWork过程为零。 MaxLoops = -1和FreeOnTerminate = False。我很拼命，希望有人可以出路。
编辑1:我正在谈论的WaitFor出现在下面列出的TEvent_Driven_Task类中。因为该类是从TSimple_Task类派生的，所以我添加了此类是出于完整性的考虑。
编辑2:由于janjan Venema指出这可能会引起问题，因此已从Application.ProcessMessages中删除了 TSimple_Task.Stop。结果是相同的(程序卡在WaitFor中)。
unit Parallel_Event_Task;
interface
uses Forms, Windows, Classes, SysUtils, SyncObjs,
Parallel_Simple_Task;
type
TEvent_Driven_Task = class (TSimple_Task)
private
FWorkEvent: TEvent; // Event signalling that some work should be done
public
constructor Create (work: TNotifyEvent; CreateSuspended: boolean = False;
max: Int32 = 1;
before: TNotifyEvent = nil; after: TNotifyEvent = nil;
terminate: boolean = True; task: integer = 1); override;
destructor Destroy; override;
procedure Activate (work: TNotifyEvent = nil);
procedure Execute; override;
procedure Stop; override;
procedure Release; override;
end; // Class: TEvent_Driven_Task //
implementation
constructor TEvent_Driven_Task.Create
(
work: TNotifyEvent; // Work to do in Execute loop
CreateSuspended: boolean = False; // False = start now, True = use Start
max: Int32 = 1; // Max loops of Execute loop, negative = infinite loop
before: TNotifyEvent = nil;// Called before Execute loop
after: TNotifyEvent = nil; // Called after Execute loop
terminate: boolean = True; // When true free the task on termination
task: integer = 1 // Task ID
);
begin
inherited Create (work, CreateSuspended, max, before, after, terminate, task);
FWorkEvent := TEvent.Create (nil, False, False, '');
end; // Create //
Destructor TEvent_Driven_Task.Destroy;
begin
inherited Destroy;
end; // Destroy //
procedure TEvent_Driven_Task.Activate (work: TNotifyEvent = nil);
begin
if Assigned (work) then OnWork := work;
FWorkEvent.SetEvent;
end; // Activate //
// Execute calls event handler OnWork in a while loop.
// Before the loop is entered, OnBeforeWork is executed, after: OnAfterWork.
procedure TEvent_Driven_Task.Execute;
var two: TWOHandleArray;
pwo: PWOHandleArray;
ret: DWORD;
begin
pwo := @two;
pwo [0] := FWorkEvent.Handle;
pwo [1] := FStopEvent.Handle;
NameThreadForDebugging (AnsiString (FTaskName));
FLoop := 0;
try
if Assigned (OnBeforeWork) then OnBeforeWork (Self);
while (not Terminated) and (Loop <> Max_Loops) do
begin
FLoop := FLoop + 1;
ret := WaitForMultipleObjects (2, pwo, FALSE, INFINITE);
if ret = WAIT_FAILED then Break;
case ret of
WAIT_OBJECT_0 + 0: if Assigned (OnWork) then OnWork (Self);
WAIT_OBJECT_0 + 1: Terminate;
end; // case
end; // while
if Assigned (OnAfterWork) then OnAfterWork (Self);
// Intercept and ignore the interruption but keep the message
except
on e: exception do FError_Mess := e.Message;
end; // try..except
end; // Execute //
procedure TEvent_Driven_Task.Stop;
begin
Terminate;
FStopEvent.SetEvent;
if not FreeOnTerminate
then WaitFor;
end; // Stop //
procedure TEvent_Driven_Task.Release;
begin
inherited Release;
FWorkEvent.Free;
end; // Release //
end. // Unit: Parallel_Simple_Task //
=============基础类别=======================
unit Parallel_Simple_Task;
interface
uses Windows, Classes, SysUtils, SyncObjs, Forms;
type
TSimple_Task = class (TThread)
protected
FStopEvent: TEvent; // Event signalling that the thread has to terminate, set by Stop
FTaskID: integer; // Task sequence number
FTaskName: string; // Task name
FLoop: integer; // Indicates number of times Work has been processed
FMax_Loops: integer; // Maximum # of iterations
FError_Mess: string; // Error message if an exception occurred, else empty
FOnBeforeWork: TNotifyEvent; // Event to be called just before thread loop is entered
FOnWork: TNotifyEvent; // Event caled in Execute loop
FOnAfterWork: TNotifyEvent; // Event to be called just after thread loop is finished
procedure set_name (value: string);
public
constructor Create (work: TNotifyEvent; CreateSuspended: boolean = False; max: Int32 = 1;
before: TNotifyEvent = nil; after: TNotifyEvent = nil;
terminate: boolean = True; task: integer = 1); reintroduce; virtual;
destructor Destroy; override;
procedure Execute; override;
procedure Stop; virtual;
procedure Release; virtual;
property TaskID: integer read FTaskID;
property TaskName: string read FTaskName write set_name;
property Loop: integer read FLoop;
property Max_Loops: integer read FMax_Loops write FMax_Loops;
property OnBeforeWork: TNotifyEvent read FOnBeforeWork write FOnBeforeWork;
property OnWork: TNotifyEvent read FOnWork write FOnWork;
property OnAfterWork: TNotifyEvent read FOnAfterWork write FOnAfterWork;
end; // Class: TSimple_Task //
implementation
constructor TSimple_Task.Create
(
work: TNotifyEvent; // Work to do in Execute loop
CreateSuspended: boolean = False; // False = start now, True = use Start
max: Int32 = 1; // Max loops of Execute loop
before: TNotifyEvent = nil;// Called before Execute loop
after: TNotifyEvent = nil; // Called after Execute loop
terminate: boolean = True; // When true free the task on termination
task: integer = 1 // Task ID
);
begin
// The thread will only be started when this constructor ends.
inherited Create (CreateSuspended);
FStopEvent := TEvent.Create (nil, True, False, '');
FError_Mess := '';
FTaskID := task;
FTaskName := '';
Max_Loops := max;
OnBeforeWork := before;
OnWork := work;
OnAfterWork := after;
FreeOnTerminate := terminate;
end; // Create //
destructor TSimple_Task.Destroy;
begin
Stop;
Release;
inherited Destroy;
end; // Destroy //
// Execute calls event handler OnWork in a while loop.
// Before the loop is entered, OnBeforeWork is executed, after: OnAfterWork.
procedure TSimple_Task.Execute;
var ret: DWORD;
begin
try
NameThreadForDebugging (AnsiString (FTaskName));
FLoop := 0;
if Assigned (OnBeforeWork) then OnBeforeWork (Self);
while (not Terminated) and (FLoop <> Max_Loops) do
begin
ret := WaitForSingleObject (FStopEvent.Handle, 0);
if ret = WAIT_OBJECT_0 then
begin
Terminate;
end else
begin
if Assigned (OnWork) then OnWork (Self);
FLoop := FLoop + 1;
end; // if
end; // while
if not Terminated and Assigned (OnAfterWork) then OnAfterWork (Self);
// Intercept and ignore the interruption but keep the message
except
on e: exception do FError_Mess := e.Message;
end; // try..except
end; // Execute //
procedure TSimple_Task.Stop;
begin
Terminate;
FStopEvent.SetEvent;
if not FreeOnTerminate
then WaitFor;
end; // Stop //
procedure TSimple_Task.Release;
begin
FStopEvent.Free;
end; // Release //
procedure TSimple_Task.set_name (value: string);
begin
FTaskName := value;
end; // set_name //
end. // Unit: Parallel_Simple_Task //
最佳答案
当底层线程对象在OS层终止时，TThread.WaitFor()等待信号通知线程句柄(TThread.Handle属性)。当TThread方法退出后(以及ExitThread()被调用并退出后)，当Execute()自身调用Win32 API TThread.DoTerminate()函数时，就会发生该信号。您所描述的内容听起来像是遇到了一个死锁，即使您已发出信号Execute()停止了循环，该死锁仍使FStopEvent方法无法正确退出。给定您已显示的代码，这意味着WaitForMultipleObjects()返回了您不想要的错误代码，或者更有可能您的OnWork事件处理程序有时无法正确退出，因此Execute()随后可以自行退出。
到目前为止，您所展示的只是任务类本身的定义，但是您尚未展示它们在项目中的实际使用方式。请显示其余的任务逻辑，并不要让人们猜测问题可能出在哪里。
我建议的第一件事是，从析构函数中取出对Stop()的调用。它不属于那里。 永不销毁仍在运行的线程。始终先停止线程并等待其终止，然后再销毁它。 TThread本身具有运行时被销毁的足够多的问题，您无需对其进行添加。
关于multithreading - 为什么线程有时卡在waitfor中？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/9152796/