补充说明：
当Task里有需要长时间执行的任务时，例如：
while ttask.CurrentTask.Status <> TTaskStatus.Canceled do
begin
sleep(10000); // 模拟尽量大点的时间
TThread.Synchronize(nil,
procedure()
begin
Form1.Memo1.Lines.Add('Task 1 ...');
end);
end;
以下代码将无法正常等待结束
for I := 0 to 2 do begin
if tasks[i].Status<>TTaskStatus.Completed then begin
application.ProcessMessages;
sleep(10);
continue;
end;
end;
Memo1.Lines.Add('All done.');
end;

除了以上，另有：
其一，使用cancel取消的任务，最终的状态将是TTaskStatus.Canceled，不是TTaskStatus.Completed
其二，使用cancel取消的任务，调用Wait系列函数时会产生一个异常，这是TTask本身的一个正常设计，无须惧怕该异常，有需要时尽管大胆地去用Wait，ITask接口里的CheckCanceled本身就是配合Cancel而设计

好玩的事情来了,改了一下代码,任务里面在while后面增加了一些延时..,改为
unit Unit1;
interface
uses
Winapi.Windows, Winapi.Messages, System.SysUtils, System.Variants,
System.Classes, Vcl.Graphics,
Vcl.Controls, Vcl.Forms, Vcl.Dialogs, Vcl.StdCtrls, System.SyncObjs,
System.Threading;
type
TForm1 = class(TForm)
Memo1: TMemo;
Button1: TButton;
Button2: TButton;
procedure Button1Click(Sender: TObject);
procedure FormCreate(Sender: TObject);
procedure Button2Click(Sender: TObject);
private
tasks: array [0 .. 2] of Itask;
{ Private declarations }
public
{ Public declarations }
end;
var
Form1: TForm1;
implementation
{$R *.dfm}
procedure TForm1.Button1Click(Sender: TObject);
begin
tasks[0].Start;
tasks[1].Start;
tasks[2].Start;
end;
procedure TForm1.Button2Click(Sender: TObject);
var
i: integer;
w_alldone:boolean;
begin
tasks[0].Cancel;
tasks[1].Cancel;
tasks[2].Cancel;
try
TTask.WaitForAll(tasks) ; //这个不行，要exception
except
end;
{for i := 0 to 2 do
begin
if tasks[i].Status <> TTaskStatus.Completed then
begin // 主线程有访问TTask的Status属性，线程内部也会访问它，但是不会引发冲突，因为查看源代码发现它有[Volatile]属性
application.ProcessMessages;
sleep(10);
continue;
end;
end;}
{w_alldone:=false;
while not w_alldone do begin
w_alldone:=true;
for i := 0 to 2 do begin
w_alldone:=w_alldone and (tasks[i].Status = TTaskStatus.Canceled);
if not w_alldone then break;
end;
application.ProcessMessages;
sleep(10);
end; }
Memo1.Lines.Add('All done.');
end;
procedure TForm1.FormCreate(Sender: TObject);
begin
tasks[0] := ttask.Create(
procedure()
begin
while ttask.CurrentTask.Status<>TTaskStatus.Canceled do
begin
sleep(10);
TThread.Synchronize(nil,
procedure()
begin
Form1.Memo1.Lines.Add('Task 1 ...');
end);
end;
TThread.Synchronize(nil,
procedure()
begin
Form1.Memo1.Lines.Add('Task 1 , exit while ...');
end);
sleep(1000);
TThread.Synchronize(nil,
procedure()
begin
Form1.Memo1.Lines.Add('Task 1 end');
end);
end);
tasks[1] := ttask.Create(
procedure()
begin
while ttask.CurrentTask.Status<> TTaskStatus.Canceled do
begin
sleep(15);
TThread.Synchronize(nil,
procedure()
begin
Form1.Memo1.Lines.Add('Task 2 ...');
end);
end;
TThread.Synchronize(nil,
procedure()
begin
Form1.Memo1.Lines.Add('Task 2 , exit while ...');
end);
sleep(1000);
TThread.Synchronize(nil,
procedure()
begin
Form1.Memo1.Lines.Add('Task 2 end');
end);
end);
tasks[2] := ttask.Create(
procedure()
begin
while ttask.CurrentTask.Status<> TTaskStatus.Canceled do
begin
sleep(12);
TThread.Synchronize(nil,
procedure()
begin
Form1.Memo1.Lines.Add('Task 3 ...');
end);
end;
TThread.Synchronize(nil,
procedure()
begin
Form1.Memo1.Lines.Add('Task 3 , exit while ...');
end);
sleep(1000);
TThread.Synchronize(nil,
procedure()
begin
Form1.Memo1.Lines.Add('Task 3 end');
end);
end);
end;
end.

用原配的
try
TTask.WaitForAll(tasks) ; //这个不行，要exception
except
end;
结果显示:
Task 2 ...
Task 1 ...
Task 3 ...
Task 1 ...
All done.
Task 2 ...
Task 1 , exit while ...
Task 2 , exit while ...
Task 3 ...
Task 3 , exit while ...
Task 1 end
Task 2 end
Task 3 end
All Done最先出来,,

用第二种方法:
for i := 0 to 2 do
begin
if tasks[i].Status <> TTaskStatus.Completed then
begin // 主线程有访问TTask的Status属性，线程内部也会访问它，但是不会引发冲突，因为查看源代码发现它有[Volatile]属性
application.ProcessMessages;
sleep(10);
continue;
end;
end;
正如吧主所言,也是刹不住车的,看结果 ...
Task 2 ...
Task 1 ...
Task 3 ...
Task 2 ...
Task 1 ...
Task 1 , exit while ...
Task 3 ...
Task 2 ...
Task 3 , exit while ...
Task 2 , exit while ...
All done.
Task 1 end
Task 3 end
Task 2 end

这个方法是吧主建议的,结果...
w_alldone:=false;
while not w_alldone do begin
w_alldone:=true;
for i := 0 to 2 do begin
w_alldone:=w_alldone and (tasks[i].Status = TTaskStatus.Canceled);
if not w_alldone then break;
end;
application.ProcessMessages;
sleep(10);
end;
Task 1 ...
Task 1 ...
Task 3 ...
Task 2 ...
All done.
Task 1 ...
Task 3 ...
Task 2 ...
Task 1 , exit while ...
Task 3 , exit while ...
Task 2 , exit while ...
Task 1 end
Task 3 end
Task 2 end

FState是set of TOptionStateFlag类型,在最后FState已经有了[Completed,Cancled]状态,但是看源代码,,,
Canceled的优先权比较大,这也解释了为什么上面的代码走不出循环了.
so 也许我们用.cancel来作为通知任务结束的方法本身就不对....
function TTask.GetStatus: TTaskStatus;
var
LFlags: TOptionStateFlags;
begin
LFlags := FState;
if TOptionStateFlag.Faulted in LFlags then
Result := TTaskStatus.Exception
else if TOptionStateFlag.Canceled in LFlags then
Result := TTaskStatus.Canceled
else if TOptionStateFlag.Complete in LFlags then
Result := TTaskStatus.Completed
else if TOptionStateFlag.ChildWait in LFlags then
Result := TTaskStatus.WaitingForChildren
else if TOptionStateFlag.CallbackRun in LFlags then
Result := TTaskStatus.Running
else if TOptionStateFlag.Started in LFlags then
Result := TTaskStatus.WaitingToRun
else
Result := TTaskStatus.Created;
end;

而且Task这东西，在delphi10的bug fix list里就有修复其中几个bug，不知还有没其他：
1.Creating TTask instances inside running TTask leads to deadlock upon application close
2.ITask.Wait Freeze/Lockup
3.TTask.WaitForAny gives exception EMonitorLockException 'Object lock not owned'