问题
I've been beating my head for over a day now, going through tons of resources trying to figure out how to receive the WM_POWERBROADCAST Windows message from within a thread.
Currently, I am using AllocateHWnd(WndMethod) inside of a stand-alone component. When I create an instance of said component in a standard VCL Forms Application, everything works fine, and I receive the WM_POWERBROADCAST message every time, as needed.
However, when I create an instance of the very same component from within a TThread, I'm no longer receiving this particular Windows message. I'm receiving all kinds of other messages, just not this particular one.
In my searching for a solution, I've found many resources related to how a Windows Service requires some extra work in order to receive this message. But I'm not using a service, at least not yet. I've also found a couple people mention that a thread needs to have a message loop in order to receive this message, and I've implemented a thread from another answer here, but again, I never receive this particular message.
Below is the complete component how I'm receiving this message, which again works perfectly if this is in a VCL application's main thread. I'm guessing the main thread needs to receive this message and forward it into the thread.
How do I make this receive the WM_POWERBROADCAST message when inside of a TThread?
unit JD.Power.Monitor;
(*
JD Power Monitor
by Jerry Dodge
Purpose: To monitor the current state of power on the computer, and trigger
events when different power related changes occur.
Component: TPowerMonitor
- Create an instance of TPowerMonitor component
- Choose desired power settings to get notified of using Settings property
- Implement event handlers for those events you wish to monitor
- Component automatically takes care of the rest of the work
*)
interface
uses
System.Classes, System.SysUtils, System.Generics.Collections,
Winapi.ActiveX, Winapi.Windows, Winapi.Messages;
type
TPowerSetting = (psACDCPowerSource, psBatteryPercentage,
psConsoleDisplayState, psGlobalUserPresence, psIdleBackgroundTask,
psMonitorPower, psPowerSaving, psPowerSchemePersonality,
psSessionDisplayStatus, psSessionUserPresence, psSystemAwayMode);
TPowerSettings = set of TPowerSetting;
TPowerSource = (poAC, poDC, poHot);
TPowerDisplayState = (pdOff, pdOn, pdDimmed);
TPowerUserPresence = (puPresent = 0, puInactive = 2);
TPowerSavingStatus = (psSaverOff, psSaverOn);
TPowerAwayMode = (paExiting, paEntering);
TPowerPersonality = (ppHighPerformance, ppPowerSaver, ppAutomatic);
TPowerMonitorSettingHandles = array[TPowerSetting] of HPOWERNOTIFY;
TPowerQueryEndSessionEvent = procedure(Sender: TObject; var EndSession: Boolean) of object;
TPowerEndSessionEvent = procedure(Sender: TObject) of object;
TPowerSettingSourceChangeEvent = procedure(Sender: TObject;
const Src: TPowerSource) of object;
TPowerSettingBatteryPercentEvent = procedure(Sender: TObject;
const Perc: Single) of object;
TPowerSettingDisplayStateEvent = procedure(Sender: TObject;
const State: TPowerDisplayState) of object;
TPowerSettingUserPresenceEvent = procedure(Sender: TObject;
const Presence: TPowerUserPresence) of object;
TPowerSettingSavingEvent = procedure(Sender: TObject;
const Status: TPowerSavingStatus) of object;
TPowerAwayModeEvent = procedure(Sender: TObject;
const Mode: TPowerAwayMode) of object;
TPowerPersonalityEvent = procedure(Sender: TObject;
const Personality: TPowerPersonality) of object;
TPowerMonitor = class(TComponent)
private
FHandle: HWND;
FSettingHandles: TPowerMonitorSettingHandles;
FSettings: TPowerSettings;
FBatteryPresent: Boolean;
FOnQueryEndSession: TPowerQueryEndSessionEvent;
FOnEndSession: TPowerEndSessionEvent;
FOnPowerStatusChange: TNotifyEvent;
FOnResumeAutomatic: TNotifyEvent;
FOnResumeSuspend: TNotifyEvent;
FOnSuspend: TNotifyEvent;
FOnSourceChange: TPowerSettingSourceChangeEvent;
FOnBatteryPercent: TPowerSettingBatteryPercentEvent;
FOnConsoleDisplayState: TPowerSettingDisplayStateEvent;
FOnGlobalUserPresence: TPowerSettingUserPresenceEvent;
FOnIdleBackgroundTask: TNotifyEvent;
FOnMonitorPower: TPowerSettingDisplayStateEvent;
FOnPowerSavingStatus: TPowerSettingSavingEvent;
FOnSessionDisplayState: TPowerSettingDisplayStateEvent;
FOnSessionUserPresence: TPowerSettingUserPresenceEvent;
FOnAwayMode: TPowerAwayModeEvent;
FOnPersonality: TPowerPersonalityEvent;
procedure UnregisterSettings;
procedure RegisterSettings;
procedure SetSettings(const Value: TPowerSettings);
protected
procedure HandlePowerSetting(const Val: PPowerBroadcastSetting);
procedure WndMethod(var Msg: TMessage);
public
constructor Create(AOwner: TComponent); override;
destructor Destroy; override;
published
property Settings: TPowerSettings read FSettings write SetSettings;
property OnQueryEndSession: TPowerQueryEndSessionEvent
read FOnQueryEndSession write FOnQueryEndSession;
property OnEndSession: TPowerEndSessionEvent
read FOnEndSession write FOnEndSession;
property OnPowerStatusChange: TNotifyEvent
read FOnPowerStatusChange write FOnPowerStatusChange;
property OnResumeAutomatic: TNotifyEvent
read FOnResumeAutomatic write FOnResumeAutomatic;
property OnResumeSuspend: TNotifyEvent
read FOnResumeSuspend write FOnResumeSuspend;
property OnSuspend: TNotifyEvent
read FOnSuspend write FOnSuspend;
property OnSourceChange: TPowerSettingSourceChangeEvent
read FOnSourceChange write FOnSourceChange;
property OnBatteryPercent: TPowerSettingBatteryPercentEvent
read FOnBatteryPercent write FOnBatteryPercent;
property OnConsoleDisplayState: TPowerSettingDisplayStateEvent
read FOnConsoleDisplayState write FOnConsoleDisplayState;
property OnGlobalUserPresence: TPowerSettingUserPresenceEvent
read FOnGlobalUserPresence write FOnGlobalUserPresence;
property OnIdleBackgroundTask: TNotifyEvent
read FOnIdleBackgroundTask write FOnIdleBackgroundTask;
property OnMonitorPower: TPowerSettingDisplayStateEvent
read FOnMonitorPower write FOnMonitorPower;
property OnPowerSavingStatus: TPowerSettingSavingEvent
read FOnPowerSavingStatus write FOnPowerSavingStatus;
property OnSessionDisplayState: TPowerSettingDisplayStateEvent
read FOnSessionDisplayState write FOnSessionDisplayState;
property OnSessionUserPresence: TPowerSettingUserPresenceEvent
read FOnSessionUserPresence write FOnSessionUserPresence;
property OnAwayMode: TPowerAwayModeEvent
read FOnAwayMode write FOnAwayMode;
property OnPersonality: TPowerPersonalityEvent
read FOnPersonality write FOnPersonality;
end;
implementation
{ TPowerMonitor }
constructor TPowerMonitor.Create(AOwner: TComponent);
begin
inherited;
FBatteryPresent:= False;
FHandle := AllocateHWnd(WndMethod);
end;
destructor TPowerMonitor.Destroy;
begin
UnregisterSettings;
DeallocateHWnd(FHandle);
inherited;
end;
procedure TPowerMonitor.SetSettings(const Value: TPowerSettings);
begin
UnregisterSettings;
FSettings := Value;
RegisterSettings;
end;
procedure TPowerMonitor.WndMethod(var Msg: TMessage);
var
Handled: Boolean;
begin
Handled := True;
case Msg.Msg of
WM_POWERBROADCAST: begin
//TODO: Why is this never received when inside of a thread?
case Msg.WParam of
PBT_APMPOWERSTATUSCHANGE: begin
//Power status has changed.
if Assigned(FOnPowerStatusChange) then
FOnPowerStatusChange(Self);
end;
PBT_APMRESUMEAUTOMATIC: begin
//Operation is resuming automatically from a low-power state.
//This message is sent every time the system resumes.
if Assigned(FOnResumeAutomatic) then
FOnResumeAutomatic(Self);
end;
PBT_APMRESUMESUSPEND: begin
//Operation is resuming from a low-power state. This message
//is sent after PBT_APMRESUMEAUTOMATIC if the resume is triggered
//by user input, such as pressing a key.
if Assigned(FOnResumeSuspend) then
FOnResumeSuspend(Self);
end;
PBT_APMSUSPEND: begin
//System is suspending operation.
if Assigned(FOnSuspend) then
FOnSuspend(Self);
end;
PBT_POWERSETTINGCHANGE: begin
//A power setting change event has been received.
HandlePowerSetting(PPowerBroadcastSetting(Msg.LParam));
end;
else begin
end;
end;
end
else Handled := False;
end;
if Handled then
Msg.Result := 0
else
Msg.Result := DefWindowProc(FHandle, Msg.Msg,
Msg.WParam, Msg.LParam);
end;
procedure TPowerMonitor.HandlePowerSetting(const Val: PPowerBroadcastSetting);
var
Pers: TPowerPersonality;
function ValAsDWORD: DWORD;
begin
Result:= DWORD(Val.Data[0]);
end;
function ValAsGUID: TGUID;
begin
Result:= StringToGUID('{00000000-0000-0000-0000-000000000000}'); //Default
if SizeOf(TGUID) = Val.DataLength then begin
Move(Val.Data, Result, Val.DataLength);
end;
end;
function IsVal(G: String): Boolean;
begin
Result:= Assigned(Val);
if Result then
Result:= IsEqualGUID(StringToGUID(G), Val.PowerSetting);
end;
function IsValGuid(G: String): Boolean;
begin
Result:= Assigned(Val);
if Result then
Result:= IsEqualGUID(StringToGUID(G), ValAsGUID);
end;
begin
if IsVal('{5d3e9a59-e9D5-4b00-a6bd-ff34ff516548}') then begin
//GUID_ACDC_POWER_SOURCE
if Assigned(FOnSourceChange) then
FOnSourceChange(Self, TPowerSource(ValAsDWORD));
end else
if IsVal('{a7ad8041-b45a-4cae-87a3-eecbb468a9e1}') then begin
//GUID_BATTERY_PERCENTAGE_REMAINING
//We assume that if we get this message, that there is a battery connected.
//Otherwise if this never occurs, then a battery is not present.
//TODO: How to handle if battery is detached and no longer present?
FBatteryPresent:= True;
if Assigned(FOnBatteryPercent) then
FOnBatteryPercent(Self, ValAsDWORD);
end else
if IsVal('{6fe69556-704a-47a0-8f24-c28d936fda47}') then begin
//GUID_CONSOLE_DISPLAY_STATE
if Assigned(FOnConsoleDisplayState) then
FOnConsoleDisplayState(Self, TPowerDisplayState(ValAsDWORD));
end else
if IsVal('{786E8A1D-B427-4344-9207-09E70BDCBEA9}') then begin
//GUID_GLOBAL_USER_PRESENCE
if Assigned(FOnGlobalUserPresence) then
FOnGlobalUserPresence(Self, TPowerUserPresence(ValAsDWORD));
end else
if IsVal('{515c31d8-f734-163d-a0fd-11a08c91e8f1}') then begin
//GUID_IDLE_BACKGROUND_TASK
if Assigned(FOnIdleBackgroundTask) then
FOnIdleBackgroundTask(Self);
end else
if IsVal('{02731015-4510-4526-99e6-e5a17ebd1aea}') then begin
//GUID_MONITOR_POWER_ON
if Assigned(FOnMonitorPower) then
FOnMonitorPower(Self, TPowerDisplayState(ValAsDWORD));
end else
if IsVal('{E00958C0-C213-4ACE-AC77-FECCED2EEEA5}') then begin
//GUID_POWER_SAVING_STATUS
if Assigned(FOnPowerSavingStatus) then
FOnPowerSavingStatus(Self, TPowerSavingStatus(ValAsDWORD));
end else
if IsVal('{245d8541-3943-4422-b025-13A784F679B7}') then begin
//GUID_POWERSCHEME_PERSONALITY
if IsValGuid('{8c5e7fda-e8bf-4a96-9a85-a6e23a8c635c}') then begin
Pers:= TPowerPersonality.ppHighPerformance;
end else
if IsValGuid('{a1841308-3541-4fab-bc81-f71556f20b4a}') then begin
Pers:= TPowerPersonality.ppPowerSaver;
end else
if IsValGuid('{381b4222-f694-41f0-9685-ff5bb260df2e}') then begin
Pers:= TPowerPersonality.ppAutomatic;
end else begin
//TODO: Handle unrecognized GUID
Pers:= TPowerPersonality.ppAutomatic;
end;
if Assigned(FOnPersonality) then
FOnPersonality(Self, Pers);
end else
if IsVal('{2B84C20E-AD23-4ddf-93DB-05FFBD7EFCA5}') then begin
//GUID_SESSION_DISPLAY_STATUS
if Assigned(FOnSessionDisplayState) then
FOnSessionDisplayState(Self, TPowerDisplayState(ValAsDWORD));
end else
if IsVal('{3C0F4548-C03F-4c4d-B9F2-237EDE686376}') then begin
//GUID_SESSION_USER_PRESENCE
if Assigned(FOnSessionUserPresence) then
FOnSessionUserPresence(Self, TPowerUserPresence(ValAsDWORD));
end else
if IsVal('{98a7f580-01f7-48aa-9c0f-44352c29e5C0}') then begin
//GUID_SYSTEM_AWAYMODE
if Assigned(FOnAwayMode) then
FOnAwayMode(Self, TPowerAwayMode(ValAsDWORD));
end else begin
//TODO: Handle Unrecognized GUID
end;
end;
function PowerSettingGUID(const Setting: TPowerSetting): TGUID;
begin
case Setting of
psACDCPowerSource: Result:= StringToGUID('{5d3e9a59-e9D5-4b00-a6bd-ff34ff516548}');
psBatteryPercentage: Result:= StringToGUID('{a7ad8041-b45a-4cae-87a3-eecbb468a9e1}');
psConsoleDisplayState: Result:= StringToGUID('{6fe69556-704a-47a0-8f24-c28d936fda47}');
psGlobalUserPresence: Result:= StringToGUID('{786E8A1D-B427-4344-9207-09E70BDCBEA9}');
psIdleBackgroundTask: Result:= StringToGUID('{515c31d8-f734-163d-a0fd-11a08c91e8f1}');
psMonitorPower: Result:= StringToGUID('{02731015-4510-4526-99e6-e5a17ebd1aea}');
psPowerSaving: Result:= StringToGUID('{E00958C0-C213-4ACE-AC77-FECCED2EEEA5}');
psPowerSchemePersonality: Result:= StringToGUID('{245d8541-3943-4422-b025-13A784F679B7}');
psSessionDisplayStatus: Result:= StringToGUID('{2B84C20E-AD23-4ddf-93DB-05FFBD7EFCA5}');
psSessionUserPresence: Result:= StringToGUID('{3C0F4548-C03F-4c4d-B9F2-237EDE686376}');
psSystemAwayMode: Result:= StringToGUID('{98a7f580-01f7-48aa-9c0f-44352c29e5C0}');
end;
end;
procedure TPowerMonitor.RegisterSettings;
var
V: TPowerSetting;
begin
for V := Low(TPowerSetting) to High(TPowerSetting) do begin
if V in FSettings then begin
FSettingHandles[V]:= RegisterPowerSettingNotification(FHandle,
PowerSettingGUID(V), 0);
end;
end;
end;
procedure TPowerMonitor.UnregisterSettings;
var
V: TPowerSetting;
begin
for V := Low(TPowerSetting) to High(TPowerSetting) do begin
if V in FSettings then begin
UnregisterPowerSettingNotification(FSettingHandles[V]);
end;
end;
end;
end.
Further, based on the other answer as mentioned, here's how I'm attempting to capture this message using only a thread, although I'm sure this is not actually what I need since WM_POWERBROADCAST is not a posted message:
unit JD.ThreadTest;
interface
uses
System.Classes, Winapi.Messages, Winapi.Windows;
type
TDataThread = class(TThread)
private
FTitle: String;
FWnd: HWND;
FWndClass: WNDCLASS;
procedure HandlePower(AMsg: TMsg);
protected
procedure Execute; override;
procedure DoTerminate; override;
public
constructor Create(const Title: String); reintroduce;
end;
implementation
constructor TDataThread.Create(const Title: String);
begin
inherited Create(True);
FTitle := Title;
with FWndClass do begin
Style := 0;
lpfnWndProc := @DefWindowProc;
cbClsExtra := 0;
cbWndExtra := 0;
hInstance := HInstance;
hIcon := 0;
hCursor := LoadCursor(0, IDC_ARROW);
hbrBackground := COLOR_WINDOW;
lpszMenuName := nil;
lpszClassName := PChar(Self.ClassName);
end;
end;
procedure TDataThread.Execute;
var
Msg: TMsg;
begin
if Winapi.Windows.RegisterClass(FWndClass) = 0 then Exit;
FWnd := CreateWindow(FWndClass.lpszClassName, PChar(FTitle), WS_DLGFRAME,
0, 0, 698, 517, 0, 0, HInstance, nil);
if FWnd = 0 then Exit;
while GetMessage(Msg, FWnd, 0, 0) = True do begin
if Terminated then Break;
case Msg.message of
WM_POWERBROADCAST: begin
HandlePower(Msg); //Never receives this message
end;
else begin
TranslateMessage(msg);
DispatchMessage(msg)
end;
end;
end;
end;
procedure TDataThread.HandlePower(AMsg: TMsg);
begin
end;
procedure TDataThread.DoTerminate;
begin
if FWnd <> 0 then DestroyWindow(FWnd);
Winapi.Windows.UnregisterClass(PChar(Self.ClassName), FWndClass.hInstance);
inherited;
end;
end.
PS: The end goal is to make this component re-usable, and to be using it inside of a thread, which will be spawned inside of a service.
EDIT
Just to show some perspective, here's a screenshot of my results, when I put my computer into sleep mode. The form on the left is my 100% working UI application, without any worker thread. It receives many messages through the WM_POWERBROADCAST message. The one on the right is where I attempt to capture this message inside of a thread - updated with the code below in Remy's answer.
Obviously the "Power Setting" specific messages are not received, because I haven't called RegisterPowerSettingNotification - but there are other cases when I should still receive this message regardless, such as PBT_APMSUSPEND.
unit JD.ThreadTest;
interface
uses
System.Classes, System.SysUtils, Winapi.Messages, Winapi.Windows;
type
TMessageEvent = procedure(Sender: TObject; Message: TMessage) of object;
TDataThread = class(TThread)
private
FTitle: String;
FWnd: HWND;
FWndClass: WNDCLASS;
FOnMessage: TMessageEvent;
FMsg: TMessage;
procedure HandleMessage(var Message: TMessage);
protected
procedure Execute; override;
procedure DoTerminate; override;
procedure DoOnMessage;
public
constructor Create(const Title: String); reintroduce;
property OnMessage: TMessageEvent read FOnMessage write FOnMessage;
end;
implementation
function DataThreadWndProc(Wnd: HWND; Msg: UINT; wParam: WPARAM; lParam: LPARAM): LRESULT; stdcall;
var
Thread: TDataThread;
Message: TMessage;
begin
if Msg = WM_NCCREATE then
begin
Thread := TDataThread(PCREATESTRUCT(lParam)^.lpCreateParams);
SetWindowLongPtr(Wnd, GWLP_USERDATA, LONG_PTR(Thread));
end else
Thread := TDataThread(GetWindowLongPtr(Wnd, GWLP_USERDATA));
if Thread <> nil then
begin
Message.Msg := Msg;
Message.WParam := wParam;
Message.LParam := lParam;
Message.Result := 0;
Thread.HandleMessage(Message);
Result := Message.Result;
end else
Result := DefWindowProc(Wnd, Msg, wParam, lParam);
end;
constructor TDataThread.Create(const Title: String);
begin
inherited Create(True);
FTitle := Title;
with FWndClass do
begin
Style := 0;
lpfnWndProc := @DataThreadWndProc;
cbClsExtra := 0;
cbWndExtra := 0;
hInstance := HInstance;
hIcon := 0;
hCursor := LoadCursor(0, IDC_ARROW);
hbrBackground := COLOR_WINDOW;
lpszMenuName := nil;
lpszClassName := 'TDataThread';
end;
end;
procedure TDataThread.Execute;
var
Msg: TMsg;
begin
if Winapi.Windows.RegisterClass(FWndClass) = 0 then Exit;
FWnd := CreateWindow(FWndClass.lpszClassName, PChar(FTitle), WS_DLGFRAME, 0, 0, 698, 517, 0, 0, HInstance, Self);
if FWnd = 0 then Exit;
while GetMessage(Msg, 0, 0, 0) do
begin
if Terminated then Exit;
TranslateMessage(msg);
DispatchMessage(msg);
end;
end;
procedure TDataThread.DoOnMessage;
begin
if Assigned(FOnMessage) then
FOnMessage(Self, FMsg);
end;
procedure TDataThread.DoTerminate;
begin
if FWnd <> 0 then DestroyWindow(FWnd);
Winapi.Windows.UnregisterClass(FWndClass.lpszClassName, HInstance);
inherited;
end;
procedure TDataThread.HandleMessage(var Message: TMessage);
begin
FMsg:= Message;
Synchronize(DoOnMessage);
case Message.Msg of
WM_POWERBROADCAST:
begin
end;
else
Message.Result := DefWindowProc(FWnd, Message.Msg, Message.WParam, Message.LParam);
end;
end;
end.
回答1:
WM_POWERBROADCAST is not a posted message, so your message loop will never see it. You need a window procedure to receive that message. Your thread code is using DefWindowProc() directly as the window procedure. Change the call to RegisterClass() to register a custom procedure instead, that then calls DefWindowProc() for unhandled messages. GetMessage() will dispatch any sent message directly to the window procedure, and DispatchMessage() will dispatch any posted messages to the same window procedure.
unit JD.ThreadTest;
interface
uses
System.Classes, Winapi.Messages, Winapi.Windows;
type
TDataThread = class(TThread)
private
FTitle: String;
FWnd: HWND;
FWndClass: WNDCLASS;
procedure HandleMessage(var Message: TMessage);
protected
procedure Execute; override;
procedure DoTerminate; override;
public
constructor Create(const Title: String); reintroduce;
end;
implementation
function DataThreadWndProc(Wnd: HWND; Msg: UINT; wParam: WPARAM; lParam: LPARAM): LRESULT; stdcall;
var
Thread: TDataThread;
Message: TMessage;
begin
if Msg = WM_NCCREATE then
begin
Thread := TDataThread(PCREATESTRUCT(lParam)^.lpCreateParams);
SetWindowLongPtr(Wnd, GWLP_USERDATA, LONG_PTR(Thread));
end else
Thread := TDataThread(GetWindowLongPtr(Wnd, GWLP_USERDATA));
if Thread <> nil then
begin
Message.Msg := Msg;
Message.WParam := wParam;
Message.LParam := lParam;
Message.Result := 0;
Thread.HandleMessage(Message);
Result := Message.Result;
end else
Result := DefWindowProc(Wnd, Msg, wParam, lParam);
end;
constructor TDataThread.Create(const Title: String);
begin
inherited Create(True);
FTitle := Title;
with FWndClass do
begin
Style := 0;
lpfnWndProc := @DataThreadWndProc;
cbClsExtra := 0;
cbWndExtra := 0;
hInstance := HInstance;
hIcon := 0;
hCursor := LoadCursor(0, IDC_ARROW);
hbrBackground := COLOR_WINDOW;
lpszMenuName := nil;
lpszClassName := 'TDataThread';
end;
end;
procedure TDataThread.Execute;
var
Msg: TMsg;
begin
if Winapi.Windows.RegisterClass(FWndClass) = 0 then Exit;
FWnd := CreateWindow(FWndClass.lpszClassName, PChar(FTitle), WS_OVERLAPPED, 0, 0, 0, 0, 0, 0, HInstance, Self);
if FWnd = 0 then Exit;
while GetMessage(Msg, 0, 0, 0) do
begin
if Terminated then Exit;
TranslateMessage(msg);
DispatchMessage(msg);
end;
end;
procedure TDataThread.DoTerminate;
begin
if FWnd <> 0 then DestroyWindow(FWnd);
Winapi.Windows.UnregisterClass(FWndClass.lpszClassName, HInstance);
inherited;
end;
procedure TDataThread.HandleMessage(var Message: TMessage);
begin
case Message.Msg of
WM_POWERBROADCAST:
begin
// ...
end;
else
Message.Result := DefWindowProc(FWnd, Message.Msg, Message.WParam, Message.LParam);
end;
end;
end.
来源：https://stackoverflow.com/questions/42739404/how-to-receive-wm-powerbroadcast-inside-of-a-thread