SCREENLOCK


Locks the working page's frame buffer

Syntax:
declare sub Screenlock ( )

Usage:
Screenlock

Description:
All of FreeBASIC's Graphics Library functions draw to a frame buffer and an automatic routine copies the frame buffer to the actual screen memory at each draw. If the user program does a lot of drawing, the automatic refreshes may take a significant amount of time.

The Screenlock function locks the automatic refresh, so several drawing operations may be done before the screen refresh is performed, thus increasing the speed of execution, and preventing the user from seeing partial results.

Frame buffer memory may be freely accessed by using pointers (see ScreenPtr) ONLY while the screen is locked. Primitive graphics statements (Line, PSet, Draw String, ...) may be used at any time.

The screen refresh remains locked until the use of Screenunlock statement, which resumes it.

Calls to Screenlock must be paired with a matching call to Screenunlock. The graphics driver keeps track of how many times Screenlock has been called using a counter. Only the first call to Screenlock actually performs a locking operation. Subsequent calls to Screenlock only increment the counter. Conversely, Screenunlock only decrements the lock counter until it reaches zero at which time the actual unlock operation will be performed. Using Screen or Screenres will release all locks and set the lock counter back to zero before changing screen modes.

It is strongly recommended that the lock on a page be held for as short a time as possible. Only screen drawing should occur while the screen is locked, input/output and waiting must be avoided. In Win32 and Linux the screen is locked by stopping the thread that processes also the OS' events. If the screen is kept locked for a long time the event queue could overflow and make the system unstable. When the induced lock time becomes too long, use preferably the method of double buffering (with Screencopy).

The automatic refresh takes place only in the visible page of the frame buffer. Screenlock has no effect when drawing to pages other than the visible one.

Examples:

'' Draws a circle on-screen at the mouse cursor
Dim As Integer mx, my
Dim As String key

ScreenRes 640, 480, 32

Do

  'process
  GetMouse(mx, my)
  key = Inkey()

  'draw
  ScreenLock()
  Cls()
  Circle (mx, my), 8, RGB(255, 255, 255)
  ScreenUnlock()

  'free up CPU time
  Sleep(18, 1)
 
Loop Until key = Chr(27) Or key = Chr(255, 107)


Platform Differences:
Dialect Differences:
Differences from QB:
See also:
Back to Screen Functions
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