CanTwoThreadsLockFocusOnTheSameView

Can two threads lock focus on the same view? Or does one thread block while the other thread completes its drawing?

The answer is: lockFocus makes no attempt to synchronize drawing between threads. Both threads will issue their drawing commands at the same time. This can have disastrous effects when drawing into NSOpenGLViews. The limitations on doing this kind of thing on normal views is less defined – but it’s probably not a good idea.

Here’s a sample view that updates itself on two threads. Simply:

Create a Cocoa app project
Create a window w/ a custom view in it
Create an NSView subclass object called “CustomView” and change your custom view’s class from “NSView” to “CustomView”
Copy the code below into your project, compile, and run.

Best of luck!

– MikeTrent

#import <Cocoa/Cocoa.h>

@interface CustomView : NSView { BOOL _forkedThread; NSTimer *_fgTimer; NSTimer *_bgTimer; BOOL _drawGreen;

int _privateData;
NSLock *_drawLock; } @end

@implementation CustomView

(void)drawRect:(NSRect)rect { BOOL resetFlag = YES; NSColor *color;

// do some drawing work, lock if necessary (see below) [_drawLock lock];
```
  if (_privateData == 1) {
      NSLog(@"looks like someone else locked focus?");
      resetFlag = NO;
  } else {
      _privateData = 1;
  }

  color = _drawGreen ? [NSColor greenColor] : [NSColor blueColor];
  [color set];
  NSRectFill(rect); 
  _drawGreen = !_drawGreen;   
        
  if (resetFlag) {
      _privateData = 0;
  }
```
[_drawLock unlock];

// manually setup a foreground refresh if (!_fgTimer) { _fgTimer = [[NSTimer scheduledTimerWithTimeInterval:(1.0/30.0) target:self selector:@selector(fgDraw:) userInfo:nil repeats:YES] retain]; }

// manually setup a background refresh if (!_forkedThread) { _forkedThread = YES; [NSApplication detachDrawingThread:@selector(startBGThread:) toTarget:self withObject:nil];
```
  // uncomment below to run w/ manual locking.
  // comment below to see if lockFocus implies locking when drawing.
  //_drawLock = [[NSLock alloc] init];   } }
```

// prep work for running a selector on another thread

(void)startBGThread:(id)sender { NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; NSRunLoop *runLoop = [NSRunLoop currentRunLoop];

_bgTimer = [[NSTimer scheduledTimerWithTimeInterval:(1.0/30.0) target:self selector:@selector(bgDraw:) userInfo:nil repeats:YES] retain];

[pool release];

while (_bgTimer) { NSAutoreleasePool * pool = [[NSAutoreleasePool alloc] init]; [runLoop runUntilDate:[NSDate dateWithTimeIntervalSinceNow:(1.0/30.0)]]; [pool release]; } }

// manually draw on foreground (normally one would use setNeedsDisplay: for this)

(void)fgDraw:(NSTimer*)timer { [self lockFocus]; [self drawRect:[self bounds]]; [self unlockFocus]; self window] flushWindow]; }

// manually draw on background

(void)bgDraw:([[NSTimer*)timer { [self lockFocus]; [self drawRect:[self bounds]]; [self unlockFocus]; [[self window] flushWindow]; }

@end

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