在iOS7中使用CGContext绘制的问题

我为iPad开发了一个绘图工具 .

在iOS6上使用该工具时，工作正常，但是当在iOS7上使用该工具时，在触摸结束后不会出现绘制线 .

我正在使用 CGContextAddLineToPoint 和 UIGraphicsGetCurrentContext 进行绘图 .

谢谢

这是代码

-(void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event{
    pointerUndone++;
//    NSLog(@"undone store %f",pointerUndone);

    UITouch *touch = [[event allTouches] anyObject];

    if (DEBUG_MESSAGES == YES) NSLog(@"Draw");

    if([touch tapCount] == 2){
        //drawImage.image = nil;//double touch for clear all current stroke
    }else if([touch tapCount] == 3){
        [self finishParentView];
    }

    self.currentStroke = [[NSMutableArray alloc] init];

    location = [touch locationInView:touch.view];
    lastClick = [NSDate date];

    lastPoint = [touch locationInView:self.view];
    lastPoint.y -= 0;

    [self.currentStroke addObject:[NSValue valueWithCGPoint:lastPoint]];

    [super touchesBegan:touches withEvent:event];
}

-(void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event{

    UITouch *touch = [touches anyObject];
    currentPoint = [touch locationInView:self.view];



    UIGraphicsBeginImageContext(CGSizeMake([UIScreen mainScreen].bounds.size.height, [UIScreen mainScreen].bounds.size.width));
    [drawImage.image drawInRect:CGRectMake(0, 0, [UIScreen mainScreen].bounds.size.height, [UIScreen mainScreen].bounds.size.width)];
    CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapButt);

    if(self.draftState == NO){
        CGContextSetLineWidth(UIGraphicsGetCurrentContext(), self.sizeStroke);
        CGContextSetRGBStrokeColor(UIGraphicsGetCurrentContext(), R/255, G/255, B/255, alpha);    //draw
    }else{

        if([[UIDevice currentDevice] userInterfaceIdiom] == UIUserInterfaceIdiomPhone)
            CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 10);
        else
            CGContextSetLineWidth(UIGraphicsGetCurrentContext(), 25);
        CGContextSetBlendMode(UIGraphicsGetCurrentContext(), kCGBlendModeClear);    //delete
    }

    CGContextBeginPath(UIGraphicsGetCurrentContext());
    CGContextMoveToPoint(UIGraphicsGetCurrentContext(), lastPoint.x, lastPoint.y);
    CGContextAddLineToPoint(UIGraphicsGetCurrentContext(), currentPoint.x, currentPoint.y);
    CGContextSetLineCap(UIGraphicsGetCurrentContext(), kCGLineCapRound);

    CGContextStrokePath(UIGraphicsGetCurrentContext());
    [drawImage setFrame:CGRectMake(0, 0, [UIScreen mainScreen].bounds.size.height, [UIScreen mainScreen].bounds.size.width)];

    //drawImage.image = UIGraphicsGetImageFromCurrentImageContext();

    drawImage.image = UIGraphicsGetImageFromContext(UIGraphicsGetCurrentContext());

    UIGraphicsEndImageContext();
    lastPoint = currentPoint;

    [self.view addSubview:drawImage];

    [self.currentStroke addObject:[NSValue valueWithCGPoint:currentPoint]];

}

-(void)touchesEnded:(NSSet *)touches withEvent:(UIEvent *)event{

    if([self.currentStroke  count]>1){
        DatabaseManager *db = [[DatabaseManager alloc] init];
        [db deleteRedone];
        NSMutableDictionary *strokeWithColor = [[NSMutableDictionary alloc] init];
        NSDictionary *colorDictionary = [[NSDictionary alloc] initWithObjects:[NSArray arrayWithObjects:[NSNumber numberWithDouble:R],[NSNumber numberWithDouble:G],[NSNumber numberWithDouble:B], nil] forKeys:[NSArray arrayWithObjects:@"R",@"G",@"B", nil]];
        [strokeWithColor setObject:[NSString stringWithFormat:@"%.2f",self.sizeStroke] forKey:@"size"];
        if(self.draftState == NO){
            [strokeWithColor setObject:@"pencil" forKey:@"type"];
            [strokeWithColor setObject:[self getSelectedColor] forKey:@"colorString"];
        }else{
            [strokeWithColor setObject:@"draft" forKey:@"type"];
            [strokeWithColor setObject:@"TRANSPARENT" forKey:@"colorString"];
        }

        [strokeWithColor setObject:colorDictionary forKey:@"color"];
        //[strokeWithColor setObject:[self getSelectedColor] forKey:@"colorString"];
        [strokeWithColor setObject:self.currentStroke forKey:@"stroke"];
        [strokeWithColor setObject:@"YES" forKey:@"visible"];

        [self.strokes addObject:strokeWithColor];
        [self registerStrokeInDataBase:self.currentStroke];
    }
}