Iphone 如何在不丢失上下文的情况下绘制上下文？

我正在网格中绘制

UIImage

s。我目前只画的变化。。。只有改变。。。但是老照片应该留在原来的地方。。。但是现在，在下一次调用

drawRect:（CGRect）rect:

- (void)drawRect:(CGRect)rect
{
    int cellSize = self.bounds.size.width / WIDTH;
    double xOffset = 0;

    CGRect cellFrame = CGRectMake(0, 0, cellSize, cellSize);
    NSUInteger cellIndex = 0;
    cellFrame.origin.x = xOffset;
    for (int i = 0; i < WIDTH; i++)
    {        
        cellFrame.origin.y = 0;
        for (int j = 0; j < HEIGHT; j++, cellIndex++)
        {
            if([[self.state.boardChanges objectAtIndex:(i*HEIGHT)+j] intValue]==1){
                if (CGRectIntersectsRect(rect, cellFrame))                {
                    NSNumber *currentCell = [self.state.board objectAtIndex:cellIndex];

                    if (currentCell.intValue == 1)
                    {
                        [image1 drawInRect:cellFrame];
                    }
                    else if (currentCell.intValue == 0)
                    {
                        [image2 drawInRect:cellFrame];
                    }
                }
            }
            cellFrame.origin.y += cellSize;
        }
        cellFrame.origin.x += cellSize;
    }   
}

---

Cocoa/UIKit可以随时丢弃以前绘制的视图内容。当要求视图绘制时，它必须绘制所提供的

rect

中的所有内容。你不能决定不画一些东西。（好吧，你可以做你想做的任何事情，但是你会得到你所看到的结果。）

找到另一个解决方案->只需截图并以drawRect为背景绘制即可

-(void)createContent{
    CGRect rect = CGRectMake(0, 0, self.bounds.size.width, self.bounds.size.height);
    UIGraphicsBeginImageContext(rect.size);
    [self.layer renderInContext:UIGraphicsGetCurrentContext()];
    CGImageRef screenshotRef = CGBitmapContextCreateImage(UIGraphicsGetCurrentContext());
    _backgroundImage = [[UIImage alloc] initWithCGImage:screenshotRef];
    CGImageRelease(screenshotRef);
    UIGraphicsEndImageContext();
}

- (void)drawRect:(CGRect)rect 
{   
    [_backgroundImage drawInRect:CGRectMake(0.0f, 0.0f, self.bounds.size.width, self.bounds.size.height)];
    ....
    ....
    ....
}

######编辑 找到了一种更复杂的方法来完成上述工作：

####编辑 另一个解决方案是使用CALayers，只更新那些更改的部分

####编辑 或：

self.layer.contents=（id）[[u previousContent CGImage]

-(void)createContent{
    CGRect rect = CGRectMake(0, 0, self.bounds.size.width, self.bounds.size.height);
    UIGraphicsBeginImageContext(rect.size);
    [self.layer renderInContext:UIGraphicsGetCurrentContext()];
    CGImageRef screenshotRef = CGBitmapContextCreateImage(UIGraphicsGetCurrentContext());
    _backgroundImage = [[UIImage alloc] initWithCGImage:screenshotRef];
    CGImageRelease(screenshotRef);
    UIGraphicsEndImageContext();
}

- (void)drawRect:(CGRect)rect 
{   
    [_backgroundImage drawInRect:CGRectMake(0.0f, 0.0f, self.bounds.size.width, self.bounds.size.height)];
    ....
    ....
    ....
}