IOS图像处理（6）在内存上下文中绘图

之前提过绘制图像首先需要取得图形上下文对象（CGContextRef），系统中维护一个CGContextRef的栈，在UI控件的drawRect方法调用前，系统会为当前绘图环境创建一个图形上下文对象并且置于CGContextRef栈顶，通过UIGraphicsGetCurrentContext()可以取得这个图像上下文对象。我们也可以创建自己的图像上下文对象，quartz 2d提供了相应的api给开发者创建各种上下文对象，当我们使用UIKit在IOS内存中绘图时，创建上下文的工作更加简单，只需要调用UIGraphicsBeginImageContextWithOptions即可创建一个基于内存绘图的上下文

- (void)drawRect:(CGRect)rect
{
    //创建一个基于位图的上下文（context）,并将其设置为当前上下文(context)
    //绘图区域大小 | 是否透明 | 缩放因子
    //UIGraphicsBeginImageContextWithOptions(rect.size, YES, 1);
    
    //透明度默认YES 缩放因子1.0
    UIGraphicsBeginImageContext(rect.size);
    
    
    //获取图像上下文对象
    CGContextRef context = UIGraphicsGetCurrentContext();
    CGContextSetFillColorWithColor(context, [UIColor magentaColor].CGColor);
    CGContextSetStrokeColorWithColor(context, [UIColor greenColor].CGColor);
    
    //绘制椭圆
    CGContextStrokeRect(context, CGRectMake(20, 20, 100, 100));
    
    //绘制矩形
    CGContextFillEllipseInRect(context, CGRectMake(150, 20, 100, 100));
    
    //绘制文字
    [@"this is a jok" drawAtPoint:CGPointMake(20, 150) withAttributes:@{NSFontAttributeName:[UIFont fontWithName:@"Arial" size:30],NSForegroundColorAttributeName:[UIColor redColor]}];
    
    //获取绘图上下文中的图片
    UIImage *image = UIGraphicsGetImageFromCurrentImageContext();
    
    //结束绘图
    UIGraphicsEndImageContext();
    
    //将图片显示到界面上
    [image drawInRect:rect];
    
    //图片存储到沙盒中
    NSString *path = [[NSHomeDirectory() stringByAppendingPathComponent:@"Documents"] stringByAppendingPathComponent:@"pic.png"];
    NSLog(@"%@",path);
    [UIImagePNGRepresentation(image) writeToFile:path atomically:YES];
}

 

从上例中我们可以看到，在内存中绘图的步骤和在view上绘图的步骤完全一样，内存中绘制完图片后也可以显示到视图上，或者存储到应用沙盒中，使用内存绘图的上下文，我们可以很方便的实现绘图板的功能

@implementation ZLTView {
    CGPoint _firstPoint;
    CGPoint _lastPoint;
    CGContextRef _imageContext;
    UIImage *_image;
}

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self) {
        
        UIGraphicsBeginImageContext(self.frame.size);
        _imageContext = UIGraphicsGetCurrentContext();
        
        CGContextSetStrokeColorWithColor(_imageContext, [UIColor purpleColor].CGColor);
        CGContextSetLineWidth(_imageContext, 5);
    }
    return self;
}


- (void)touchesBegan:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    CGPoint touchPoint = [touch locationInView:self];
    _firstPoint = touchPoint;
}

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch *touch = [touches anyObject];
    CGPoint touchPoint = [touch locationInView:self];
    _lastPoint = touchPoint;
    
    
    CGContextMoveToPoint(_imageContext, _firstPoint.x, _firstPoint.y);
    CGContextAddLineToPoint(_imageContext, _lastPoint.x, _lastPoint.y);
    CGContextDrawPath(_imageContext, kCGPathStroke);
    _image = UIGraphicsGetImageFromCurrentImageContext();

    [self setNeedsDisplay];
    _firstPoint = _lastPoint;
}

- (void)drawRect:(CGRect)rect{
    [_image drawInRect:rect];
}