在矩形上绘制圆圈

Question

I have UIView class and in method I want to draw first rectangles and sometimes circle

- (void)drawRect:(CGRect)rect
{
    CGContextRef context = UIGraphicsGetCurrentContext();

    if ([WhatToDraw isEqual:@"Fields"]) {
        [self DrawField:context];
            }
    if ([WhatToDraw isEqual:@"Ball"]) {
        [self DrawBall:context x:20 y:20];
    }

}

-(void)DrawBall:(CGContextRef)context x:(float) x y:(float) y
{
    UIGraphicsPushContext(context);
    CGRect  rect = CGRectMake(x, y, 25, 25);
    CGContextClearRect(context, rect);
    CGContextFillEllipseInRect(context, rect);
}

-(void)DrawField:(CGContextRef)context 
{

    columns = 6;
    float offset = 5;
    float boardWidth = self.frame.size.width;
    float allOffset = (columns + 2) * offset;
    float currentX = 10;
    float currentWidth = (boardWidth - allOffset) / columns;
    float currentHeight = currentWidth;
    self.fieldsArray = [[NSMutableArray alloc] init];
    //create a new dynamic button board
    for (int columnIndex = 0; columnIndex<columns; columnIndex++) {
        float currentY = offset;
        for (int rowIndex=0; rowIndex<columns; rowIndex++) {
            UIGraphicsPushContext(context);
            //create new field


            CGContextSetRGBFillColor(context, 1.0, 0.0, 0.0, 1.0);
            CGContextBeginPath(context);
            CGRect rect = CGRectMake(currentX, currentY, currentWidth, currentHeight);
            CGContextAddRect(context, rect);
            CGContextFillPath(context);

            currentY = currentY + offset + currentHeight;
        }
        currentX = currentX + offset + currentWidth;
    }  
}

I also have method changing what to draw

-(void)Draw:(NSString*)Thing
{
    self.WhatToDraw = Thing;
    [self setNeedsDisplay];
}

Drawing rectangles (Fields) is ok, but when I click button to draw circle all rectangles disappear and only circle was drawn. How can I draw circle on existing rectangle ?

Answer 1

The Problem

Your problem is that when a UIView redraws a region as marked by setNeedsDisplay or setNeedsDisplayInRect it will completely clear that region before executing your drawing code. This means that unless you draw both the rectangles and circle in a single drawing operation within drawRect you will never see the two both drawn in the area you choose to redraw, whether it be the entire view bounds with setNeedsDisplay or a specific area with setNeedsDisplayInRect .

The Solutions

There's no reason why you can't draw both the rectangles and circle each time within drawRect and optimise the performance of the drawing by only redrawing the regions necessary with setNeedsDisplayInRect .

Alternatively you could break up the content using CALayers and have the rectangles in one layer and the circle in another. This would allow you to leverage the animation capabilities of Core Animation . Core animation provides a simple and effective way to manipulate onscreen layers with implicit animations such as moving, resizing, changing colour etc.

Answer 2

我的猜测，你的DrawBall方法中的CGContextClearRect调用是矩形消失的责任...来自Apple文档： 如果提供的上下文是窗口或位图上下文，Quartz会有效地清除矩形。