在示例应用程序中绘制正方形时出现问题
您好，我正在制作一个示例应用程序，其中我想创建一个正方形，我使用了以下代码
- (void)viewDidLoad {
[super viewDidLoad];
[self drawRect:CGRectMake(0, 0, 300, 200)];
[[self view] setNeedsDisplay];
}
- (void) drawRect:(CGRect)rect
{
NSLog(@"drawRect");
CGFloat centerx = rect.size.width/2;
CGFloat centery = rect.size.height/2;
CGFloat half = 100/2;
CGRect theRect = CGRectMake(-half, -half, 100, 100);
// Grab the drawing context
CGContextRef context = UIGraphicsGetCurrentContext();
// like Processing pushMatrix
CGContextSaveGState(context);
CGContextTranslateCTM(context, centerx, centery);
// Uncomment to see the rotated square
//CGContextRotateCTM(context, rotation);
// Set red stroke
CGContextSetRGBStrokeColor(context, 1.0, 0.0, 0.0, 1.0);
{
CGContextSetRGBFillColor(context, 0.0, 1.0, 0.0, 1.0);
}
// Draw a rect with a red stroke
CGContextFillRect(context, theRect);
CGContextStrokeRect(context, theRect);
// like Processing popMatrix
CGContextRestoreGState(context);
[[self view] setNeedsDisplay];
}
，但屏幕上没有绘制任何内容，不知道问题出在哪里。当我调试它时，CGContextRef 上下文始终为 0x0，我不知道为什么它的 0x0 总是我在代码中丢失的东西。
Hi i am making a sample app in which i wanto create a square for which i used the following code
- (void)viewDidLoad {
[super viewDidLoad];
[self drawRect:CGRectMake(0, 0, 300, 200)];
[[self view] setNeedsDisplay];
}
- (void) drawRect:(CGRect)rect
{
NSLog(@"drawRect");
CGFloat centerx = rect.size.width/2;
CGFloat centery = rect.size.height/2;
CGFloat half = 100/2;
CGRect theRect = CGRectMake(-half, -half, 100, 100);
// Grab the drawing context
CGContextRef context = UIGraphicsGetCurrentContext();
// like Processing pushMatrix
CGContextSaveGState(context);
CGContextTranslateCTM(context, centerx, centery);
// Uncomment to see the rotated square
//CGContextRotateCTM(context, rotation);
// Set red stroke
CGContextSetRGBStrokeColor(context, 1.0, 0.0, 0.0, 1.0);
{
CGContextSetRGBFillColor(context, 0.0, 1.0, 0.0, 1.0);
}
// Draw a rect with a red stroke
CGContextFillRect(context, theRect);
CGContextStrokeRect(context, theRect);
// like Processing popMatrix
CGContextRestoreGState(context);
[[self view] setNeedsDisplay];
}
But nothing is drawn on screen , dont know wheres the issue is .When i debug it the CGContextRef context was always 0x0 , i dont know why its 0x0 always am i missing something in my code.
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评论（2）
看起来您正在尝试在
UIViewController的子类中进行绘制。您需要子类化UIView来重写drawRect:方法，然后使用有效的图形上下文自动调用该方法。您自己几乎从未调用过此方法。It looks like you're trying to draw in a subclass of
UIViewController. You need to subclassUIViewto override thedrawRect:method, which is then called automatically with a valid graphics context in place. You almost never call this method yourself.引用 Apple 文档：
“要在 iOS 应用程序中绘制到屏幕上，您需要设置一个 UIView 对象并实现其 drawRect: 方法来执行绘制。当视图在屏幕上可见且其内容需要时，将调用视图的 drawRect: 方法。在调用自定义的drawRect:方法之前，视图对象会自动配置其绘图环境，以便您的代码可以立即开始绘图。作为此配置的一部分，UIView 对象会为当前绘图创建一个图形上下文（CGContextRef 不透明类型）。环境。您可以通过调用 UIKit 函数 UIGraphicsGetCurrentContext 在 drawRect: 方法中获取此图形上下文。”
因此，本质上，您的代码已步入正轨，您只需将其放在正确的位置即可。它需要位于 View 对象中。
To quote the Apple docs:
"To draw to the screen in an iOS application, you set up a UIView object and implement its drawRect: method to perform drawing. The view’s drawRect: method is called when the view is visible onscreen and its contents need updating. Before calling your custom drawRect: method, the view object automatically configures its drawing environment so that your code can start drawing immediately. As part of this configuration, the UIView object creates a graphics context (a CGContextRef opaque type) for the current drawing environment. You obtain this graphics context in your drawRect: method by calling the UIKit function UIGraphicsGetCurrentContext."
So essentailly, your code is on track, you just need to get it in the right place. It needs to be in the View object.