CGContextFillEllipseInRect：無效的上下文

Question

當我啓動我的應用程序，我不斷收到這個CGContextFillEllipseInRect：無效的上下文錯誤。我的應用程序只是爲了讓這個圈子變得「可捏造」。

調試器顯示我：

[Session started at 2010-05-23 18:21:25 +0800.] 
2010-05-23 18:21:27.140 Erase[8045:207] I'm being redrawn. 
Sun May 23 18:21:27 Sidwyn-Kohs-MacBook-Pro.local Erase[8045] <Error>: CGContextFillEllipseInRect: invalid context 
2010-05-23 18:21:27.144 Erase[8045:207] New value of counter is 1 
2010-05-23 18:21:27.144 Erase[8045:207] I'm being redrawn. 
Sun May 23 18:21:27 Sidwyn-Kohs-MacBook-Pro.local Erase[8045] <Error>: CGContextFillEllipseInRect: invalid context 
2010-05-23 18:21:27.144 Erase[8045:207] New value of counter is 2 
2010-05-23 18:21:27.144 Erase[8045:207] I'm being redrawn. 
Sun May 23 18:21:27 Sidwyn-Kohs-MacBook-Pro.local Erase[8045] <Error>: CGContextFillEllipseInRect: invalid context 
2010-05-23 18:21:27.144 Erase[8045:207] New value of counter is 3 
2010-05-23 18:21:27.145 Erase[8045:207] I'm being redrawn. 
Sun May 23 18:21:27 Sidwyn-Kohs-MacBook-Pro.local Erase[8045] <Error>: CGContextFillEllipseInRect: invalid context 
2010-05-23 18:21:27.145 Erase[8045:207] New value of counter is 4 
2010-05-23 18:21:27.148 Erase[8045:207] I'm being redrawn. 
Sun May 23 18:21:27 Sidwyn-Kohs-MacBook-Pro.local Erase[8045] <Error>: CGContextFillEllipseInRect: invalid context 
2010-05-23 18:21:27.149 Erase[8045:207] New value of counter is 5 
2010-05-23 18:21:27.150 Erase[8045:207] I'm being redrawn. 
Sun May 23 18:21:27 Sidwyn-Kohs-MacBook-Pro.local Erase[8045] <Error>: CGContextFillEllipseInRect: invalid context 
2010-05-23 18:21:27.150 Erase[8045:207] New value of counter is 6 

我的執行文件是：

// 
// ImageView.m 
// Erase 
// 

#import "ImageView.h" 
#import "EraseViewController.h" 

@implementation ImageView 


-(void)setNewRect:(CGRect)anotherRect{ 
    newRect = anotherRect; 
} 

- (id)initWithFrame:(CGRect)frame { 
    if (self = [super initWithFrame:frame]) { 
     newRect = CGRectMake(0,0,0,0); 
     ref = UIGraphicsGetCurrentContext(); 
    } 
    return self; 
} 



- (void)drawRect:(CGRect)rect { 
    static int counter = 0; 
    CGRect veryNewRect = CGRectMake(30.0, 210.0, 60.0, 60.0); 
    NSLog(@"I'm being redrawn."); 

    if (counter == 0){ 
     CGContextFillEllipseInRect(ref, veryNewRect); 
    } 
    else{ 
     CGContextFillEllipseInRect(ref, rect); 
    } 

    counter++; 
    NSLog(@"New value of counter is %d", counter); 
} 

- (void)setNeedsDisplay{ 
    [super setNeedsDisplay]; 
    [self drawRect:newRect]; 
} 

- (void)dealloc { 
    [super dealloc]; 
} 


@end 

我有兩個問題： 1）爲什麼更新計數器6倍？我刪除了行[super setNeedsDisplay];，但它變成了4次。 2）什麼是無效的上下文錯誤？

謝謝你們。

Answer 1

1. 我不確定這六個調用，但至少有兩個調用正在發生，因爲框架調用drawRect:，你也是如此。你不需要自己撥打drawRect:（因此你不需要覆蓋setNeedsDisplay:）。
2. 您應該從drawRect:內撥打UIGraphicsGetCurrentContext。不要緩存它。它可能會失敗，因爲框架不能保證在每次調用時使用相同的上下文，或者因爲您直接調用了drawRect:，或者兩者都調用;我不確定哪個。