故事背景如下,我有一个UITextField的子类MyTextField,创建了MyTextField的实例tf。我希望tf对应的键盘显示done按钮。代码如下:
1 @interface MyTextField : UITextField
2 @end
3
4 MyTextField* tf = [[MyTextField alloc] initWithFrame:someframe];
5 tf.returnKeyType = UIReturnKeyDone;
然后我希望,当done被按下的时候,键盘关闭,通常的做法是给TextField一个delegate,监听textFieldShouldReturn,当done被按下时,会回调delegate的textFieldShouldReturn方法,代码如下:
8 @interface TextFieldDelegate : NSObject<UITextFieldDelegate>
9 @end
10
11 @implementation TextFieldDelegate
12 - (BOOL)textFieldShouldReturn:(UITextField *)textField
13 {
14 [textField resignFirstResponder];
15 return YES;
16 }
17 @end
18
19 tf.delegate = [[TextFieldDelegate alloc] init];
这是比较常规的做法,但是要新建一个类,新建一个对象,用完了还要释放这个对象,成本很大。这时我产生了个偷懒的想法,我自己的事情我就自己做吧,我做我自己的代理,self.delegate = self。代码如下,
23 @interface MyTextField : UITextField<UITextFieldDelegate>
24 @end
25
26 @implementation MyTextField
27 - (BOOL)textFieldShouldReturn:(UITextField *)textField
28 {
29 [textField resignFirstResponder];
30 return YES;
31 }
32 @end
33
34 MyTextField* tf = [[MyTextField alloc] initWithFrame:someframe];
35 tf.returnKeyType = UIReturnKeyDone;
36 tf.delegate = tf;
37
干净利落,然后恶梦就开始了。运行这段代码,点击输入框,先是程序就再不响应了,XCode也没什么有用的提示,试一两次,XCode也不再响应了。
这个问题不是我自己解决的,我求助了google。在MyTextField的实现中加入以下函数,
39 - (BOOL)respondsToSelector:(SEL)aSelector
40 {
41 return [super respondsToSelector:aSelector];
42 }
在这个函数中打个断点,再次执行,程序运行到断点后继续执行,会不断的运行到这个断点,然后不断继续,会发现,最终程序在这里无限递归了,参数始终是keyboardInputChangedSelection,直到栈溢出,程序崩溃。
现在我们知道原因了,为什么呢?猜想一下,这个函数的内部大概会是这样实现
45 - (BOOL)respondsToSelector:(SEL)aSelector
46 {
47 switch(aSelector)
48 {
49 ...
50 case keyboardInputChangedSelection:
51 return [self.delegate respondsToSelector:aSelector];
52 ...
53 }
54 }
这个问题已经不在我们能看到的代码层面了,所以即便能解决,也只能非常暴力的重写respondsToSelector,这个和我们要偷懒的初衷不符。
总结一下,self.delegate = self这种写法,在遇到
56 -(void)method
57 {
58 [self.delegate method];
59 }
这种用法的时候,会发生无限递归导致崩溃,一定要避免。
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