为什么valueForKey：在UITextField上为UITextInputTraits属性抛出异常？ [英] Why does valueForKey: on a UITextField throws an exception for UITextInputTraits properties?

问题描述

执行此操作：

  @try 
 {
 NSLog（@1。autocapitalizationType =％ d，[self.textField autocapitalizationType]）; 
 NSLog（@2。autocapitalizationType =％@，[self.textField valueForKey：@autocapitalizationType]）; 
} 
 @catch（NSException * exception）
 {
 NSLog（@3。％@，exception）; 
} 
  

输出：

  1。 autocapitalizationType = 0 
 3. [< UITextField 0x6c15df0> valueForUndefinedKey：]：此类不是键值编码兼容的键autocapitalizationType。 
  

我期待：

  1。 autocapitalizationType = 0 
 2. autocapitalizationType = 0 
  

此异常仅在属性的 UITextInputTraits 协议。 UITextField 的属性可以通过 clearButtonMode > valueForKey：。

那么为什么不能用键来访问 UITextInputTraits 属性如果你深入到UIKit框架并打开 UITextField.h

code>，您会发现：

  @interface UITextField：UIControl< UITextInput，NSCoding> {
 @private 
 
 UITextInputTraits * _traits; 
 UITextInputTraits * _nonAtomTraits; 
  

您还会发现 clearButtonMode 在UITextField头文件中声明为 @property ，但 autocapitalizationType （其余的 UITextInputTraits 协议）不是。

你和我不能看到 UITextField.m ，所以我们可以得出真正的结论是，苹果实现了 UITextField 的 UITextInputTraits 方式不符合KVC标准。可能胶水代码在某处转换 [myTextField autocapitalizationType] 到适当的值，但任何幕后魔法发生停止短于 valueForKey： / code>。

Running this:

@try
{
    NSLog(@"1. autocapitalizationType = %d", [self.textField autocapitalizationType]);
    NSLog(@"2. autocapitalizationType = %@", [self.textField valueForKey:@"autocapitalizationType"]);
}
@catch (NSException *exception)
{
    NSLog(@"3. %@", exception);
}

Outputs this:

1. autocapitalizationType = 0
3. [<UITextField 0x6c15df0> valueForUndefinedKey:]: this class is not key value coding-compliant for the key autocapitalizationType.

I was expecting:

1. autocapitalizationType = 0
2. autocapitalizationType = 0

This exception only happens with properties that are part of the UITextInputTraits protocol. Regular properties of a UITextField such has clearButtonMode can be accessed through valueForKey:.

So why can't you access UITextInputTraits properties with key-value coding?

解决方案

If you delve into the UIKit framework and open up UITextField.h, you'll find:

@interface UITextField : UIControl <UITextInput, NSCoding> {
  @private

    UITextInputTraits  *_traits;
    UITextInputTraits  *_nonAtomTraits;

You'll also find that clearButtonMode is declared as a @property in the UITextField header file, but that autocapitalizationType (and the rest of the UITextInputTraits protocol) are not.

You and I don't get to see UITextField.m, so all we can really conclude is that Apple implemented the UITextField's UITextInputTraits protocol in a way that's not KVC compliant. Presumably glue code somewhere converts [myTextField autocapitalizationType] into the appropriate value, but whatever behind-the-scenes magic is taking place stops short of valueForKey:.

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