1.UITextField 的私有属性 _placeholderLabel 被禁止访问
历史代码

[self.inputField setValue:[UIColor whiteColor] forKeyPath:@"_placeholderLabel.textColor"];

Xcode升级到11.3后，之前正常运行的代码启动后就会发生闪退

*** Terminating app due to uncaught exception 'NSGenericException', reason: 'Access to UITextField's _placeholderLabel ivar is prohibited. This is an application bug'
*** First throw call stack:
(
    0   CoreFoundation                      0x00007fff23c7127e __exceptionPreprocess + 350
    1   libobjc.A.dylib                     0x00007fff513fbb20 objc_exception_throw + 48
    2   CoreFoundation                      0x00007fff23c710bc +[NSException raise:format:] + 188
    3   UIKitCore                           0x00007fff4838e451 -[UITextField valueForKey:] + 84
    4   Foundation                          0x00007fff257138bb -[NSObject(NSKeyValueCoding) setValue:forKeyPath:] + 251
    ......
    12  UIKitCore                           0x00007fff47a0ef01 -[UIViewController _sendViewDidLoadWithAppearanceProxyObjectTaggingEnabled] + 83
    13  UIKitCore                           0x00007fff47a13e5a -[UIViewController loadViewIfRequired] + 1084
    14  UIKitCore                           0x00007fff479781e4 -[UINavigationController _updateScrollViewFromViewController:toViewController:] + 160
    15  UIKitCore                           0x00007fff479784e8 -[UINavigationController _startTransition:fromViewController:toViewController:] + 144
    16  UIKitCore                           0x00007fff479793b6 -[UINavigationController _startDeferredTransitionIfNeeded:] + 868
    17  UIKitCore                           0x00007fff4797a721 -[UINavigationController __viewWillLayoutSubviews] + 150
    18  UIKitCore                           0x00007fff4795b553 -[UILayoutContainerView layoutSubviews] + 217
    19  UIKitCore                           0x00007fff485784bd -[UIView(CALayerDelegate) layoutSublayersOfLayer:] + 2478
    20  QuartzCore                          0x00007fff2b131db1 -[CALayer layoutSublayers] + 255
    21  QuartzCore                          0x00007fff2b137fa3 _ZN2CA5Layer16layout_if_neededEPNS_11TransactionE + 517
    22  QuartzCore                          0x00007fff2b1438da _ZN2CA5Layer28layout_and_display_if_neededEPNS_11TransactionE + 80
    23  QuartzCore                          0x00007fff2b08a848 _ZN2CA7Context18commit_transactionEPNS_11TransactionEd + 324
    24  QuartzCore                          0x00007fff2b0bfb51 _ZN2CA11Transaction6commitEv + 643
    25  UIKitCore                           0x00007fff480aa575 __34-[UIApplication _firstCommitBlock]_block_invoke_2 + 81
    26  CoreFoundation                      0x00007fff23bd429c __CFRUNLOOP_IS_CALLING_OUT_TO_A_BLOCK__ + 12
    27  CoreFoundation                      0x00007fff23bd3a08 __CFRunLoopDoBlocks + 312
    28  CoreFoundation                      0x00007fff23bce894 __CFRunLoopRun + 1284
    29  CoreFoundation                      0x00007fff23bce066 CFRunLoopRunSpecific + 438
    30  GraphicsServices                    0x00007fff384c0bb0 GSEventRunModal + 65
    31  UIKitCore                           0x00007fff48092d4d UIApplicationMain + 1621
    32  XtioniPhone                         0x000000010f398bd0 main + 112
    33  libdyld.dylib                       0x00007fff5227ec25 start + 1
    34  ???                                 0x0000000000000001 0x0 + 1
)
libc++abi.dylib: terminating with uncaught exception of type NSException

修改方法：UITextField有个attributedPlaceholder的属性，我们可以自定义这个富文本来达到我们需要的结果。iOS 13 通过 KVC 方式修改私有属性，有 Crush 风险，谨慎使用。

NSMutableAttributedString *placeholderString = [[NSMutableAttributedString alloc] initWithString:placeholder attributes:@{NSForegroundColorAttributeName : self.placeholderColor}];
_textField.attributedPlaceholder = placeholderString;