修改字体颜色：
[textField setValue:[UIColor redColor] forKeyPath:@"_placeholderLabel.textColor"];
复制代码修改字体大小：
[textField setValue:[UIFont boldSystemFontOfSize:16] forKeyPath:@"_placeholderLabel.font"];
复制代码转载于:https://juejin.im/post/5a30e526f265da433562c0fc
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