修改UITextField的placeholder字體顏色並垂直居中

修改字體和顏色須要用到kvc：

yourTextField.placeholder = @"username is in here!";

[yourTextField setValue:[UIColor redColor] forKeyPath:@"_placeholderLabel.textColor"];

[yourTextField setValue:[UIFont boldSystemFontOfSize:16] forKeyPath:@"_placeholderLabel.font"];

有一個問題，當字體太小的時候，placeholder的字體會顯示不是垂直居中的，這須要用到NSParagraphStyle，

個人解決辦法是：

NSMutableParagraphStyle *style = [yourTextField.defaultTextAttributes[NSParagraphStyleAttributeName] mutableCopy];

style.minimumLineHeight = yourTextField.font.lineHeight - (yourTextField.font.lineHeight - [UIFont systemFontOfSize:14.0].lineHeight) / 2.0;

yourTextField.attributedPlaceholder = [[NSAttributedString alloc] initWithString:@"YourTextField Placeholder text"

attributes:@{

NSForegroundColorAttributeName: [UIColor colorWithRed:255/255.0f green:255/255.0f blue:255/255.0f alpha:0.7f],

NSFontAttributeName : [UIFont systemFontOfSize:14.0],

NSParagraphStyleAttributeName : style

}

];

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