剛從事這個行業的時候，想在UITextView上加Placeholder這個功能上為難了一下，然后各種查，也在他人的博客、簡書上留言來尋找答案。但是實現起來都差強人意。后來偶然的情況下找到了封裝好的類簇，用起來非常方便。最近有挺多新手也遇到這樣的問題，通過我之前在他人博客上留的郵箱老找到我的QQ詢問我是如何解決的。我想想還是把他放在網絡上比較好。

UITextView+Placeholder.h
#import <UIKit/UIKit.h>

@interface UITextView (Placeholder)

@property (nonatomic, readonly) UILabel *placeholderLabel;

@property (nonatomic, strong) NSString *placeholder;
@property (nonatomic, strong) NSAttributedString *attributedPlaceholder;
@property (nonatomic, strong) UIColor *placeholderColor;

+ (UIColor *)defaultPlaceholderColor;

@end

UITextView+Placeholder.m

#import <objc/runtime.h>
#import "UITextView+Placeholder.h"

@implementation UITextView (Placeholder)

#pragma mark - Swizzle Dealloc

+ (void)load {
// is this the best solution?
method_exchangeImplementations(class_getInstanceMethod(self.class, NSSelectorFromString(@"dealloc")),
                               class_getInstanceMethod(self.class, @selector(swizzledDealloc)));
}

- (void)swizzledDealloc {
[[NSNotificationCenter defaultCenter] removeObserver:self];
UILabel *label = objc_getAssociatedObject(self, @selector(placeholderLabel));
if (label) {
    for (NSString *key in self.class.observingKeys) {
        @try {
            [self removeObserver:self forKeyPath:key];
        }
        @catch (NSException *exception) {
            // Do nothing
        }
    }
}
[self swizzledDealloc];
}

#pragma mark - Class Methods
#pragma mark `defaultPlaceholderColor`

+ (UIColor *)defaultPlaceholderColor {
static UIColor *color = nil;
static dispatch_once_t onceToken;
dispatch_once(&onceToken, ^{
    UITextField *textField = [[UITextField alloc] init];
    textField.placeholder = @" ";
    color = [textField valueForKeyPath:@"_placeholderLabel.textColor"];
});
return color;
}

#pragma mark - `observingKeys`

+ (NSArray *)observingKeys {
return @[@"attributedText",
         @"bounds",
         @"font",
         @"frame",
         @"text",
         @"textAlignment",
         @"textContainerInset"];
}

#pragma mark - Properties
#pragma mark `placeholderLabel`

- (UILabel *)placeholderLabel {
UILabel *label = objc_getAssociatedObject(self, @selector(placeholderLabel));
if (!label) {
    NSAttributedString *originalText = self.attributedText;
    self.text = @" "; // lazily set font of `UITextView`.
    self.attributedText = originalText;

    label = [[UILabel alloc] init];
    label.textColor = [self.class defaultPlaceholderColor];
    label.numberOfLines = 0;
    label.userInteractionEnabled = NO;
    objc_setAssociatedObject(self, @selector(placeholderLabel), label, OBJC_ASSOCIATION_RETAIN_NONATOMIC);

    [[NSNotificationCenter defaultCenter] addObserver:self
                                             selector:@selector(updatePlaceholderLabel)
                                                 name:UITextViewTextDidChangeNotification
                                               object:self];

    for (NSString *key in self.class.observingKeys) {

        [self addObserver:self forKeyPath:key options:NSKeyValueObservingOptionNew context:nil];
    }
}
  return label;
}

#pragma mark `placeholder`

- (NSString *)placeholder {
    return self.placeholderLabel.text;
}

- (void)setPlaceholder:(NSString *)placeholder {

    self.placeholderLabel.text = placeholder;
    [self updatePlaceholderLabel];
}

- (NSAttributedString *)attributedPlaceholder {
return self.placeholderLabel.attributedText;
}

- (void)setAttributedPlaceholder:(NSAttributedString *)attributedPlaceholder {
self.placeholderLabel.attributedText = attributedPlaceholder;
[self updatePlaceholderLabel];
}

#pragma mark `placeholderColor`

- (UIColor *)placeholderColor {
return self.placeholderLabel.textColor;
}

- (void)setPlaceholderColor:(UIColor *)placeholderColor {
self.placeholderLabel.textColor = placeholderColor;
}
#pragma mark - KVO

- (void)observeValueForKeyPath:(NSString *)keyPath
                  ofObject:(id)object
                    change:(NSDictionary *)change
                   context:(void *)context {
[self updatePlaceholderLabel];
}

#pragma mark - Update

- (void)updatePlaceholderLabel {
if (self.text.length) {
    [self.placeholderLabel removeFromSuperview];
    return;
}

[self insertSubview:self.placeholderLabel atIndex:0];

self.placeholderLabel.font = self.font;
self.placeholderLabel.textAlignment = self.textAlignment;

// `NSTextContainer` is available since iOS 7
CGFloat lineFragmentPadding;
UIEdgeInsets textContainerInset;

#pragma deploymate push "ignored-api-availability"
// iOS 7+
if (NSFoundationVersionNumber > NSFoundationVersionNumber_iOS_6_1) {
    lineFragmentPadding = self.textContainer.lineFragmentPadding;
    textContainerInset = self.textContainerInset;
}
#pragma deploymate pop

// iOS 6
else {
    lineFragmentPadding = 5;
    textContainerInset = UIEdgeInsetsMake(8, 0, 8, 0);
}

CGFloat x = lineFragmentPadding + textContainerInset.left;
CGFloat y = textContainerInset.top;
CGFloat width = CGRectGetWidth(self.bounds) - x - lineFragmentPadding - textContainerInset.right;
CGFloat height = [self.placeholderLabel sizeThatFits:CGSizeMake(width, 0)].height;
self.placeholderLabel.frame = CGRectMake(x, y, width, height);
}

@end

iOS中UITextField帶有PlaceHolder屬性，可以方便用于提示輸入。但是同樣可以進行文本輸入的UITextView控件則沒有PlaceHolder屬性，還是有些不方便的。
使用方法：把文件引入工程中然后直接用屬性placeHolder就可以實現效果。

核心思路就是使用2個UITextView來模擬PlaceHolder的效果，其中做為輸入區域的TextView在表面，背景要設為透明，作為PlaceHolder角色的TextView則在底層，兩者通過UITextViewDelegate來動態控制。