第一版本（并不能完美的適配8.0iOS手機(jī)）

一開始采用kvc的方法，主要使用[self setValue:label forKey:@"_placeholderLabel"]對(duì)UITextView的私有屬性進(jìn)行賦值。

//  UITextView+Extension.m
//  UITextVIew-placeholder

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

@implementation UITextView (Extension)

+ (void)load {
    
    // 獲取類方法 class_getClassMethod
    // 獲取對(duì)象方法 class_getInstanceMethod
    
    Method setFontMethod = class_getInstanceMethod(self, @selector(setFont:));
    Method was_setFontMethod = class_getInstanceMethod(self, @selector(was_setFont:));
    
    // 交換方法的實(shí)現(xiàn)
    method_exchangeImplementations(setFontMethod, was_setFontMethod);
}

- (void)was_setPlaceholderWithText:(NSString *)text Color:(UIColor *)color{
    //防止重復(fù)設(shè)置 cell復(fù)用等問(wèn)題
    for (UIView *view in self.subviews) {
        if ([view isKindOfClass:[UILabel class]]) {
            [view removeFromSuperview];
        }
    }

    //設(shè)置占位label
    UILabel *label = [[UILabel alloc] init];
    label.text = text;
    label.font = self.font;
    label.textColor = color;
    
    [self addSubview:label];
    [self setValue:label forKey:@"_placeholderLabel"];
    
    //第一次會(huì)有位移
    self.text = @"1";
    self.text = @"";
}

- (void)was_setFont:(UIFont *)font{
    //調(diào)用原方法 setFont:
    [self was_setFont:font];
    //設(shè)置占位字符串的font
    UILabel *label = [self valueForKey:@"_placeholderLabel"];
    label.font = font;
    NSLog(@"%s", __func__);
}
@end

之后在bugly上發(fā)現(xiàn)報(bào)錯(cuò)：

設(shè)備機(jī)型
iPhone 6 Plus

系統(tǒng)版本
8.1.3 (12B466)

崩潰內(nèi)容：
[<UITextView 0x12d165200> setValue:forUndefinedKey:]: this class is not key value coding-compliant for the key _placeholderLabel.
LilyForParent -[UITextView(Extension) was_setPlaceholderWithText:Color:] (UITextView+Extension.m:)
bugly顯示有錯(cuò)誤日志，并不是iOS8.0以上系統(tǒng)都支持，不能找到
_placeholderLabel這個(gè)關(guān)鍵字，所以感覺(jué)KVC不能使用，也不知道是不是其他的原因。

改進(jìn)型

思路

通過(guò)runtime關(guān)聯(lián)對(duì)象方法，關(guān)聯(lián)一個(gè)私有Label變量，實(shí)現(xiàn)占位字符串的功能。

使用方法交換將TextView的font與placeholderlabel的font綁定，同時(shí)變化大小。

//
//  UITextView+Placeholder.m

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

static const char *kPlaceholderKey = "kPlaceholderKey";
static const char *kPlaceholderLabelKey = "kPlaceholderLabelKey";

@interface UITextView (PlaceholderLabel)<UITextViewDelegate>

@property (nonatomic, strong) UILabel *placeholderLabel;

@end

@implementation UITextView (Placeholder)

+ (void)load {
    
    // 獲取類方法 class_getClassMethod
    // 獲取對(duì)象方法 class_getInstanceMethod
    
    Method setFontMethod = class_getInstanceMethod(self, @selector(setFont:));
    Method was_setFontMethod = class_getInstanceMethod(self, @selector(was_setFont:));
    
    // 交換方法的實(shí)現(xiàn)
    method_exchangeImplementations(setFontMethod, was_setFontMethod);
}

- (void)was_setFont:(UIFont *)font{
    //調(diào)用原方法 setFont:
    [self was_setFont:font];
    //設(shè)置占位字符串的font
    if (self.placeholderLabel != nil) {
        self.placeholderLabel.font = font;
        NSLog(@"%s", __func__);
    }
}

- (UILabel *)placeholderLabel{
    return objc_getAssociatedObject(self, kPlaceholderLabelKey);
}

- (void)setPlaceholderLabel:(UILabel *)placeholderLabel{
    objc_setAssociatedObject(self, kPlaceholderLabelKey, placeholderLabel, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
}

- (NSString *)placeholder{
    return objc_getAssociatedObject(self, kPlaceholderKey);
}

- (void)was_setPlaceholderWithText:(NSString *)placeholder Color:(UIColor *)color{
    objc_setAssociatedObject(self, kPlaceholderKey, placeholder, OBJC_ASSOCIATION_COPY_NONATOMIC);
    
    UILabel *label = [[UILabel alloc] init];
    label.text = placeholder;
    label.textColor = color;
    label.numberOfLines = 0;
    [label sizeToFit];
    [self addSubview:label];
    
    self.placeholderLabel = label;
    
    self.delegate = self;
}

- (void)layoutSubviews{
    [super layoutSubviews];
    //5  7.75是實(shí)驗(yàn)后得出的完美位置，如有需求可以修改
    self.placeholderLabel.frame = CGRectMake(5, 7.75, self.frame.size.width, self.frame.size.height);
    [self.placeholderLabel sizeToFit];
}

- (void)textViewDidChange:(UITextView *)textView{
    self.placeholderLabel.hidden = !(self.text.length == 0);
}

@end

使用方法

直接將UITextView+Placeholder.h和UITextView+Placeholder.m拖入項(xiàng)目中，在需要使用TextView的控制器內(nèi)導(dǎo)入，調(diào)用下面方法可以

- (void)was_setPlaceholderWithText:(NSString *)placeholder Color:(UIColor *)color;

• 如果有好的建議可以直接留言或者發(fā)送郵件：andysection@gmail.com

Thank you for your reading !

Github：Demo