一行代码搞定UITextView的placeholder

 1 /** 注意先设置textView的字体 */
 2 
 3 #import "UITextView+placeholder.h"
 4 #import <objc/runtime.h>
 5 
 6 #define LEFT_MARGIN 5
 7 #define TOP_MARGIN  8
 8 
 9 @implementation UITextView (placeholder)
10 
11 - (NSString *)placeholder{
12      return self.label.text;
13 }
14 
15 - (void)setPlaceholder:(NSString *)placeholder{
16 
17     //赋值修改高度
18     self.label.text = placeholder;
19     [self changeLabelFrame];
20     
21     //监听文本改变,如果没有设置placeholder就不会监听
22     [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(textDidChange:) name:UITextViewTextDidChangeNotification object:nil];
23 }
24 
25 //文本修改
26 - (void)textDidChange:(NSNotification *)notify{
27    self.label.hidden = self.text.length;
28 }
29 
30 - (UILabel *)label{
31     
32     UILabel *label =  objc_getAssociatedObject(self, @"label");
33     
34     if (label == nil) {
35         //没有就创建,并设置属性
36         label = [[UILabel alloc] init];
37         label.font = self.font;
38         label.textColor = [UIColor grayColor];
39         label.textAlignment = NSTextAlignmentLeft;
40         label.numberOfLines = 0;
41         
42         [self addSubview:label];
43         
44         //关联到自身
45         objc_setAssociatedObject(self, @"label", label, OBJC_ASSOCIATION_RETAIN);
46        
47     }
48     
49     return label;
50 }
51 
52 //计算frame
53 - (void)changeLabelFrame{
54     //文字可显示区域
55     CGSize size = CGSizeMake(self.bounds.size.width - 2*LEFT_MARGIN, CGFLOAT_MAX);
56     //计算文字所占区域
57     CGSize labelSize = [self.placeholder boundingRectWithSize:size options:NSStringDrawingUsesLineFragmentOrigin attributes:@{NSFontAttributeName : self.label.font} context:nil].size;
58     
59     self.label.frame = CGRectMake(LEFT_MARGIN, TOP_MARGIN, labelSize.width, labelSize.height);
60 }
61 @end