UITextView实现占位文字

UITextField *textField默认的情况
1.只能显示一行文字
2.有占位文字

UITextView *textView默认的情况
2.能显示任意行文字
2.没有占位文字

文本输入控件,最终希望拥有的功能
1.能显示任意行文字
2.有占位文字

最终的方案:
1.继承自UITextView
2.在UITextView能显示任意行文字的基础上,增加”有占位文字”的功能

二、首先新建一个ZJPlaceholderTextView，.h文件如下

#import <UIKit/UIKit.h>

@interface ZJPlaceholderTextView : UITextView
/** 占位文字 */
@property (nonatomic, copy) NSString *placeholder;
/** 占位文字的颜色 */
@property (nonatomic, strong) UIColor *placeholderColor;

第二步、.m文件如下:

#import "ZJPlaceholderTextView.h"

@interface ZJPlaceholderTextView()
/** 占位文字label */
@property (nonatomic, weak) UILabel *placeholderLabel;
@end

@implementation ZJPlaceholderTextView

- (UILabel *)placeholderLabel
{
    if (!_placeholderLabel) {
        // 添加一个用来显示占位文字的label
        UILabel *placeholderLabel = [[UILabel alloc] init];
        placeholderLabel.numberOfLines = 0;
        placeholderLabel.x = 4;
        placeholderLabel.y = 7;
        [self addSubview:placeholderLabel];
        _placeholderLabel = placeholderLabel;
    }
    return _placeholderLabel;
}

- (instancetype)initWithFrame:(CGRect)frame
{
    if (self = [super initWithFrame:frame]) {
        // 垂直方向上永远有弹簧效果
        self.alwaysBounceVertical = YES;

        // 默认字体
        self.font = [UIFont systemFontOfSize:15];

        // 默认的占位文字颜色
        self.placeholderColor = [UIColor grayColor];

        // 监听文字改变
        [[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(textDidChange) name:UITextViewTextDidChangeNotification object:nil];
    }
    return self;
}

- (void)dealloc
{
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}

/**
 * 监听文字改变
 */
- (void)textDidChange
{
    // 只要有文字, 就隐藏占位文字label
    self.placeholderLabel.hidden = self.hasText;
}

/**
 * 更新占位文字的尺寸
 */
- (void)layoutSubviews
{
    [super layoutSubviews];

    self.placeholderLabel.width = self.width - 2 * self.placeholderLabel.x;
    [self.placeholderLabel sizeToFit];
}

#pragma mark - 重写setter
- (void)setPlaceholderColor:(UIColor *)placeholderColor
{
    _placeholderColor = placeholderColor;

    self.placeholderLabel.textColor = placeholderColor;
}

- (void)setPlaceholder:(NSString *)placeholder
{
    _placeholder = [placeholder copy];

    self.placeholderLabel.text = placeholder;

    [self setNeedsLayout];
}

- (void)setFont:(UIFont *)font
{
    [super setFont:font];

    self.placeholderLabel.font = font;

    [self setNeedsLayout];
}

- (void)setText:(NSString *)text
{
    [super setText:text];

    [self textDidChange];
}

- (void)setAttributedText:(NSAttributedString *)attributedText
{
    [super setAttributedText:attributedText];

    [self textDidChange];
}

/**
 * setNeedsDisplay方法 : 会在恰当的时刻自动调用drawRect:方法
 * setNeedsLayout方法 : 会在恰当的时刻调用layoutSubviews方法
 */

@end