iOS 利用runtime给UITextView加上占位符placeholder属性

我们都知道UITextField有个placeholder属性来展示没有文字输入时的占位符，可是UITextView却没有，偏偏这个确实我们平常开发经常会遇到的需求。当然解决方式多种多样，今天我们来说一种通过runtime添加属性的方式来实现。
我们首先给UITetxView创建一个类别Category，然后利用runtime来添加属性。代码如下：
首先 .h文件

#import 

@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

.m文件：

#import "UITextView+Placeholder.h"
#import 
@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

使用的时候导入 #import "UITextView+Placeholder.h"

    _textView = [[UITextView alloc]init];
    _textView.placeholder = @"占位符显示内容";
    _textView.placeholderColor = [UIColor colorWithHexString:@"#999999"];
    _textView.placeholderLabel.font = [UIFont systemFontOfSize:15];
    _textView.font = [UIFont systemFontOfSize:15];
    _textView.textContainerInset = UIEdgeInsetsMake(10, 10, 10, 10);
    [self.view addSubview:_textView];
   [_textView mas_makeConstraints:^(MASConstraintMaker *make) {
        make.left.equalTo(self.view.mas_left).offset(0);
        make.top.equalTo(bg.mas_bottom).offset(0);
        make.width.mas_equalTo(xScreenWidth);
        make.height.mas_equalTo(170);
    }];

ok～～，完美解决问题。代码可以直接粘过去用。。。。