iOS开发-带Placeholder的UITextView实现

刚从事这个行业的时候，想在UITextView上加Placeholder这个功能上为难了一下，然后各种查，也在他人的博客、简书上留言来寻找答案。但是实现起来都差强人意。后来偶然的情况下找到了封装好的类簇，用起来非常方便。最近有挺多新手也遇到这样的问题，通过我之前在他人博客上留的邮箱老找到我的QQ询问我是如何解决的。我想想还是把他放在网络上比较好。

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带有PlaceHolder属性，可以方便用于提示输入。但是同样可以进行文本输入的UITextView控件则没有PlaceHolder属性，还是有些不方便的。
使用方法：把文件引入工程中然后直接用属性placeHolder就可以实现效果。

核心思路就是使用2个UITextView来模拟PlaceHolder的效果，其中做为输入区域的TextView在表面，背景要设为透明，作为PlaceHolder角色的TextView则在底层，两者通过UITextViewDelegate来动态控制。