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。
