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 <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
.m文件：
#import "UITextView+Placeholder.h"
#import <objc/runtime.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
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。。。。
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