这个线程有很多答案，但是这是我喜欢的版本。
它扩展了现有的UITextView类，因此易于重用，并且不会拦截类似的事件textViewDidChange（如果它们已经在其他地方拦截了这些事件，则可能会破坏用户的代码）。
使用我的代码（如下所示），您可以轻松地将占位符添加到您的任何对象中，UITextViews如下所示：
self.textViewComments.placeholder = @"(Enter some comments here.)";
当您设置此新的占位符值时，它会UILabel在您的上方安静地添加一个UITextView，然后根据需要隐藏/显示它：
好的，要进行这些更改，请添加一个包含以下代码的“ UITextViewHelper.h”文件：
// UITextViewHelper.h
// Created by Michael Gledhill on 13/02/15.
#import <Foundation/Foundation.h>
@interface UITextView (UITextViewHelper)
@property (nonatomic, strong) NSString* placeholder;
@property (nonatomic, strong) UILabel* placeholderLabel;
@property (nonatomic, strong) NSString* textValue;
-(void)checkIfNeedToDisplayPlaceholder;
@end
...以及一个包含以下内容的UITextViewHelper.m文件：
// UITextViewHelper.m
// Created by Michael Gledhill on 13/02/15.
//
// This UITextView category allows us to easily display a PlaceHolder string in our UITextView.
// The downside is that, your code needs to set the "textValue" rather than the "text" value to safely set the UITextView's text.
//
#import "UITextViewHelper.h"
#import <objc/runtime.h>
@implementation UITextView (UITextViewHelper)
#define UI_PLACEHOLDER_TEXT_COLOR [UIColor colorWithRed:170.0/255.0 green:170.0/255.0 blue:170.0/255.0 alpha:1.0]
@dynamic placeholder;
@dynamic placeholderLabel;
@dynamic textValue;
-(void)setTextValue:(NSString *)textValue
{
// Change the text of our UITextView, and check whether we need to display the placeholder.
self.text = textValue;
[self checkIfNeedToDisplayPlaceholder];
}
-(NSString*)textValue
{
return self.text;
}
-(void)checkIfNeedToDisplayPlaceholder
{
// If our UITextView is empty, display our Placeholder label (if we have one)
if (self.placeholderLabel == nil)
return;
self.placeholderLabel.hidden = (![self.text isEqualToString:@""]);
}
-(void)onTap
{
// When the user taps in our UITextView, we'll see if we need to remove the placeholder text.
[self checkIfNeedToDisplayPlaceholder];
// Make the onscreen keyboard appear.
[self becomeFirstResponder];
}
-(void)keyPressed:(NSNotification*)notification
{
// The user has just typed a character in our UITextView (or pressed the delete key).
// Do we need to display our Placeholder label ?
[self checkIfNeedToDisplayPlaceholder];
}
#pragma mark - Add a "placeHolder" string to the UITextView class
NSString const *kKeyPlaceHolder = @"kKeyPlaceHolder";
-(void)setPlaceholder:(NSString *)_placeholder
{
// Sets our "placeholder" text string, creates a new UILabel to contain it, and modifies our UITextView to cope with
// showing/hiding the UILabel when needed.
objc_setAssociatedObject(self, &kKeyPlaceHolder, (id)_placeholder, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
self.placeholderLabel = [[UILabel alloc] initWithFrame:self.frame];
self.placeholderLabel.numberOfLines = 1;
self.placeholderLabel.text = _placeholder;
self.placeholderLabel.textColor = UI_PLACEHOLDER_TEXT_COLOR;
self.placeholderLabel.backgroundColor = [UIColor clearColor];
self.placeholderLabel.userInteractionEnabled = true;
self.placeholderLabel.font = self.font;
[self addSubview:self.placeholderLabel];
[self.placeholderLabel sizeToFit];
// Whenever the user taps within the UITextView, we'll give the textview the focus, and hide the placeholder if necessary.
[self addGestureRecognizer:[[UITapGestureRecognizer alloc] initWithTarget:self action:@selector(onTap)]];
// Whenever the user types something in the UITextView, we'll see if we need to hide/show the placeholder label.
[[NSNotificationCenter defaultCenter] addObserver:self selector: @selector(keyPressed:) name:UITextViewTextDidChangeNotification object:nil];
[self checkIfNeedToDisplayPlaceholder];
}
-(NSString*)placeholder
{
// Returns our "placeholder" text string
return objc_getAssociatedObject(self, &kKeyPlaceHolder);
}
#pragma mark - Add a "UILabel" to this UITextView class
NSString const *kKeyLabel = @"kKeyLabel";
-(void)setPlaceholderLabel:(UILabel *)placeholderLabel
{
// Stores our new UILabel (which contains our placeholder string)
objc_setAssociatedObject(self, &kKeyLabel, (id)placeholderLabel, OBJC_ASSOCIATION_RETAIN_NONATOMIC);
[[NSNotificationCenter defaultCenter] addObserver:self selector: @selector(keyPressed:) name:UITextViewTextDidChangeNotification object:nil];
[self checkIfNeedToDisplayPlaceholder];
}
-(UILabel*)placeholderLabel
{
// Returns our new UILabel
return objc_getAssociatedObject(self, &kKeyLabel);
}
@end
是的，这是很多代码，但是一旦将其添加到项目中并包含.h文件...
#import "UITextViewHelper.h"
...您可以轻松地在中使用占位符UITextViews。
虽然有一个陷阱。
如果您这样做：
self.textViewComments.placeholder = @"(Enter some comments here.)";
self.textViewComments.text = @"Ooooh, hello there";
...占位符将出现在文本顶部。当您设置text值时，不会调用任何常规通知，因此无法确定如何调用我的函数来决定是否显示/隐藏占位符。
解决方案是设置textValue而不是text：
self.textViewComments.placeholder = @"(Enter some comments here.)";
self.textViewComments.textValue = @"Ooooh, hello there";
或者，您可以设置text值，然后调用checkIfNeedToDisplayPlaceholder。
self.textViewComments.text = @"Ooooh, hello there";
[self.textViewComments checkIfNeedToDisplayPlaceholder];
我喜欢这样的解决方案，因为它们可以“填补” Apple提供给我们的东西与我们（作为开发人员）实际需要的东西之间的差距在应用程序中。您只需编写一次此代码，然后将其添加到“ helper” .m / .h文件库中，随着时间的流逝，SDK实际上会变得越来越令人沮丧。
（我写了一个类似的帮助程序，为我的UITextViews添加了一个“清除”按钮，这是烦人的事，UITextField但在UITextView... 中却不存在）
