【optimize】Use observer object replace textView itself

[leaflinfeng]

Swizzling dealloc and implement kvo method in category is not safe. We can use another observer object and associate it on textView. It can unobserve when textView dealloc.

Some code

@interface UITextView ()
@property (nonatomic, strong) _UITextViewObserver *xx_observer;
@end 

@interface _UITextViewObserver ()
@property (nonatomic, weak) UITextView *textView;
@end

@implementation _UITextViewObserver
+ (NSArray *)observingKeys {
    return @[
        @"attributedText",
        @"bounds",
        @"font",
        @"frame",
        @"text",
        @"textAlignment",
        @"textContainerInset",
        @"textContainer.exclusionPaths"
    ];
}
+ (instancetype)observerWithTextView:(UITextView *)textView
{
    _UITextViewObserver *observer = [_UITextViewObserver new];
    [[NSNotificationCenter defaultCenter] addObserver:observer selector:@selector(update) name:UITextViewTextDidChangeNotification object:textView];
    for (NSString *key in [self.class observingKeys]) {
        [textView addObserver:observer forKeyPath:key options:NSKeyValueObservingOptionNew context:nil];
    }
    observer.textView = textView;
    return observer;
}

- (void)update
{
    [self.textView updatePlaceholderTextView];
}

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context
{
    [self update];
}

- (void)dealloc
{
    for (NSString *key in [self.class observingKeys]) {
        [self.textView removeObserver:self forKeyPath:key];
    }
    [[NSNotificationCenter defaultCenter] removeObserver:self];
}
@end