擷取鍵值監聽到的值

本文中我是想讓一個collectionview内容變化時高度跟随變化,第二步取值時是需要注意的

1.監聽

[selectedColl addObserver:self forKeyPath:@"contentSize" options:NSKeyValueObservingOptionNew context:nil];
           

2.實作 observeValueForKeyPath方法

- (void)observeValueForKeyPath:(NSString *)keyPath ofObject:(id)object change:(NSDictionary<NSKeyValueChangeKey,id> *)change context:(void *)context {
    NSLog(@"keyPath:%@ object:%@ change%@ context%@", keyPath, object, change, context);
    //NSConcreteValue
    id value = [object valueForKey:keyPath];
    NSAssert([value isKindOfClass:[NSValue class]],
             @"監聽到的value不是NSValue類型");
    NSAssert(strcmp(@encode(CGSize), [value objCType]) == 0,
             @"監聽到的value不是CGSize類型");
    //取出value
    CGSize unwrappedValue;
    [value getValue:&unwrappedValue];
    selectedH.constant = unwrappedValue.height;
}
           

取值方法來自于https://stackoverflow.com/questions/4615966/what-is-an-nsconcretevalue-and-how-do-i-extract-its-value

3.頁面銷毀時移除監聽

- (void)dealloc {
    [selectedColl removeObserver:self forKeyPath:@"contentSize"];
    NSLog(@"頁面銷毀");
}
           

完