观察 UIView 的窗口和超级视图属性的变化

Question

我正在寻找一种在从可见视图层次结构中添加或删除通用 UIView 时收到通知的方法。 KVO 看起来是在这种情况下使用的完美工具，但观察视图窗口或超级视图属性的更改不会执行任何操作。对frame或backgroundColor等属性的更改按预期工作，但对与视图层次结构相关的属性的更改似乎从未调用observeValueForKeyPath。

我通过调用automaticallyNotifyObserversForKey来检查UIView是否支持这些属性上的KVO，并且UIView对这两个属性都报告为YES，这让我不知所措。所以我的问题是：

1）有没有办法使用 KVO 来通知与视图层次结构中添加/删除视图相关的事件？

2）如果没有，是否有另一种方法可以通知此类事件，而不涉及子类化 UIView？

Answer 1

重写此方法：

- (void)didMoveToSuperview
{
  UIView *superView = [self superview];
}

您可以在自定义视图中重写这些方法以供其他用途：

- (void)willMoveToSuperview:(UIView *)newSuperview;
- (void)didMoveToSuperview;
- (void)willMoveToWindow:(UIWindow *)newWindow;
- (void)didMoveToWindow;

Answer 2

这是一个方法。很恶心吗？是的。我推荐这种行为吗？不，但我们都是成年人了。

要点是，您使用 method_setImplementation 更改 -[UIView didAddSubview:] 的实现，以便每当调用它时您都会收到通知（并且您可以对 willRemoveSubview: 执行相同的操作>）。不幸的是，您将被要求更改所有视图层次结构。您必须添加自己的过滤才能找到您感兴趣的特定视图。

static void InstallAddSubviewListener(void (^listener)(id _self, UIView* subview))
{
    if ( listener == NULL )
    {
        NSLog(@"listener cannot be NULL.");
        return;
    }

    Method addSubviewMethod = class_getInstanceMethod([UIView class], @selector(didAddSubview:));
    IMP originalImp = method_getImplementation(addSubviewMethod);

    void (^block)(id, UIView*) = ^(id _self, UIView* subview) {
        originalImp(_self, @selector(didAddSubview:), subview);
        listener(_self, subview);
    };

    IMP newImp = imp_implementationWithBlock((__bridge void*)block);
    method_setImplementation(addSubviewMethod, newImp);
}

要使用，请执行以下操作：

InstallAddSubviewListener(^(id _self, UIView *subview) {
    NSLog(@"-[UIView didAddSubview:]   self=%@, view=%@", _self, subview);
});

Answer 3

基于 @doug-richardson 的代码，为什么不做一些更干净的东西来允许 KVO 的 superview 属性呢？

//Make views announce their change of superviews
    Method method = class_getInstanceMethod([UIView class], @selector(willMoveToSuperview:));
    IMP originalImp = method_getImplementation(method);

    void (^block)(id, UIView*) = ^(id _self, UIView* superview) {
        [_self willChangeValueForKey:@"superview"];
        originalImp(_self, @selector(willMoveToSuperview:), superview);
        [_self didChangeValueForKey:@"superview"];
    };

    IMP newImp = imp_implementationWithBlock((__bridge void*)block);
    method_setImplementation(method, newImp);

Answer 4

这是我的解决方案，使用上面的想法，修复了一些错误，并使其可扩展。您可以使用它来监视超级视图随某些类及其子类的 KVO 的变化，它应该是即插即用的。

用 C 语言编写，易于理解且快速。您可以将它们修改为 NSObject 的一些光滑类别。

使用示例：

add_superview_kvo(UILabel.class);

然后您必须按照正常使用将自己的观察者添加到实例中。

// simple datatype to neatly manage the various runtime elements
typedef struct {
    Class target;
    SEL cmd;
    Method method;
    IMP imp;
} override_t;

// call to initialize an override struct
static override_t
_override_init(Class target, SEL sel) {
    BOOL instance = YES; // should be able to handle class methods by changing this
    override_t o = {
        .target = target,
        .cmd = sel,
        // note this can return a method from the superclass
        .method = instance ? class_getInstanceMethod(target,sel) : class_getClassMethod(target,sel),
        .imp = method_getImplementation(o.method)
    };
    return o;
};

// applies the runtime patch
static void
_override_patch(override_t o, id _Nonnull block) {
    IMP imp = imp_implementationWithBlock(block);
    // first we try to add the method to the class, if we are
    // dealing with an inherited method from a superclass, our
    // new method will drop right in
    if (!class_addMethod(o.target, o.cmd, imp, method_getTypeEncoding(o.method))){
        // this means we got the original method from the
        // class we're manipulating, so we just overwrite
        // its version
        method_setImplementation(o.method,imp);
    }
}

// pass the class in here that you want to monitor for superview changes
// if you pass in UIView.class it will monitor all views... this may
// generate unnecessary overhead, so you can focus it on a class that
// you want (you will get that class and all its subclasses)
void
add_superview_kvo(Class target)
{
    NSString *keyPath = @"superview";

    override_t override = _override_init(target,@selector(willMoveToSuperview:));
    _override_patch(override,^void(id _self, UIView *superview) {
        [_self willChangeValueForKey:keyPath];
        // note that this is the correct way to call an imp, it must be cast
        ((void(*)(id,SEL,id))override.imp)(_self, override.cmd, superview);
    });

    override = _override_init(target,@selector(didMoveToSuperview));
    _override_patch(override,^void(id _self) {
        ((void(*)(id,SEL))override.imp)(_self, override.cmd);
        [_self didChangeValueForKey:keyPath];
    });
}