Question

我正在尝试为UIActivityIndicatorView创建一个类类别 - 我想在屏幕中心设置它。

所以我宣布：

@implementation UIActivityIndicatorView(Normalize)

-(UIActivityIndicatorView *) setAtScreenCenter{
    CGRect r = [UIScreen mainScreen].applicationFrame;
    CGRect wheelR = CGRectMake(r.size.width / 2 - 12, r.size.height / 2 - 12, 24, 24);
    self = [[UIActivityIndicatorView alloc] initWithFrame:wheelR];
    self.activityIndicatorViewStyle = UIActivityIndicatorViewStyleWhite;
    self.autoresizingMask = (UIViewAutoresizingFlexibleLeftMargin |
                                      UIViewAutoresizingFlexibleRightMargin |
                                      UIViewAutoresizingFlexibleTopMargin |
                                      UIViewAutoresizingFlexibleBottomMargin);
    return self;
}
@end

使用方法： [activityWheel setAtScreenCenter];

但是，我收到了编译错误：

Cannot assign to 'self' outside of a method in the init family

Answer 1

您的setAtScreenCenter是一条实例消息。您必须将其发送到现有UIActivityIndicatorView。为什么要尝试在此方法中创建 new UIActivityIndicatorView？

请改为尝试：

self.frame = wheelR;

您可能对坐标如何工作感到困惑。视图的框架与其superview的坐标系相关。它与屏幕坐标系无关。即使您的超级视图是全屏的，它的坐标系也会与屏幕的坐标系不同，除了一个方向。您可以像这样转换坐标：

CGRect frame = [UIScreen mainScreen].applicationFrame;
CGPoint p = CGPointMake(CGRectGetMidX(frame), CGRectGetMidY(frame));
p = [self.window convertPoint:p fromWindow:nil]; // convert screen -> window
p = [self.superview convertPoint:p fromView:nil]; // convert window -> my superview
self.center = p;

Answer 2

警告不仅仅是迂腐，编译器实际上是在拯救你自己。您应该声明要返回的新UIActivityIndicator var，而不是尝试分配给self。该方法的更好版本如下所示：

+(UIActivityIndicatorView *)setAtScreenCenter{
    CGRect r = [UIScreen mainScreen].applicationFrame;
    CGRect wheelR = CGRectMake(r.size.width / 2 - 12, r.size.height / 2 - 12, 24, 24);
    UIActivityIndicatorView *indicator = [[UIActivityIndicatorView alloc] initWithFrame:wheelR];
    indicator.activityIndicatorViewStyle = UIActivityIndicatorViewStyleWhite;
    indicator.autoresizingMask = (UIViewAutoresizingFlexibleLeftMargin |
                                  UIViewAutoresizingFlexibleRightMargin |
                                  UIViewAutoresizingFlexibleTopMargin |
                                  UIViewAutoresizingFlexibleBottomMargin);
    return indicator;
}

作为一个类方法，我们不再需要处理赋值给self，并且始终可以保证满足方法的返回类型（如果这是一个真正的初始化器，它将返回id更容易的子类化。）

Answer 3

这不是一个真正的init方法。因此，你不应该在这里指定自己。只需跳过该行并使该方法成为void方法（无返回值）。

@implementation UIActivityIndicatorView(Normalize)

-(void) setAtScreenCenter{
    CGRect r = [UIScreen mainScreen].applicationFrame;
    CGRect wheelR = CGRectMake(r.size.width / 2 - 12, r.size.height / 2 - 12, 24, 24);
    self.frame = wheelR;
    self.activityIndicatorViewStyle = UIActivityIndicatorViewStyleWhite;
    self.autoresizingMask = (UIViewAutoresizingFlexibleLeftMargin |
                                      UIViewAutoresizingFlexibleRightMargin |
                                      UIViewAutoresizingFlexibleTopMargin |
                                      UIViewAutoresizingFlexibleBottomMargin);

}
@end

现在您可以在现有的UIActivityIndicatorView上调用它来设置它想要的位置：

[activityWheel setAtScreenCenter];

使用init方法创建类类别

3 个答案: