Ios 为什么按钮不显示？

上面的代码不起作用，LeftBarButtonim不显示。但是当我在函数UIMapViewController.vieDidLoad中设置LeftBarButtonim时，LeftBarButtonim显示。为什么？ 流动代码位于UIMapViewController.vieDidLoad中

UIMapViewController *mapViewContr = [[[UIMapViewController alloc] initwithClientKey:clientKey] autorelease];
UINavigationController *nc = [[[UINavigationController alloc] initWithRootViewController:mapViewContr] autorelease];

UIButton *button = [UIButton buttonWithType:UIButtonTypeCustom];
button.frame = CGRectMake(0, 0, 44, 44);
[button setImage:[UIImage imageNamed:@"info2.png"] forState:UIControlStateNormal];
UIBarButtonItem *lbutton = [[[UIBarButtonItem alloc] initWithCustomView:button] autorelease];
nc.navigationItem.leftBarButtonItem = lbutton;

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在您调用的第一个代码块中：

    UIButton *_button = [UIButton buttonWithType:UIButtonTypeCustom];
    _button.frame = CGRectMake(0, 0, 44, 44);
    //    [button setBackgroundImage:[UIImage imageNamed:@"info2.png"] forState:UIControlStateNormal];
    [_button setImage:[UIImage imageNamed:@"info2.png"] forState:UIControlStateNormal];
    UIBarButtonItem *lbutton = [[[UIBarButtonItem alloc] initWithCustomView:_button] autorelease];
    self.navigationItem.leftBarButtonItem = lbutton;

这应该是：

nc.navigationItem.leftBarButtonItem = lbutton;

您总是希望引用视图控制器的

navigationItem

，而不是导航控制器

mapViewContr.navigationItem.leftBarButtonItem = lbutton;