UINavigationItem BackButtonItem not displayed

Question

使用下面的代碼我正在實現我自己的導航欄。出於某種原因，當我運行我的應用程序時，導航欄上沒有顯示後退（左箭頭）按鈕。但是，如果我將代碼更改爲leftBarButtonItem，則按鈕確實會出現。

// Draw Navigation Bar 
UINavigationBar *navigationBar = [[UINavigationBar alloc] initWithFrame:CGRectMake(0, 0, 320, 44)]; 
[navigationBar setDelegate:self]; 

UINavigationItem *navigationItem = [[UINavigationItem alloc] init]; 

UIBarButtonItem *backButton = [[UIBarButtonItem alloc] initWithTitle:@"Back" 
                   style:UIBarButtonItemStylePlain 
                   target:nil 
                   action:nil]; 
navigationItem.backBarButtonItem = backButton; 
[navigationBar pushNavigationItem:navigationItem animated:NO]; 
[self.view addSubview:navigationBar]; 
[navigationBar release]; 
[backButton release]; 

Answer 1

想通了：

// Draw Navigation Bar 
UINavigationBar *navigationBar = [[UINavigationBar alloc] initWithFrame:CGRectMake(0, 0, 320, 44)]; 
[navigationBar setDelegate:self]; 

UINavigationItem *navigationItem = [[UINavigationItem alloc] init]; 

UIButton *button = [UIButton buttonWithType:101]; 
// add selector 
[button setTitle:@"Back" forState:UIControlStateNormal]; 

UIBarButtonItem *backButton = [[UIBarButtonItem alloc] initWithCustomView:button]; 

navigationItem.leftBarButtonItem = backButton; 
[navigationBar pushNavigationItem:navigationItem animated:NO]; 
[self.view addSubview:navigationBar]; 
[navigationBar release]; 
[backButton release]; 

Answer 2

backBarButtonItem由父級ViewController設置。 換句話說，它不是由您看到它的ViewController設置的，而是由它指向的ViewController設置的。所以如果你的ViewController是第一個，它就不會有後退按鈕。

此外，你正在自己創建一個NavigtionBar。這通常不是要走的路，UINavigationBar不是像按鈕或標籤那樣的UI元素。您應該使用UINavigationController來處理ViewControllers的所有推送和彈出。