如何自定义 navigation controller 的 pop 转场动画？

今天要自定义从 ViewController A 到 ViewControllerB 的转场( segue )动画，这两个 controller 都在一个 UInavigationController 管理之下。有 A push 到 B 的专场做好了，但是发现怎么都拦截不到点击系统自带导航栏上返回键的事件来 prepare 我自己写的 unwind segue。Google 了半个小时，找到了一个网页，标题跟我的问题一样一样的，好开心啊。结果点进去就黑线了，所有的评论都说 build-in back 事件是拦截不到的...

“I've already wasted so much time on this that I am almost tempted to
go down the route of subclassing UINavigationController, trying to
trigger a segue manually on a pop.”

”Oh. So your question is not actually about segue unwinding. That's disappointing. :)“

“If you want a button in the UINavigationBar to do something other than "go back", don't use a back button.”

好吧，那按照大家说的把 back button 换掉。StackOverFlow 都说这么干,

UIBarButtonItem *backButtonItem = [[UIBarButtonItem alloc] initWithTitle:@"back" style:UIBarButtonItemStylePlain target:self action:@selector(backButtonPressed)];
self.navigationItem.backButtonBarItem = backButtonItem;

结果怎么都替换不掉 build-in back button，也没有的任何警告。试来试去，又半小时没了。

解决方案

正确的姿势是，将 build-in back 隐藏掉，然后添加一个 leftBarButtonItem，问题解决。

- (void)viewWillAppear:(BOOL)animated
{
    [super viewWillAppear:animated];
    
    [self.navigationItem setLeftBarButtonItem:[[UIBarButtonItem alloc] initWithTitle:@"Library" style:UIBarButtonItemStyleDone target:self action:@selector(onLibraryButtonClicked:)]];
    self.navigationItem.hidesBackButton = YES;
}


- (IBAction)onLibraryButtonClicked:(id)sender {
    [self performSegueWithIdentifier:@"segueUnwindFromMapToCollection" sender:self];
}

The Life Cycle of a Segue

好在折腾了这么久也对 segue 的 life time 有了更深的了解。Apple官方文档如下,

To understand how custom segues work, you need to understand the life cycle of a segue object. Segue objects are instances of UIStoryboardSegue or one of its subclasses. Your app never creates segue objects directly; they are always created on your behalf by iOS when a segue is triggered. Here’s what happens:

1. The destination controller is created and initialized.

• The segue object is created and its initWithIdentifier:source:destination: method is called. The identifier is the unique string you provided for the segue in Interface Builder, and the two other parameters represent the two controller objects in the transition.
• The source view controller’s prepareForSegue:sender: method is called. See Configuring the Destination Controller When a Segue is Triggered.
• The segue object’s perform method is called. This method performs a transition to bring the destination view controller on-screen.
• The reference to the segue object is released, causing it to be deallocated.

另外得到的两个结论：

1. custom segue perform 方法中，destinationViewController 的 viewDidLoad: 方法是在其 view 被加到 view tree 上之后被调用的。（通常在做动画时，会先将 destinationViewController.view 作为 subView 加在前一个 sourcdViewController.view 上去。）
2. custom segue perform 方法中，viewWillAppear: 是在 destinationViewController 被 push 之后调用的。

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