原文地址 Cyrus' blog

最近幾天看了好多人的面經，時不時就看到一個問題：

在A頁面跳轉到B頁面的過程中 A 的 viewDidDisappear 方法和 B 的 viewDidAppear 誰先調用？

其實我以前一直理所當然的認為它們的順序就是 A-viewDidDisappear --> B-viewDidAppear，至于為什么會這么想，就是感覺

今天寫了個Demo去測試了一下，發現完全不是自己想象的那樣，每一種轉場(Push,Present，TabBar切換)都是不一樣的。

Demo 地址 Appear&Disappear-Sample

測試

我自定義了兩個類，繼承自UIViewController，為了方便觀察分別叫做A_ViewController,B_ViewController，然后重寫了一系列 appear 和 disappear 方法。

- (void)viewWillAppear:(BOOL)animated {
    [super viewWillAppear:animated];
    NSLog(@"A  view will appear    %@",self);
}

- (void)viewDidAppear:(BOOL)animated {
    [super viewDidAppear:animated];
    NSLog(@"A  view did  appear    %@",self);
}

- (void)viewWillDisappear:(BOOL)animated {
    [super viewWillDisappear:animated];
    NSLog(@"A  view will disappear %@",self);
}

- (void)viewDidDisappear:(BOOL)animated {
    [super viewDidDisappear: animated];
    NSLog(@"A  view did  disappear %@",self);
}

然后分別用 push， present， 以及 tabbar 的切換來測試，然后查看打印的結果。

Push & Pop

--------------------------PUSH-----------------------------

 A  view will disappear <A_ViewController: 0x7fa11067a730>
 B  view will appear    <B_ViewController: 0x7fa110516530>
 A  view did  disappear <A_ViewController: 0x7fa11067a730>
 B  view did  appear    <B_ViewController: 0x7fa110516530>

--------------------------POP-----------------------------

 B  view will disappear <B_ViewController: 0x7fa110516530>
 A  view will appear    <A_ViewController: 0x7fa11067a730>
 B  view did  disappear <B_ViewController: 0x7fa110516530>
 A  view did  appear    <A_ViewController: 0x7fa11067a730>
 

在這里的調用順序是和我之前預想的一樣的，也就是

A-willDisappear --> B-willAppear --> A-didDisappear --> B-didAppear

Present & Dismiss

在模態的轉場中就有一些不一樣的事了。我們知道模態轉場有好多種style，其中有兩種我們需要特別注意的UIModalPresentationCustom,UIModalPresentationOverFullScreen。我們先看一下除了這兩種style以外的轉場，他們的調用順序是怎樣的。

-----------------------NormalPresent------------------------

 A  view will disappear <A_ViewController: 0x7fc2a14ac490>
 B  view will appear    <B_ViewController: 0x7fc2a152a880>
 B  view did  appear    <B_ViewController: 0x7fc2a152a880>
 A  view did  disappear <A_ViewController: 0x7fc2a14ac490>

-------------------------DISMISS----------------------------

 B  view will disappear <B_ViewController: 0x7fc2a152a880>
 A  view will appear    <A_ViewController: 0x7fc2a14ac490>
 A  view did  appear    <A_ViewController: 0x7fc2a14ac490>
 B  view did  disappear <B_ViewController: 0x7fc2a152a880>

這里其實已經和push的轉場不一樣了。普通模態的轉場的順序是這樣的

A-willDisappear --> B-willAppear --> B-didAppear --> A-didDisappear

再看看前面提到的那兩種特殊的模態轉場。因為在這兩種方式的轉場下A的view是不會消失的，所以所有與A有關的 appear 或 disappear 方法都不會被調用

-----------------------CustomPresent------------------------

 B  view will appear    <B_ViewController: 0x7f960249bac0>
 B  view did  appear    <B_ViewController: 0x7f960249bac0>

-------------------------DISMISS----------------------------

 B  view will disappear <B_ViewController: 0x7f960249bac0>
 B  view did  disappear <B_ViewController: 0x7f960249bac0>

所以在這兩種style下只有B-willAppear --> B-didAppear

TabBar切換

TabBar 的切換也是一個很神奇的順序

 B  view will appear    <B_ViewController: 0x7f88cbed7160>
 A  view will disappear <A_ViewController: 0x7f88cbed6680>
 A  view did  disappear <A_ViewController: 0x7f88cbed6680>
 B  view did  appear    <B_ViewController: 0x7f88cbed7160>

與push不同的是，TabBar下，會先調用 B 的 willAppear，再調用 A 的 willDisappear

所以TabBar的順序是這樣的：

B-willAppear --> A-willDisappear --> A-didDisappear --> B-didAppear

總結

在每一種轉場下，appear 與 disappear 都有一些不一樣的順序，一定要分清楚，不能一概而論。

Push： A-willDisappear-->B-willAppear-->A-didDisappear-->B-didAppear

Present： A-willDisappear-->B-willAppear-->B-didAppear-->A-didDisappear

CustomPresent： B-willAppear-->B-didAppear

TabBar： B-willAppear-->A-willDisappear-->A-didDisappear-->B-didAppear

疑惑