iOS 转场时 appear 与 disappear 的调用顺序

原文地址 Cyrus' blog

最近几天看了好多人的面经，时不时就看到一个问题：

在A页面跳转到B页面的过程中 A 的 viewDidDisappear 方法和 B 的 viewDidAppear 谁先调用？

其实我以前一直理所当然的认为它们的顺序就是 A-viewDidDisappear --> B-viewDidAppear，至于为什么会这么想，就是感觉

今天写了个Demo去测试了一下，发现完全不是自己想象的那样，每一种转场(Push,Present，TabBar切换)都是不一样的。

Demo 地址 Appear&Disappear-Sample

测试

---

我自定义了两个类，继承自UIViewController，为了方便观察分别叫做A_ViewController,B_ViewController，然后重写了一系列 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);
}

然后分别用 push， present， 以及 tabbar 的切换来测试，然后查看打印的结果。

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>
 

在这里的调用顺序是和我之前预想的一样的，也就是

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

Present & Dismiss

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在模态的转场中就有一些不一样的事了。我们知道模态转场有好多种style，其中有两种我们需要特别注意的UIModalPresentationCustom,UIModalPresentationOverFullScreen。我们先看一下除了这两种style以外的转场，他们的调用顺序是怎样的。

-----------------------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>

这里其实已经和push的转场不一样了。普通模态的转场的顺序是这样的

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

再看看前面提到的那两种特殊的模态转场。因为在这两种方式的转场下A的view是不会消失的，所以所有与A有关的 appear 或 disappear 方法都不会被调用

-----------------------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>

所以在这两种style下只有B-willAppear --> B-didAppear

TabBar切换

---

TabBar 的切换也是一个很神奇的顺序

 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>

与push不同的是，TabBar下，会先调用 B 的 willAppear，再调用 A 的 willDisappear

所以TabBar的顺序是这样的：

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

总结

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在每一种转场下，appear 与 disappear 都有一些不一样的顺序，一定要分清楚，不能一概而论。

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

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为什么要把这个过程弄的这么复杂？目前还没有想法，如果有知道的朋友，希望告知。