一个iOS开发者的android修行之路一：UIViewCont

先说说自己学习安卓的一个初衷，在移动app开发的过程中，经常和android开发人员讨论一些技术的实现，有时候交流起来就特费劲，不是他听不懂我说的术语，就是他听不懂我讲的什么，反正就是有点语言不通，比如：iOS中的集合视图UITableView、UICollectionView中每个子项一般叫做cell，讨论的时候也叫做cell中干啥干啥，而安卓中的集合视图ListView,GridView,RecyclerView的每一个子项叫做item，这个还是小的差异；还有就是讨论某个UI界面数据刷新是什么时机进行刷新的，我说我们这边是在viewWillAppear的时候进行刷新，然后android那边就懵逼了，直到我换了一种说法，就是在这个页面即将显示的时候刷新数据，对方才听懂；而有时候我们听到对方的一些名词的时候，会想这Activity是什么东东？Fragment又是什么？于是我们会联想这对应到我们iOS这边相应的是什么。总之，诸如此类的听到对方说的一些名词自己云里雾里的。其实，一个更为重要的原因是想要成为移动开发的leader，你不懂一些android的开发，以及了解android和iOS的一些差异，你是没法胜任这个工作的。有时候，android遇到一些坑时，你必须知道如何高效的解决。如何和小组开发人员探讨某个问题的最佳实践。

Activity的生命周期

一个优秀app必须兼顾两个必要条件：1、优秀的界面，2、良好的用户交互体验。
在iOS中是由UIViewController主要负责加载界面呈现给用户，以及处理用户的交互逻辑；而安卓负责界面展示和交互功能就是Android系统四大组件之一的活动Activity。
1. 我们先看下iOS的UIViewController的生命周期函数：

class BaseViewController: UIViewController {

  // 前两个init方法是UIViewController的初始化方法
  // 非storyboard加载会调用此方法
  override init(nibName nibNameOrNil: String?, bundle nibBundleOrNil: Bundle?) {
    super.init(nibName: nibNameOrNil, bundle: nibBundleOrNil)
    print("\(self)\(#function)")
  }
  
  // 使用storyboard加载的UIViewController会调用此初始化方法
  required init?(coder aDecoder: NSCoder) {
    super.init(coder: aDecoder)
    print("\(self)\(#function)")
  }
  
  // 视图的@IBOutlet和@IBAction链接完成再次方法中
  override func awakeFromNib() {
    super.awakeFromNib()
    print("\(self)\(#function)")
  }
  
  // Creates the view that the controller manages.
  // 此方法一般调用的少
  override func loadView() {
    super.loadView()
    print("\(self)\(#function)")
  }
  
  // Called after the controller'��s view is loaded into memory.
  // 控制器视图加载完成（加载到了内存）的时候调用,只被调用一次，我们一般在这个方法中进行数据的网路请求，约束的设置，以及视图的一些操作等等。
  override func viewDidLoad() {
    super.viewDidLoad()
    print("\(self)\(#function)")
  }
  
  // MARK: -分割线----上面的方法只会调用一次，下面的方法除了deinit方法可能会调用多次
  
  // Notifies the view controller that its view is about to be added to a view hierarchy.
  // 视图即将显示的时候调用
  override func viewWillAppear(_ animated: Bool) {
    super.viewWillAppear(animated)
    print("\(self)\(#function)")
  }
  
  // Called to notify the view controller that its view is about to layout its subviews.
  // 控制器视图即将布局他的子视图时调用
  override func viewWillLayoutSubviews() {
    super.viewWillLayoutSubviews()
    print("\(self)\(#function)")
  }
  
  // Called to notify the view controller that its view has just laid out its subviews.
  // 控制器视图已经布局完其子视图后调用，在iphone X的适配就是在此方法中获取view.safeAreaInsets进行处理的，
  override func viewDidLayoutSubviews() {
    super.viewDidLayoutSubviews()
    print("\(self)\(#function)")
  }
  
  // Notifies the view controller that its view was added to a view hierarchy.
  // 试图已经完全显示的时候调用
  override func viewDidAppear(_ animated: Bool) {
    super.viewDidAppear(animated)
    print("\(self)\(#function)")
  }
  
  // Notifies the view controller that its view is about to be removed from a view hierarchy.
  // 视图即将被移除的时候调用
  override func viewWillDisappear(_ animated: Bool) {
    super.viewWillDisappear(animated)
    print("\(self)\(#function)")
  }
  
  // Notifies the view controller that its view was removed from a view hierarchy.
  // 视图已经被完全移除的时候调用
  override func viewDidDisappear(_ animated: Bool) {
    super.viewDidAppear(animated)
    print("\(self)\(#function)")
  }
  
  // Sent to the view controller when the app receives a memory warning.
  // 手机收到内存警告的时候调用
  override func didReceiveMemoryWarning() {
    super.didReceiveMemoryWarning()
    print("\(self)\(#function)")
  }
  
  // 视图被销毁的时候调用，在这里可以执行的操作有：KVO移除，通知的移除，定时器的销毁
  deinit {
    print("\(self)\(#function)")
  }

}

下面是从ViewController跳转到SecondViewControler在跳转到ThirdController控制台的log：

<LifecycleDemo.ViewController: 0x7fd122d0b5d0>init(coder:)
<LifecycleDemo.ViewController: 0x7fd122d0b5d0>awakeFromNib()
<LifecycleDemo.ViewController: 0x7fd122d0b5d0>loadView()
<LifecycleDemo.ViewController: 0x7fd122d0b5d0>viewDidLoad()
<LifecycleDemo.ViewController: 0x7fd122d0b5d0>viewWillAppear
<LifecycleDemo.ViewController: 0x7fd122d0b5d0>viewWillLayoutSubviews()
<LifecycleDemo.ViewController: 0x7fd122d0b5d0>viewDidLayoutSubviews()
<LifecycleDemo.ViewController: 0x7fd122d0b5d0>viewWillLayoutSubviews()
<LifecycleDemo.ViewController: 0x7fd122d0b5d0>viewDidLayoutSubviews()
<LifecycleDemo.ViewController: 0x7fd122d0b5d0>viewDidAppear
<LifecycleDemo.SecondViewController: 0x7fd122c16480>init(coder:)
<LifecycleDemo.SecondViewController: 0x7fd122c16480>awakeFromNib()
<LifecycleDemo.SecondViewController: 0x7fd122c16480>loadView()
<LifecycleDemo.SecondViewController: 0x7fd122c16480>viewDidLoad()
<LifecycleDemo.ViewController: 0x7fd122d0b5d0>viewWillDisappear
<LifecycleDemo.SecondViewController: 0x7fd122c16480>viewWillAppear
<LifecycleDemo.SecondViewController: 0x7fd122c16480>viewWillLayoutSubviews()
<LifecycleDemo.SecondViewController: 0x7fd122c16480>viewDidLayoutSubviews()
<LifecycleDemo.SecondViewController: 0x7fd122c16480>viewWillLayoutSubviews()
<LifecycleDemo.SecondViewController: 0x7fd122c16480>viewDidLayoutSubviews()
<LifecycleDemo.ViewController: 0x7fd122d0b5d0>viewDidDisappear
<LifecycleDemo.SecondViewController: 0x7fd122c16480>viewDidAppear
<LifecycleDemo.ThirdViewController: 0x7fd122c2ff00>init(nibName:bundle:)
<LifecycleDemo.ThirdViewController: 0x7fd122c2ff00>loadView()
<LifecycleDemo.ThirdViewController: 0x7fd122c2ff00>viewDidLoad()
<LifecycleDemo.SecondViewController: 0x7fd122c16480>viewWillDisappear
<LifecycleDemo.ThirdViewController: 0x7fd122c2ff00>viewWillAppear
<LifecycleDemo.ThirdViewController: 0x7fd122c2ff00>viewWillLayoutSubviews()
<LifecycleDemo.ThirdViewController: 0x7fd122c2ff00>viewDidLayoutSubviews()
<LifecycleDemo.ThirdViewController: 0x7fd122c2ff00>viewWillLayoutSubviews()
<LifecycleDemo.ThirdViewController: 0x7fd122c2ff00>viewDidLayoutSubviews()
<LifecycleDemo.SecondViewController: 0x7fd122c16480>viewDidDisappear
<LifecycleDemo.ThirdViewController: 0x7fd122c2ff00>viewDidAppear
<LifecycleDemo.ThirdViewController: 0x7fd122c2ff00>viewWillDisappear
<LifecycleDemo.SecondViewController: 0x7fd122c16480>viewWillAppear
<LifecycleDemo.ThirdViewController: 0x7fd122c2ff00>viewDidDisappear
<LifecycleDemo.SecondViewController: 0x7fd122c16480>viewDidAppear
<LifecycleDemo.ThirdViewController: 0x7fd122c2ff00>deinit
<LifecycleDemo.SecondViewController: 0x7fd122c16480>viewWillDisappear
<LifecycleDemo.ViewController: 0x7fd122d0b5d0>viewWillAppear
<LifecycleDemo.SecondViewController: 0x7fd122c16480>viewDidDisappear
<LifecycleDemo.ViewController: 0x7fd122d0b5d0>viewDidAppear
<LifecycleDemo.SecondViewController: 0x7fd122c16480>deinit

2. 我们再来看下android的Activity的生命周期函数：

public class BaseActivity extends AppCompatActivity {

    String str = this.getClass().getName();

    /*活动第一次被创建的时候调用，布局的加载，事件的绑定可以在这里完成,此时活动还是不可见的*/
    @Override
    public void onCreate(Bundle savedInstanceState)  {
        super.onCreate(savedInstanceState);
        Log.i(str,"onCreate");
    }

    /*活动由不可见变为可见的时候调用*/
    @Override
    protected void onStart() {
        super.onStart();
        Log.i(str,"onStart");
    }

    /*活动已经完全可见，并且可以与用户交互，活动处于返回栈的栈顶*/
    @Override
    protected void onResume() {
        super.onResume();
        Log.i(str,"onResume");
    }

    /*在系统准备启动或者恢复一个activity时调用*/
    @Override
    protected void onPause() {
        super.onPause();
        Log.i(str,"onPause");
    }

    /*当activity 完全不可见的时候调用*/
    @Override
    protected void onStop() {
        super.onStop();
        Log.i(str,"onStop");
    }

    /*活动被重新启动的时候调用*/
    @Override
    protected void onRestart() {
        super.onRestart();
        Log.i(str,"onRestart");
    }

    /*acitivity被销毁前调用，调用之后，activity变为销毁状态*/
    @Override
    protected void onDestroy() {
        super.onDestroy();
        Log.i(str,"onDestroy");
    }

}

下面是一个MainActivity跳转到SecondActivity，然后在回来的生命函数调用在控制台输出的log：

I/com.example.xcqromance.lifecycledemo.MainActivity: onCreate
I/com.example.xcqromance.lifecycledemo.MainActivity: onStart
I/com.example.xcqromance.lifecycledemo.MainActivity: onResume
I/com.example.xcqromance.lifecycledemo.MainActivity: onPause
I/com.example.xcqromance.lifecycledemo.SecondActivity: onCreate
I/com.example.xcqromance.lifecycledemo.SecondActivity: onStart
I/com.example.xcqromance.lifecycledemo.SecondActivity: onResume
I/com.example.xcqromance.lifecycledemo.MainActivity: onStop
I/com.example.xcqromance.lifecycledemo.SecondActivity: onPause
I/com.example.xcqromance.lifecycledemo.MainActivity: onRestart
I/com.example.xcqromance.lifecycledemo.MainActivity: onStart
I/com.example.xcqromance.lifecycledemo.MainActivity: onResume
I/com.example.xcqromance.lifecycledemo.SecondActivity: onStop
I/com.example.xcqromance.lifecycledemo.SecondActivity: onDestroy

通过上面的对比，我整理出iOS和android的生命周期函数有及其相似的地方和不同之处： iOS和android生命周期函数对比图

GithubDemo地址，里面有上述操作的代码，文章写的有点粗糙，同时欢迎大家一切探讨。一起添加iOS和android的生命周期的异同。