IOS UIView的生命周期的實例詳解
任何對象的者有一個生命周期，即都存在一個實例化到銷毀的過程。
UIView對象也不例外，那么UIView從init/new開始后，直到dealloc結束的過程中都經歷了哪些過程呢？
首先自定義繼承自UIView的對象LifeView
#import <UIKit/UIKit.h> @interface LifeView : UIView @end
#import "LifeView.h" @interface LifeView () { NSInteger count; } @end @implementation LifeView - (instancetype)initWithFrame:(CGRect)frame { self = [super initWithFrame:frame]; if (self) { NSLog(@"<-- 1 %s , count = %@-->", __func__, @(count++)); } return self; } - (void)willMoveToSuperview:(nullable UIView *)newSuperview { NSLog(@"<-- 2 %s , count = %@-->", __func__, @(count++)); } - (void)didMoveToSuperview { NSLog(@"<-- 3 %s , count = %@-->", __func__, @(count++)); } - (void)willMoveToWindow:(nullable UIWindow *)newWindow { NSLog(@"<-- 4/7 %s , count = %@-->", __func__, @(count++)); } - (void)didMoveToWindow { NSLog(@"<-- 5/8 %s , count = %@-->", __func__, @(count++)); } - (void)layoutSubviews { NSLog(@"<-- 6 %s , count = %@-->", __func__, @(count++)); } - (void)removeFromSuperview { NSLog(@"<-- 9 %s , count = %@-->", __func__, @(count++)); } - (void)dealloc { NSLog(@"<-- 10 %s , count = %@-->", __func__, @(count++)); } @end 其次，在B視圖控制器中實例化，并添加到父視圖
- (void)viewDidLoad { [super viewDidLoad]; // Do any additional setup after loading the view. self.title = @"life view"; LifeView *lifeView = [[LifeView alloc] initWithFrame:CGRectMake(10.0, 80.0, 100.0, 100.0)]; [self.view addSubview:lifeView]; lifeView.tag = 1000; lifeView.backgroundColor = [UIColor orangeColor]; } 當B視圖控制器被push，或present出來時，被調用的LifeView的相關方法，如下所示：
// 實例化時 2017-06-16 00:37:10.694 DemoViewLife[3963:121184] <-- 1 -[LifeView initWithFrame:] , count = 0--> 2017-06-16 00:37:10.695 DemoViewLife[3963:121184] <-- 2 -[LifeView willMoveToSuperview:] , count = 1--> 2017-06-16 00:37:10.695 DemoViewLife[3963:121184] <-- 3 -[LifeView didMoveToSuperview] , count = 2--> 2017-06-16 00:37:10.697 DemoViewLife[3963:121184] <-- 4/7 -[LifeView willMoveToWindow:] , count = 3--> 2017-06-16 00:37:10.697 DemoViewLife[3963:121184] <-- 5/8 -[LifeView didMoveToWindow] , count = 4--> 2017-06-16 00:37:10.701 DemoViewLife[3963:121184] <-- 6 -[LifeView layoutSubviews] , count = 5-->
當B視圖控制器被pop，或dismiss時，被調用的LifeView的相關方法，如下所示：
// 銷毀時 2017-06-16 00:37:25.514 DemoViewLife[3963:121184] <-- 4/7 -[LifeView willMoveToWindow:] , count = 6--> 2017-06-16 00:37:25.514 DemoViewLife[3963:121184] <-- 5/8 -[LifeView didMoveToWindow] , count = 7--> 2017-06-16 00:37:25.515 DemoViewLife[3963:121184] <-- 9 -[LifeView removeFromSuperview] , count = 8--> 2017-06-16 00:37:25.516 DemoViewLife[3963:121184] <-- 9 -[LifeView removeFromSuperview] , count = 9--> 2017-06-16 00:37:25.518 DemoViewLife[3963:121184] <-- 10 -[LifeView dealloc] , count = 10-->
從打印信息可以看到方法"removeFromSuperview"被調用了兩次，這是因為在B視圖控制器中，主動調用了該方法。如果沒有主動調用該方法的話，只會被調用一次。
另外被執行過兩次的方法還有"willMoveToWindow"和"didMoveToWindow"。我們可以很好地進行區分，當newWindow存在時，則是實例化創建被第一次調用；而當newWindow為nil時，則是被銷毀了，即結束生命周期。
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