uikit——UIView——view hierarchy

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view hierarchy

@property(nullable, nonatomic,readonly) UIView       *superview;@property(nonatomic,readonly,copy) NSArray<__kindof UIView *> *subviews;@property(nullable, nonatomic,readonly) UIWindow     *window;- (void)removeFromSuperview;- (void)insertSubview:(UIView *)view atIndex:(NSInteger)index;- (void)exchangeSubviewAtIndex:(NSInteger)index1 withSubviewAtIndex:(NSInteger)index2;- (void)addSubview:(UIView *)view;- (void)insertSubview:(UIView *)view belowSubview:(UIView *)siblingSubview;- (void)insertSubview:(UIView *)view aboveSubview:(UIView *)siblingSubview;- (void)bringSubviewToFront:(UIView *)view;- (void)sendSubviewToBack:(UIView *)view;- (BOOL)isDescendantOfView:(UIView *)view;  // returns YES for self.

解释：

view hierarchy本质是tree
superview：view parent
subviews：view children，添加subview会进行判断，nil不会添加，同一subview不会重复添加
window：view所在view hierarchy(tree) root，即view controller.view.superview

view hierarchy图层

superview位于下层，subviews位于上层
同一subviews，索引小的位于下层，索引大的位于上层，索引也称为view z-index或z-order

- (void)drawView{    int width = self.view.bounds.size.width;    int halfHeight = self.view.bounds.size.height / 2;        {        UIView *view = [[UIView alloc] initWithFrame:CGRectMake(0, 0, width, halfHeight)];        UIView *view1 = [[UIView alloc] initWithFrame:CGRectMake(40, 30, 200, 200)];        UIView *view2 = [[UIView alloc] initWithFrame:CGRectMake(80, 60, 200, 200)];        UIView *view1_1 = [[UIView alloc] initWithFrame:CGRectMake(50, 50, 80, 80)];        UIView *view1_2 = [[UIView alloc] initWithFrame:CGRectMake(100, 100, 80, 80)];                view.backgroundColor = [UIColor purpleColor];        view1.backgroundColor = [UIColor blueColor];        view2.backgroundColor = [UIColor orangeColor];        view1_1.backgroundColor = [UIColor redColor];        view1_2.backgroundColor = [UIColor cyanColor];                [self.view addSubview:view];        [view addSubview:view1];        [view addSubview:view2];        [view1 addSubview:view1_1];        [view1 addSubview:view1_2];    }        {        UIView *view = [[UIView alloc] initWithFrame:CGRectMake(0, halfHeight, width, halfHeight)];        UIView *view1 = [[UIView alloc] initWithFrame:CGRectMake(40, 30, 200, 200)];        UIView *view2 = [[UIView alloc] initWithFrame:CGRectMake(80, 60, 200, 200)];        UIView *view1_1 = [[UIView alloc] initWithFrame:CGRectMake(50, 50, 80, 80)];        UIView *view1_2 = [[UIView alloc] initWithFrame:CGRectMake(100, 100, 80, 80)];                view.backgroundColor = [UIColor brownColor];        view1.backgroundColor = [UIColor blueColor];        view2.backgroundColor = [UIColor orangeColor];        view1_1.backgroundColor = [UIColor redColor];        view1_2.backgroundColor = [UIColor cyanColor];                [self.view addSubview:view];        [view addSubview:view2];        [view addSubview:view1];        [view1 addSubview:view1_1];        [view1 addSubview:view1_2];    }}

view hierarchy遍历

view hierarchy指以receiver为root view hierarchy，view hierarchy先根遍历

通常情况下，遍历子树按从左到右顺利遍历(subview index从小到大)，用代码表示：

- (void)traverse{    [self visit];        for(int i = 0; i < self.subviews.count; ++i)    {        UIView *view = [self.subviews objectAtIndex:i];        [view traverse];    }}

特殊情况下，为了性能优化，遍历子树按从右到左顺利遍历(subview index从大到小)，用代码表示：

- (void)traverse{    [self visit];        for(int i = self.subviews.count - 1; i >= 0; --i)    {        UIView *view = [self.subviews objectAtIndex:i];        [view traverse];    }}

view的draw(layout)为了性能优化，遍历子树按从右到左顺利遍历(subview index从大到小)，先draw(layout)上层图层，然后draw(layout)下层图层，如果下层图层区域被上层图层不透明覆盖，则不必draw(layout)这些区域，极大提高了draw(layout)性能

view hierarchy crash

view hierarchy本质为tree，因此切不可形成闭环(循环)，一旦形成闭环(循环)，就破坏tree结构，先根遍历无穷递归，crash

- (void)circleCrash{    UIView *view1 = [[UIView alloc] initWithFrame:CGRectMake(50, 50, 200, 200)];    UIView *view2 = [[UIView alloc] initWithFrame:CGRectMake(100, 100, 200, 200)];    UIView *subView = [[UIView alloc] initWithFrame:CGRectMake(100, 100, 100, 100)];        [self.view addSubview:view1];    [self.view addSubview:view2];    [view1 addSubview:subView];    [subView addSubview:self.view];}

self.view->view1->subview->self.view形成闭环(循环)

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