解析LayoutSubviews

从百度上搜索了一下layoutSubviews的用处,以下是搜索的结果,当然,笔者是会一一验证的.

1、init初始化不会触发layoutSubviews

2、addSubview会触发layoutSubviews

3、设置view的Frame会触发layoutSubviews，当然前提是frame的值设置前后发生了变化

4、滚动一个UIScrollView会触发layoutSubviews

5、旋转Screen会触发父UIView上的layoutSubviews事件

6、改变一个UIView大小的时候也会触发父UIView上的layoutSubviews事件

在开始验证之前,先看看layoutSubviews到底是啥来着:)

Lays out subviews.
The default implementation of this method does nothing on iOS 5.1 and earlier. Otherwise, the default implementation uses any constraints you have set to determine the size and position of any subviews.

在iOS5.1或之前的版本中,这个方法什么也没干.这个方法的默认实现是用参数来设定subviews的尺寸和位置的.

Subclasses can override this method as needed to perform more precise layout of their subviews. You should override this method only if the autoresizing and constraint-based behaviors of the subviews do not offer the behavior you want. You can use your implementation to set the frame rectangles of your subviews directly.

如果你需要更加精确的布局,可以在子类里面重写这个方法.仅仅在以下情况下:自动布局达不到你想要效果时你才有必要重写这个方法.你可以直接设置subviews的尺寸.

You should not call this method directly. If you want to force a layout update, call the setNeedsLayout method instead to do so prior to the next drawing update. If you want to update the layout of your views immediately, call the layoutIfNeeded method.

你不能直接调用这个方法.如果你需要强制layout刷新,调用setNeedsLayout来代替.如果你想要立即刷新你的view,调用layoutIfNeeded

大概总结以下就是:

你不要直接调用方法layoutSubviews,如果想要刷新,请调用setNeedsLayout或者layoutIfNeeded

好了,开始验证:)

现在提供继承至UIView的类如下:

//
//  TestView.h
//  LayoutSubviews
//
//  Copyright (c) 2014年 Y.X. All rights reserved.
//

#import <UIKit/UIKit.h>

@interface TestView : UIView

@end

//
//  TestView.m
//  LayoutSubviews
//
//  Copyright (c) 2014年 Y.X. All rights reserved.
//

#import "TestView.h"

@implementation TestView

- (id)initWithFrame:(CGRect)frame
{
    self = [super initWithFrame:frame];
    if (self)
    {
        NSLog(@"initWithFrame:%@" ,NSStringFromCGRect(frame));
    }
    return self;
}

- (void)layoutSubviews
{
    NSLog(@"layoutSubviews %@", self);
    [super layoutSubviews];
}

@end

测试代码:

//
//  RootViewController.m
//  LayoutSubviews
//
//  Copyright (c) 2014年 Y.X. All rights reserved.
//

#import "RootViewController.h"
#import "TestView.h"

@interface RootViewController ()

@property (nonatomic, strong) NSTimer   *timer;
@property (nonatomic, strong) TestView  *largeView;
@property (nonatomic, strong) TestView  *smallView;

@end

@implementation RootViewController

- (void)viewDidLoad
{
    [super viewDidLoad];

    // 1、init初始化不会触发layoutSubviews [正确的]
    // 2、addSubview会触发layoutSubviews [不完全正确,当frame为0时是不会触发的]
    // 3、设置view的Frame会触发layoutSubviews，当然前提是frame的值设置前后发生了变化 [正确]

//    [self test_1];
//    [self test_2];
//    [self test_3];

    // 4、滚动一个UIScrollView会触发layoutSubviews[错误,不用滚动就会触发]
//    [self test_4];

    // 5、改变一个UIView大小的时候也会触发父UIView上的layoutSubviews事件
    [self test_5];
}

- (void)test_1
{
    /*
     解释:

     走了initWithFrame:方法,但是又有frame值为{{0, 0}, {0, 0}},并不需要绘制任何的东西,
     所以即使添加了test,也没必要绘制它,同时也验证了addSubview会触发layoutSubviews是错
     误的,只有当被添加的view有着尺寸的时候才会触发layoutSubviews
     */

    TestView *test = [TestView new];
    [self.view addSubview:test];
}

- (void)test_2
{
    TestView *test = [TestView new];
    test.frame = CGRectMake(0, 0, 100, 100);
    [self.view addSubview:test];
}

- (void)test_3
{
    /*
     解释:

     layoutSubviews这个方法自身无法调用,是被父类添加的时候才执行的方法
     */

    TestView *test = [TestView new];
    test.frame = CGRectMake(0, 0, 50, 50);
    UIView *showView = [[UIView alloc] initWithFrame:CGRectMake(0, 0, 50, 50)];
    [test addSubview:showView];
}

- (void)test_4
{
    CGRect rect    = self.view.bounds;
    CGFloat height = rect.size.height;
    CGFloat width  = rect.size.width;

    UIScrollView *rootScroll = [[UIScrollView alloc] initWithFrame:self.view.bounds];
    NSArray *data            = @[@"", @"", @"", @""];
    [data enumerateObjectsUsingBlock:^(id obj, NSUInteger idx, BOOL *stop) {
        TestView *tmp        = [[TestView alloc] initWithFrame:CGRectMake(width*idx, 0,
                                                                          width, height)];
        [rootScroll addSubview:tmp];
    }];
    rootScroll.contentSize   = CGSizeMake(width * data.count, height);
    [self.view addSubview:rootScroll];
}

- (void)test_5
{
    _timer = [NSTimer scheduledTimerWithTimeInterval:1.f
                                              target:self
                                            selector:@selector(timerEvent:)
                                            userInfo:nil
                                             repeats:YES];
    _largeView = [[TestView alloc] initWithFrame:self.view.bounds];
    [self.view addSubview:_largeView];

    _smallView = [[TestView alloc] initWithFrame:CGRectMake(0, 0, 100, 100)];
    [_largeView addSubview:_smallView];
}

- (void)timerEvent:(id)sender
{
    _smallView.frame = CGRectMake(arc4random()%100 + 20,
                                  arc4random()%100 + 20,
                                  arc4random()%100 + 20,
                                  arc4random()%100 + 20);
    NSLog(@"_smallView %@", _smallView);
    NSLog(@"_smallView %@", _largeView);
}

@end

测试后的结论是这样子的:

1. 一个view是不能够自己调用layoutSubviews,如果要调用,需要调用setNeedsLayout或者layoutIfNeeded

2. 如果view的frame值为0,即使被添加了耶不会调用layoutSubviews

3. 如果一个view的frame值改变了,那么它的父类的layoutSubviews也会被执行

解析LayoutSubviews