UITableViewHeaderFooterView的复用问题

前言

关于UITableViewCell和UICollectionViewCell的复用想必大家已经很清楚了，在此就不再啰嗦。不懂的朋友可以百度、Google吧！

然而，关于UITableViewHeaderFooterView之前就发现了有这样API, 如下：

// like dequeueReusableCellWithIdentifier:, but for headers/footers
- (UITableViewHeaderFooterView *)dequeueReusableHeaderFooterViewWithIdentifier:(NSString *)identifier

发现后我就这样使用：

- (UIView *)tableView:(UITableView *)tableView viewForHeaderInSection:(NSInteger)section {
    // headerView复用
    UITableViewHeaderFooterView *headerView = [tableView dequeueReusableHeaderFooterViewWithIdentifier:@"HeaderViewID"];
    if (headerView == nil) {
        headerView = [[RefreshHeaderView alloc] initWithReuseIdentifier:@"HeaderViewID"];
        NSLog(@"section: %ld", (long)section);
    }
    return headerView;
}

其实当时也就想：苹果官方出了这样的API就是希望我们复用UITableViewHeaderFooterView吧！
当时也没有多想，没有考虑过复用和不复用的区别。昨天和一个公司朋友讨论关于这个问题，今天就研究了下，且看接下来的内容。

正文

其实UITableViewHeaderFooterView的原理与她们一样。

复用的测试代码如下：

- (UIView *)tableView:(UITableView *)tableView viewForHeaderInSection:(NSInteger)section {
    // headerView复用测试情况
    UITableViewHeaderFooterView *headerView = [tableView dequeueReusableHeaderFooterViewWithIdentifier:@"HeaderViewID"];
    if (headerView == nil) {
        headerView = [[RefreshHeaderView alloc] initWithReuseIdentifier:@"HeaderViewID"];
        NSLog(@"section: -------> %ld", (long)section);
    }
    return headerView;
}

---------------------------------
#import 

@interface RefreshHeaderView : UITableViewHeaderFooterView

@end


#import "RefreshHeaderView.h"

@interface RefreshHeaderView ()

/**image*/
@property (nonatomic, strong) UIImageView *imageView;

@end

@implementation RefreshHeaderView

- (instancetype)initWithReuseIdentifier:(NSString *)reuseIdentifier
{
    self = [super initWithReuseIdentifier:reuseIdentifier];
    if (self) {
        self.contentView.backgroundColor = [UIColor whiteColor];
        self.imageView = [[UIImageView alloc] init];
        self.imageView.frame = CGRectMake(10, 10, 30, 30);
        self.imageView.image = [UIImage imageNamed:@"stock"];
        [self.contentView addSubview:self.imageView];
        
        for (int i = 0; i < 10; i++) {
            UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(50 + i * 20, 0, 100, 50)];
            label.text = @"呵呵";
            [self.contentView addSubview:label];
        }
    }
    return self;
}

@end

复用的测试Log如下：

2017-01-19 09:38:07.516709 QJNetworking[4169:1311469] section: -------> 0
2017-01-19 09:38:07.518637 QJNetworking[4169:1311469] section: -------> 1
2017-01-19 09:38:07.519517 QJNetworking[4169:1311469] section: -------> 2
2017-01-19 09:38:07.520233 QJNetworking[4169:1311469] section: -------> 3
2017-01-19 09:38:07.520863 QJNetworking[4169:1311469] section: -------> 4
2017-01-19 09:38:07.521519 QJNetworking[4169:1311469] section: -------> 5
2017-01-19 09:38:07.522134 QJNetworking[4169:1311469] section: -------> 6
2017-01-19 09:38:07.522838 QJNetworking[4169:1311469] section: -------> 7
2017-01-19 09:38:07.523539 QJNetworking[4169:1311469] section: -------> 8
2017-01-19 09:38:07.524204 QJNetworking[4169:1311469] section: -------> 9
2017-01-19 09:38:07.524827 QJNetworking[4169:1311469] section: -------> 10
2017-01-19 09:38:07.525417 QJNetworking[4169:1311469] section: -------> 11
2017-01-19 09:38:07.526031 QJNetworking[4169:1311469] section: -------> 12
2017-01-19 09:38:11.855421 QJNetworking[4169:1311469] section: -------> 13
2017-01-19 09:38:11.963743 QJNetworking[4169:1311469] section: -------> 14
2017-01-19 09:38:12.986768 QJNetworking[4169:1311469] section: -------> 23
2017-01-19 09:38:13.520631 QJNetworking[4169:1311469] section: -------> 29
2017-01-19 09:38:15.380638 QJNetworking[4169:1311469] section: -------> 57
2017-01-19 09:38:15.480633 QJNetworking[4169:1311469] section: -------> 62
2017-01-19 09:38:15.547375 QJNetworking[4169:1311469] section: -------> 65
2017-01-19 09:38:15.635934 QJNetworking[4169:1311469] section: -------> 68
2017-01-19 09:38:16.047329 QJNetworking[4169:1311469] section: -------> 80
2017-01-19 09:38:16.097419 QJNetworking[4169:1311469] section: -------> 82
2017-01-19 09:38:16.180931 QJNetworking[4169:1311469] section: -------> 85
2017-01-19 09:38:17.654964 QJNetworking[4169:1311469] section: -------> 85
2017-01-19 09:38:17.729023 QJNetworking[4169:1311469] section: -------> 82
2017-01-19 09:38:17.812837 QJNetworking[4169:1311469] section: -------> 79
2017-01-19 09:38:18.281034 QJNetworking[4169:1311469] section: -------> 68
2017-01-19 09:38:18.365286 QJNetworking[4169:1311469] section: -------> 65
2017-01-19 09:38:18.480834 QJNetworking[4169:1311469] section: -------> 62
2017-01-19 09:38:18.997506 QJNetworking[4169:1311469] section: -------> 51
2017-01-19 09:38:19.114332 QJNetworking[4169:1311469] section: -------> 48
2017-01-19 09:38:19.469727 QJNetworking[4169:1311469] section: -------> 41
2017-01-19 09:38:19.553230 QJNetworking[4169:1311469] section: -------> 38
2017-01-19 09:38:19.680752 QJNetworking[4169:1311469] section: -------> 30
2017-01-19 09:38:19.747531 QJNetworking[4169:1311469] section: -------> 27
2017-01-19 09:38:19.797485 QJNetworking[4169:1311469] section: -------> 25
2017-01-19 09:38:19.849612 QJNetworking[4169:1311469] section: -------> 23
2017-01-19 09:38:20.068577 QJNetworking[4169:1311469] section: -------> 17
2017-01-19 09:38:20.118201 QJNetworking[4169:1311469] section: -------> 15
2017-01-19 09:38:20.164471 QJNetworking[4169:1311469] section: -------> 13
2017-01-19 09:38:20.179870 QJNetworking[4169:1311469] section: -------> 11
2017-01-19 09:38:20.196951 QJNetworking[4169:1311469] section: -------> 9
2017-01-19 09:38:20.213055 QJNetworking[4169:1311469] section: -------> 7
2017-01-19 09:38:20.249866 QJNetworking[4169:1311469] section: -------> 4
2017-01-19 09:38:20.262628 QJNetworking[4169:1311469] section: -------> 2
2017-01-19 09:38:20.833058 QJNetworking[4169:1311469] section: -------> 12

了解Cell的朋友应该知道从上面的数据中可以看出，UITableViewHeaderFooterView的复用原理完全与Cell一致。

接下来我们看看不复用的测试代码

不复用的测试代码：

- (UIView *)tableView:(UITableView *)tableView viewForHeaderInSection:(NSInteger)section {

    UIView *view = [[UIView alloc] init];
    NSLog(@"section: -------> %ld", (long)section);
    UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(10, 10, 30, 30)];
    imageView.image = [UIImage imageNamed:@"stock"];
    [view addSubview:imageView];
    for (int i = 0; i < 10; i++) {
        UILabel *label = [[UILabel alloc] initWithFrame:CGRectMake(50 + i * 20, 0, 100, 50)];
        label.text = @"呵呵";
        [view addSubview:label];
    }
    return view;
}

接下来，我们看看大家最关心的内存和CPU使用情况

不复用的内存和CPU使用状态

unreuse.gif

复用的内存和CPU使用状态

reuse.gif

看了以上结果，想必大家都明白了吧。

总结：

UITableViewHeaderFooterView复用原理与Cell相同
UITableViewHeaderFooterView由于复用机制，显然在CPU使用上更节省。

总之，正如官方一样，推荐大家使用UITableViewHeaderFooterView复用机制。

如果本人有不对的地方，希望朋友可以纠正，谢谢！

UITableViewHeaderFooterView的复用问题

总结：

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