RunLoop 预处理缓存任务

将一个RunLoop的一些耗时任务分拆到多个RunLoop里面处理

需求：cell 里面需要加载大图（3M），保证Cell滚动的时候，UITableView不卡顿

卡顿原因：

在一次runloop中绘制大量的图片，所以UI会卡顿

解决方案：(一次runloop，只绘制一张图片)

• 1、监听Runloop 循环，循环一次，调用一次回调函数
• 2、回调函数里面做加载图片的事情（执行一次任务）
• 3、提供一个添加任务的方法，不要直接加载图片，你将加载图片的代码都给RunLoop
• 4、回调函数里面将任务一个一个取出来
• 5、手动向 RunLoop 中添加 Source 任务（由应用发起和处理的是 Source 0 任务）。通过开启定时，NSTimeInterval = 0.001

代码链接

#import "ViewController.h"

typedef BOOL(^RunloopBlock)(void);

static NSString *IDENTIFIER = @"IDENTIFIER";
static CGFloat CELL_HEIGHT = 135.f;

@interface ViewController () <UITableViewDataSource, UITableViewDelegate>

@property (nonatomic, strong) NSMutableArray *tasks;
@property (nonatomic, strong) NSMutableArray *tasksKeys;
@property (nonatomic, assign) NSUInteger max;
@property (nonatomic, strong) NSTimer * timer;
@property (nonatomic, strong) UITableView *exampleTableView;

@end


@implementation ViewController

- (void)viewDidLoad {
    [super viewDidLoad];
    _max = 20;
    _tasks = [NSMutableArray array];
    _tasksKeys = [NSMutableArray array];
    
    self.exampleTableView = [[UITableView alloc] initWithFrame:self.view.bounds];
    self.exampleTableView.delegate = self;
    self.exampleTableView.dataSource = self;
    [self.exampleTableView registerClass:[UITableViewCell class] forCellReuseIdentifier:IDENTIFIER];
    [self.view addSubview:self.exampleTableView];
    
    _timer = [NSTimer scheduledTimerWithTimeInterval:0.001 target:self selector:@selector(_timerFiredMethod) userInfo:nil repeats:YES];
    
    [self addRunloopObserver];
}

- (void)_timerFiredMethod{
    
}

+ (void)addImage1With:(UITableViewCell *)cell{
    
    UIImageView *imageView = [[UIImageView alloc] initWithFrame:CGRectMake(5, 20, 85, 85)];
    imageView.tag = 1;
    NSString *path1 = [[NSBundle mainBundle] pathForResource:@"spaceship" ofType:@"png"];
    UIImage *image = [UIImage imageWithContentsOfFile:path1];
    imageView.contentMode = UIViewContentModeScaleAspectFit;
    imageView.image = image;
    [UIView transitionWithView:cell.contentView duration:0.3 options:(UIViewAnimationOptionCurveEaseInOut | UIViewAnimationOptionTransitionCrossDissolve) animations:^{
        [cell.contentView addSubview:imageView];
    } completion:nil];
}

+ (void)addImage2With:(UITableViewCell *)cell{
    
    UIImageView *imageView1 = [[UIImageView alloc] initWithFrame:CGRectMake(105, 20, 85, 85)];
    imageView1.tag = 2;
    NSString *path1 = [[NSBundle mainBundle] pathForResource:@"spaceship" ofType:@"png"];
    UIImage *image1 = [UIImage imageWithContentsOfFile:path1];
    imageView1.contentMode = UIViewContentModeScaleAspectFit;
    imageView1.image = image1;
    [UIView transitionWithView:cell.contentView duration:0.3 options:(UIViewAnimationOptionCurveEaseInOut | UIViewAnimationOptionTransitionCrossDissolve) animations:^{
        [cell.contentView addSubview:imageView1];
    } completion:nil];
}

+ (void)addImage3With:(UITableViewCell *)cell{
    UIImageView *imageView2 = [[UIImageView alloc] initWithFrame:CGRectMake(200, 20, 85, 85)];
    imageView2.tag = 3;
    NSString *path1 = [[NSBundle mainBundle] pathForResource:@"spaceship" ofType:@"png"];
    UIImage *image2 = [UIImage imageWithContentsOfFile:path1];
    imageView2.contentMode = UIViewContentModeScaleAspectFit;
    imageView2.image = image2;
    [UIView transitionWithView:cell.contentView duration:0.3 options:(UIViewAnimationOptionCurveEaseInOut | UIViewAnimationOptionTransitionCrossDissolve) animations:^{
        [cell.contentView addSubview:imageView2];
    } completion:nil];
}

#pragma mark - UITableViewDelegate

- (CGFloat)tableView:(UITableView *)tableView heightForRowAtIndexPath:(NSIndexPath *)indexPath {
    return CELL_HEIGHT;
}

#pragma mark - UITableViewDataSource

- (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section {
    return 399;
}

- (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath {
    UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:IDENTIFIER];
    cell.selectionStyle = UITableViewCellSelectionStyleNone;
    
    for (NSInteger i = 1; i <= 3; i++) {
        
        [[cell.contentView viewWithTag:i] removeFromSuperview];
    }
    //不要直接加载图片!! 你将加载图片的代码!都给RunLoop!!
    [self addTask:^BOOL{
        [ViewController addImage1With:cell];
        return YES;
    } withKey:indexPath];
    [self addTask:^BOOL{
        [ViewController addImage2With:cell];
        return YES;
    } withKey:indexPath];
    [self addTask:^BOOL{
        [ViewController addImage3With:cell];
        return YES;
    } withKey:indexPath];
    
    return cell;
}


#pragma mark - <RunLoop>

- (void)addTask:(RunloopBlock)unit withKey:(id)key{
    [self.tasks addObject:unit];
    [self.tasksKeys addObject:key];
    
    if (self.tasks.count > self.max) {
        [self.tasks removeObjectAtIndex:0];
        [self.tasksKeys removeObjectAtIndex:0];
    }
    
}

//定义一个回调函数  一次RunLoop来一次
static void Callback(CFRunLoopObserverRef observer, CFRunLoopActivity activity, void *info){
    ViewController * vc = (__bridge ViewController *)(info);
    if (vc.tasks.count == 0) {
        return;
    }
    BOOL result = NO;
    while (result == NO && vc.tasks.count) {
        RunloopBlock unit = vc.tasks.firstObject;
        //执行任务
        result = unit();
        [vc.tasks removeObjectAtIndex:0];
        [vc.tasksKeys removeObjectAtIndex:0];
    }
    
}

// 添加一个监听者
- (void)addRunloopObserver{
    CFRunLoopRef runloop = CFRunLoopGetCurrent();
    CFRunLoopObserverContext context = {
        0,
        ( __bridge void *)(self),
        &CFRetain,
        &CFRelease,
        NULL
    };
    static CFRunLoopObserverRef defaultModeObsever;
    defaultModeObsever = CFRunLoopObserverCreate(NULL,
                                                 kCFRunLoopBeforeWaiting,
                                                 YES,
                                                 NSIntegerMax - 999,
                                                 &Callback,
                                                 &context
                                                 );
    
    //添加当前RunLoop的观察者
    CFRunLoopAddObserver(runloop, defaultModeObsever, kCFRunLoopDefaultMode);
    CFRelease(defaultModeObsever);
    
}

另一种解决方案：

利用RunLoop空闲时间执行预缓存任务（孙源大神的UITableView-FDTemplateLayoutCell）

需求：

利用RunLoop空闲时间执行预缓存任务（cell高度的计算）

解决方案：

• 1、分解成多个RunLoop Source任务
• 2、RunLoop 处于休眠状态，则唤醒它处理事件
• 3、每个 RunLoopObserver 回调时都把第一个任务拿出来分发

假设列表有 20 个 cell，加载后展示了前 5 个，那么开启估算后 table view 只计算了这 5 个的高度，此时剩下 15 个就是“预缓存”的任务，而我们并不希望这 15 个计算任务在同一个 RunLoop 迭代中同步执行，这样会卡顿 UI，所以应该把它们分别分解到 15 个 RunLoop 迭代中执行，这时就需要手动向 RunLoop 中添加 Source 任务（由应用发起和处理的是 Source 0 任务）

NSMutableArray *mutableIndexPathsToBePrecached = self.fd_allIndexPathsToBePrecached.mutableCopy;
CFRunLoopObserverRef observer = CFRunLoopObserverCreateWithHandler(kCFAllocatorDefault, kCFRunLoopBeforeWaiting, true, 0, ^(CFRunLoopObserverRef observer, CFRunLoopActivity _) {
    if (mutableIndexPathsToBePrecached.count == 0) {
        CFRunLoopRemoveObserver(runLoop, observer, runLoopMode);
        CFRelease(observer); // 注意释放，否则会造成内存泄露
        return;
    }
    NSIndexPath *indexPath = mutableIndexPathsToBePrecached.firstObject;
    [mutableIndexPathsToBePrecached removeObject:indexPath];
    [self performSelector:@selector(fd_precacheIndexPathIfNeeded:)
                 onThread:[NSThread mainThread]
               withObject:indexPath
            waitUntilDone:NO
                    modes:@[NSDefaultRunLoopMode]];
});

每个任务都被分配到下个“空闲” RunLoop 迭代中执行，其间但凡有滑动事件开始，Mode 切换成 UITrackingRunLoopMode，所有的“预缓存”任务的分发和执行都会自动暂定，最大程度保证滑动流畅。