tailang

##Creating a timer with Grand Central Dispatch

At the following is the implementation file of a sample class that shows, how to make a timer with the help of Grand Central Dispatch. The timer fires on a global queue, just change the queue to the main queue or any custom queue and the timer fires on this queue and not on the global queue anymore.

#import <Foundation/Foundation.h>

@interface SampleClass : NSObject
- (void)startTimer;

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//实现
/// Network thread entry point.
+ (void)_networkThreadMain:(id)object {
    @autoreleasepool {
        [[NSThread currentThread] setName:@"com.xxx.xxx"];
        NSRunLoop *runLoop = [NSRunLoop currentRunLoop];
        [runLoop addPort:[NSMachPort port] forMode:NSDefaultRunLoopMode];
        [runLoop run];
 }

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- (instancetype)initWithRequest:(NSURLRequest *)request
                        options:(YYWebImageOptions)options
                          cache:(nullable YYImageCache *)cache
                       cacheKey:(nullable NSString *)cacheKey
                       progress:(nullable YYWebImageProgressBlock)progress
                      transform:(nullable YYWebImageTransformBlock)transform
                     completion:(nullable YYWebImageCompletionBlock)completion NS_DESIGNATED_INITIALIZER;

- (instancetype)init UNAVAILABLE_ATTRIBUTE;

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// this is the cell height cache; obviously you don't want a static size array in production
var itemHeights = [CGFloat](count: 1000, repeatedValue: UITableViewAutomaticDimension) 

func tableView(tableView: UITableView, willDisplayCell cell: UITableViewCell, forRowAtIndexPath indexPath: NSIndexPath) {
  if itemHeights[indexPath.row] == UITableViewAutomaticDimension {
    itemHeights[indexPath.row] = cell.bounds.height
  }
  // fetch more cells, etc.
}

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dispatch_async(dispatch_get_global_queue(0, 0), ^{
        _link = [CADisplayLink displayLinkWithTarget:self selector:@selector(tick:)];
        // NOTE: 子线程的runloop默认不创建； 在子线程获取 currentRunLoop 对象的时候，就会自动创建RunLoop
        // 这里不加到 main loop，必须创建一个 runloop
        NSRunLoop *runloop = [NSRunLoop currentRunLoop];
        [_link addToRunLoop:runloop forMode:NSRunLoopCommonModes];
        // 必须 timer addToRunLoop 后，再run
        [runloop run];
 });

tailang

// 添加一个监听者
- (void)addObserver {
    
    // 1. 创建监听者
    /**
     *  创建监听者
     *
     *  @param allocator#>  分配存储空间
     *  @param activities#> 要监听的状态