UITableView's reusableTableC

@implementation UITableView (T)

• (NSMutableArray *)reusableCells
  {
  return [self performSelector:@selector(_reusableTableCells)];
  }

@end

@interface ViewController () <UITableViewDataSource, UITableViewDelegate>

@property (nonatomic, strong) UITableView *tableView;

@end

@implementation ViewController

• (void)viewDidLoad
  {
  [super viewDidLoad];

  self.tableView = [[UITableView alloc] initWithFrame:self.view.bounds];
  self.tableView.delegate = self;
  self.tableView.dataSource = self;

  [self.view addSubview:self.tableView];

  [self reloadData];

  [self performSelector:@selector(reloadData) withObject:nil afterDelay:.8];
  }

• (void)reloadData
  {
  NSLog(@"--------------------");
  NSLog(@"before reloadData");
  NSLog(@"%@", [self.tableView valueForKey:@"_reusableTableCells"]);

  [self.tableView reloadData];

  NSLog(@"after reloadData");
  NSLog(@"%@", [self.tableView valueForKey:@"_reusableTableCells"]);
  }

• (NSInteger)numberOfSectionsInTableView:(UITableView *)tableView
  {
  return 1;
  }

• (NSInteger)tableView:(UITableView *)tableView numberOfRowsInSection:(NSInteger)section
  {
  return 3;
  }

• (UITableViewCell *)tableView:(UITableView *)tableView cellForRowAtIndexPath:(NSIndexPath *)indexPath
  {
  UITableViewCell *cell = [tableView dequeueReusableCellWithIdentifier:@"test"];
  if (!cell) {
  cell = [[UITableViewCell alloc] initWithStyle:UITableViewCellStyleDefault reuseIdentifier:@"test"];
  }
  NSLog(@"make cell row:%d", indexPath.row);

  return cell;
  }
  @end

2014-10-17 19:23:09.334 test[52333:4372534] --------------------
2014-10-17 19:23:09.335 test[52333:4372534] before reloadData
2014-10-17 19:23:09.335 test[52333:4372534] (null)
2014-10-17 19:23:09.335 test[52333:4372534] after reloadData
2014-10-17 19:23:09.335 test[52333:4372534] {
}
2014-10-17 19:23:09.344 test[52333:4372534] make cell row:0
2014-10-17 19:23:09.347 test[52333:4372534] make cell row:1
2014-10-17 19:23:09.347 test[52333:4372534] make cell row:2
2014-10-17 19:23:10.136 test[52333:4372534] --------------------
2014-10-17 19:23:10.136 test[52333:4372534] before reloadData
2014-10-17 19:23:10.137 test[52333:4372534] {
}
2014-10-17 19:23:10.137 test[52333:4372534] after reloadData
2014-10-17 19:23:10.137 test[52333:4372534] {

    test =     (
        "<UITableViewCell: 0x7c3a4af0; frame = (0 88; 320 44); hidden = YES; autoresize = W; layer = <CALayer: 0x7c380f20>>",
        "<UITableViewCell: 0x7c3a4290; frame = (0 44; 320 44); hidden = YES; autoresize = W; layer = <CALayer: 0x7c3a4420>>",
        "<UITableViewCell: 0x7c39dfd0; frame = (0 0; 320 44); hidden = YES; autoresize = W; layer = <CALayer: 0x7c39e300>>"
    );

    其中test为NSMutableDictionary的key.
     (
        "<UITableViewCell: 0x7c3a4af0; frame = (0 88; 320 44); hidden = YES; autoresize = W; layer = <CALayer: 0x7c380f20>>",
        "<UITableViewCell: 0x7c3a4290; frame = (0 44; 320 44); hidden = YES; autoresize = W; layer = <CALayer: 0x7c3a4420>>",
        "<UITableViewCell: 0x7c39dfd0; frame = (0 0; 320 44); hidden = YES; autoresize = W; layer = <CALayer: 0x7c39e300>>"
    )
   为键对应的值，数据结构为数组。通过 对比数组中frame可以发现：
数组第一个元素：frame = (0 88; 320 44);
数组第二个元素：frame = (0 44; 320 44);
数组第三个元素：frame = (0 0; 320 44);

最先离开屏幕的元素，最近加入到数组中。同时，从数组中取元素的顺序也是，最先加入数组的元素，最先被取出来。这其实就是**队列**的思想。
同时我们也可以从方法的命名看出来问题：
dequeueReusableCellWithIdentifier：

}
2014-10-17 19:23:10.137 test[52333:4372534] make cell row:0
2014-10-17 19:23:10.138 test[52333:4372534] make cell row:1
2014-10-17 19:23:10.138 test[52333:4372534] make cell row:2

参考：
https://gist.github.com/hirohisa/36b61d628a3a1e6866fe
https://github.com/Microsoft/WinObjC/blob/master/Frameworks/UIKit/UITableView.mm