-[UITableView dequeueReusableCellWithIdentifier:] 的意义何在？

来源：互联网发布：主机屋注册域名编辑：程序博客网时间：2024/04/25 12:48

通过 Apple 官方文档，以及 Sample Code 中 tableView:cellForRowAtIndexPath: 的实现代码，对 dequeueReusableCellWithIdentifier: 的理解是：首次调用该方法返回 nil，然后以其参数指定的 identifier 作为 reuseIdentifier 新创建一个 UITableViewCell 对象，再然后每次调用它都返回刚创建的那个 cell。

但是，事实不是这样的！

-[UITableView dequeueReusableCellWithIdentifier:] 的目的是节约内存。比如，假设屏幕同时可显示 4 个或 5 个 cell，那么，在一个 1000 行的 table 中，系统只需要分配 4 个或 5 个 cell 对象的内存空间，而不是 1000 个，也不是 1 个。

做了个实验，创建了个 table，有 20 个 section，每个 section 包含 20 行，每行高 100 逻辑单位。分别用代码创建或者从 Nib 中加载 cell 的方式，对比对 cell 是否（正确）使用 reuseIdentifier 时内存中总共创建的 cell 数目，可以看出：不使用 reuseIdentifier 时，每次都会新创建一个 cell，当 table 的 entry 特别多时，就非常耗内存；而正确地使用了 reuseIdentifier 后，只在开始时在内存中创建了 6 个 cell，然后循环使用它们。

1）不使用 reuseIdentifier

Section: 0 Row: 0 Count: 1 Index in cellsArray: 0

Section: 0 Row: 1 Count: 2 Index in cellsArray: 1

Section: 0 Row: 2 Count: 3 Index in cellsArray: 2

Section: 0 Row: 3 Count: 4 Index in cellsArray: 3

Section: 0 Row: 4 Count: 5 Index in cellsArray: 4

Section: 0 Row: 5 Count: 6 Index in cellsArray: 5

Section: 0 Row: 6 Count: 7 Index in cellsArray: 6

Section: 0 Row: 7 Count: 8 Index in cellsArray: 7

Section: 0 Row: 8 Count: 9 Index in cellsArray: 8

Section: 0 Row: 9 Count: 10 Index in cellsArray: 9

Section: 0 Row: 10 Count: 11 Index in cellsArray: 10

Section: 0 Row: 11 Count: 12 Index in cellsArray: 11

Section: 0 Row: 12 Count: 13 Index in cellsArray: 12

Section: 0 Row: 13 Count: 14 Index in cellsArray: 13

Section: 0 Row: 14 Count: 15 Index in cellsArray: 14

Section: 0 Row: 15 Count: 16 Index in cellsArray: 15

Section: 0 Row: 16 Count: 17 Index in cellsArray: 16

Section: 0 Row: 17 Count: 18 Index in cellsArray: 17

Section: 0 Row: 18 Count: 19 Index in cellsArray: 18

Section: 0 Row: 19 Count: 20 Index in cellsArray: 19

Section: 1 Row: 0 Count: 21 Index in cellsArray: 20

Section: 1 Row: 1 Count: 22 Index in cellsArray: 21

Section: 1 Row: 2 Count: 23 Index in cellsArray: 22

Section: 1 Row: 3 Count: 24 Index in cellsArray: 23

Section: 1 Row: 4 Count: 25 Index in cellsArray: 24

Section: 1 Row: 5 Count: 26 Index in cellsArray: 25

Section: 1 Row: 6 Count: 27 Index in cellsArray: 26

Section: 1 Row: 7 Count: 28 Index in cellsArray: 27

Section: 1 Row: 8 Count: 29 Index in cellsArray: 28

Section: 1 Row: 9 Count: 30 Index in cellsArray: 29

Section: 1 Row: 10 Count: 31 Index in cellsArray: 30

Section: 1 Row: 11 Count: 32 Index in cellsArray: 31

Section: 1 Row: 12 Count: 33 Index in cellsArray: 32

Section: 1 Row: 13 Count: 34 Index in cellsArray: 33

Section: 1 Row: 14 Count: 35 Index in cellsArray: 34

Section: 1 Row: 15 Count: 36 Index in cellsArray: 35

Section: 1 Row: 16 Count: 37 Index in cellsArray: 36

Section: 1 Row: 17 Count: 38 Index in cellsArray: 37

Section: 1 Row: 18 Count: 39 Index in cellsArray: 38

Section: 1 Row: 19 Count: 40 Index in cellsArray: 39

Section: 2 Row: 0 Count: 41 Index in cellsArray: 40

Section: 2 Row: 1 Count: 42 Index in cellsArray: 41

Section: 2 Row: 2 Count: 43 Index in cellsArray: 42

Section: 2 Row: 3 Count: 44 Index in cellsArray: 43

Section: 2 Row: 4 Count: 45 Index in cellsArray: 44

Section: 2 Row: 5 Count: 46 Index in cellsArray: 45

Section: 2 Row: 6 Count: 47 Index in cellsArray: 46

Section: 2 Row: 7 Count: 48 Index in cellsArray: 47

Section: 2 Row: 8 Count: 49 Index in cellsArray: 48

Section: 2 Row: 9 Count: 50 Index in cellsArray: 49

Section: 2 Row: 10 Count: 51 Index in cellsArray: 50

Section: 2 Row: 11 Count: 52 Index in cellsArray: 51

Section: 2 Row: 12 Count: 53 Index in cellsArray: 52

Section: 2 Row: 13 Count: 54 Index in cellsArray: 53

Section: 2 Row: 14 Count: 55 Index in cellsArray: 54

Section: 2 Row: 15 Count: 56 Index in cellsArray: 55

Section: 2 Row: 16 Count: 57 Index in cellsArray: 56

Section: 2 Row: 17 Count: 58 Index in cellsArray: 57

Section: 2 Row: 18 Count: 59 Index in cellsArray: 58

Section: 2 Row: 19 Count: 60 Index in cellsArray: 59

Section: 3 Row: 0 Count: 61 Index in cellsArray: 60

Section: 3 Row: 1 Count: 62 Index in cellsArray: 61

Section: 3 Row: 2 Count: 63 Index in cellsArray: 62

Section: 3 Row: 3 Count: 64 Index in cellsArray: 63

2）正确使用 reuseIdentifier

Section: 0 Row: 0 Count: 1 Index in cellsArray: 0

Section: 0 Row: 1 Count: 2 Index in cellsArray: 1

Section: 0 Row: 2 Count: 3 Index in cellsArray: 2

Section: 0 Row: 3 Count: 4 Index in cellsArray: 3

Section: 0 Row: 4 Count: 5 Index in cellsArray: 4

Section: 0 Row: 5 Count: 6 Index in cellsArray: 5

Section: 0 Row: 6 Count: 6 Index in cellsArray: 0

Section: 0 Row: 7 Count: 6 Index in cellsArray: 1

Section: 0 Row: 8 Count: 6 Index in cellsArray: 2

Section: 0 Row: 9 Count: 6 Index in cellsArray: 3

Section: 0 Row: 10 Count: 6 Index in cellsArray: 4

Section: 0 Row: 11 Count: 6 Index in cellsArray: 5

Section: 0 Row: 12 Count: 6 Index in cellsArray: 0

Section: 0 Row: 13 Count: 6 Index in cellsArray: 1

Section: 0 Row: 14 Count: 6 Index in cellsArray: 2

Section: 0 Row: 15 Count: 6 Index in cellsArray: 3

Section: 0 Row: 16 Count: 6 Index in cellsArray: 4

Section: 0 Row: 17 Count: 6 Index in cellsArray: 5

Section: 0 Row: 18 Count: 6 Index in cellsArray: 0

Section: 0 Row: 19 Count: 6 Index in cellsArray: 1

Section: 1 Row: 0 Count: 6 Index in cellsArray: 2

Section: 1 Row: 1 Count: 6 Index in cellsArray: 3

Section: 1 Row: 2 Count: 6 Index in cellsArray: 4

Section: 1 Row: 3 Count: 6 Index in cellsArray: 5

Section: 1 Row: 4 Count: 6 Index in cellsArray: 0

Section: 1 Row: 5 Count: 6 Index in cellsArray: 1

Section: 1 Row: 6 Count: 6 Index in cellsArray: 2

Section: 1 Row: 7 Count: 6 Index in cellsArray: 3

Section: 1 Row: 8 Count: 6 Index in cellsArray: 4

Section: 1 Row: 9 Count: 6 Index in cellsArray: 5

Section: 1 Row: 10 Count: 6 Index in cellsArray: 0

Section: 1 Row: 11 Count: 6 Index in cellsArray: 1

Section: 1 Row: 12 Count: 6 Index in cellsArray: 2

Section: 1 Row: 13 Count: 6 Index in cellsArray: 3

Section: 1 Row: 14 Count: 6 Index in cellsArray: 4

Section: 1 Row: 15 Count: 6 Index in cellsArray: 5

Section: 1 Row: 16 Count: 6 Index in cellsArray: 0

Section: 1 Row: 17 Count: 6 Index in cellsArray: 1

Section: 1 Row: 18 Count: 6 Index in cellsArray: 2

Section: 1 Row: 19 Count: 6 Index in cellsArray: 3

Section: 2 Row: 0 Count: 6 Index in cellsArray: 4

Section: 2 Row: 1 Count: 6 Index in cellsArray: 5

Section: 2 Row: 2 Count: 6 Index in cellsArray: 0

Section: 2 Row: 3 Count: 6 Index in cellsArray: 1

Section: 2 Row: 4 Count: 6 Index in cellsArray: 2

Section: 2 Row: 5 Count: 6 Index in cellsArray: 3

Section: 2 Row: 6 Count: 6 Index in cellsArray: 4

Section: 2 Row: 7 Count: 6 Index in cellsArray: 5

Section: 2 Row: 8 Count: 6 Index in cellsArray: 0

Section: 2 Row: 9 Count: 6 Index in cellsArray: 1

Section: 2 Row: 10 Count: 6 Index in cellsArray: 2

Section: 2 Row: 11 Count: 6 Index in cellsArray: 3

Section: 2 Row: 12 Count: 6 Index in cellsArray: 4

Section: 2 Row: 13 Count: 6 Index in cellsArray: 5

Section: 2 Row: 14 Count: 6 Index in cellsArray: 0

Section: 2 Row: 15 Count: 6 Index in cellsArray: 1

Section: 2 Row: 16 Count: 6 Index in cellsArray: 2

Section: 2 Row: 17 Count: 6 Index in cellsArray: 3

Section: 2 Row: 18 Count: 6 Index in cellsArray: 4

Section: 2 Row: 19 Count: 6 Index in cellsArray: 5

Section: 3 Row: 0 Count: 6 Index in cellsArray: 0

Section: 3 Row: 1 Count: 6 Index in cellsArray: 1

Section: 3 Row: 2 Count: 6 Index in cellsArray: 2

Section: 3 Row: 3 Count: 6 Index in cellsArray: 3

【备注】打印结果的 4 列数字分别表示：当前 section 序数、当前 row 在 section 中的序号、至当前总共创建的 cell 数目、当前行使用的 cell 是哪个（为了方便实验，代码中维护了一个数组，用来保存创建的 cell 对象）。

【代码】dequeueReusableCellWithIdentifier.zip

【链接】iPhone - dequeueReusableCellWithIdentifier usage