1
00:00:04,700 --> 00:00:06,890
in this video we're gonna have a look at

2
00:00:06,890 --> 00:00:09,680
what the Android view model class is and

3
00:00:09,680 --> 00:00:11,210
see how it fits into our apps

4
00:00:11,210 --> 00:00:13,610
architecture now a good starting point

5
00:00:13,610 --> 00:00:16,070
is the Google documentation which you've

6
00:00:16,070 --> 00:00:19,010
got opened at this page it's the Android

7
00:00:19,010 --> 00:00:22,640
arch lifecycle view model page so view

8
00:00:22,640 --> 00:00:24,550
model is part of the new architecture

9
00:00:24,550 --> 00:00:27,470
components so it's a lifecycle aware

10
00:00:27,470 --> 00:00:29,900
class and we'll see that it saves a lot

11
00:00:29,900 --> 00:00:31,369
of work and we have to do with the

12
00:00:31,369 --> 00:00:33,470
activity lifecycle you know the

13
00:00:33,470 --> 00:00:35,600
reference documentation mensen mentions

14
00:00:35,600 --> 00:00:37,519
that so let's see what it has to say

15
00:00:37,519 --> 00:00:39,890
specifically about view model so firstly

16
00:00:39,890 --> 00:00:42,080
we have a look at it the first SAR

17
00:00:42,080 --> 00:00:44,239
paragraph there the first sentence says

18
00:00:44,239 --> 00:00:46,670
that it's responsible for preparing and

19
00:00:46,670 --> 00:00:49,159
managing the data for an activity or

20
00:00:49,159 --> 00:00:50,540
fragment you know we'll be using

21
00:00:50,540 --> 00:00:52,790
fragments a bit later and it's good to

22
00:00:52,790 --> 00:00:54,260
know that we can use a view model with

23
00:00:54,260 --> 00:00:57,140
them as well as with an activity so next

24
00:00:57,140 --> 00:00:58,970
it goes on to say that a view model is

25
00:00:58,970 --> 00:01:01,970
always created in association with a

26
00:01:01,970 --> 00:01:03,949
scope in other words a fragment or an

27
00:01:03,949 --> 00:01:06,440
activity and it will be retained as long

28
00:01:06,440 --> 00:01:08,930
as the scope is alive personally I think

29
00:01:08,930 --> 00:01:10,609
that gives the wrong impression and

30
00:01:10,609 --> 00:01:12,680
implies that the view model will be

31
00:01:12,680 --> 00:01:16,250
destroyed when the activity is but the

32
00:01:16,250 --> 00:01:17,960
next paragraph may be down still further

33
00:01:17,960 --> 00:01:20,090
clarifies that and actually says this

34
00:01:20,090 --> 00:01:22,009
means that a view model will not be

35
00:01:22,009 --> 00:01:25,130
destroyed if its owner is destroyed for

36
00:01:25,130 --> 00:01:28,130
a configuration change so all the state

37
00:01:28,130 --> 00:01:29,750
that we've had to concern ourselves with

38
00:01:29,750 --> 00:01:32,960
saving will be retained if we put it

39
00:01:32,960 --> 00:01:35,179
into a view model because the view model

40
00:01:35,179 --> 00:01:37,789
survives a configuration change nothing

41
00:01:37,789 --> 00:01:40,310
will be lost so that's good we'll see

42
00:01:40,310 --> 00:01:42,170
how that really simplifies things when

43
00:01:42,170 --> 00:01:45,409
we use one in our apps interestingly for

44
00:01:45,409 --> 00:01:47,390
the new instance of the owner we'll just

45
00:01:47,390 --> 00:01:50,719
reconnect to the existing view model so

46
00:01:50,719 --> 00:01:51,590
we don't have to worry about

47
00:01:51,590 --> 00:01:53,899
reconnecting our activity if it's

48
00:01:53,899 --> 00:01:55,520
destroyed as a result of a configuration

49
00:01:55,520 --> 00:01:58,249
change Android takes care of all of that

50
00:01:58,249 --> 00:02:00,439
for us now we're going to come back to

51
00:02:00,439 --> 00:02:03,109
the next paragraph in a minute but the

52
00:02:03,109 --> 00:02:05,179
one after that makes it really clear

53
00:02:05,179 --> 00:02:08,270
what the role of view model is basically

54
00:02:08,270 --> 00:02:09,710
comes out goes under here to save you

55
00:02:09,710 --> 00:02:12,380
models only responsibility is to manage

56
00:02:12,380 --> 00:02:15,680
data for the UI that's clear enough and

57
00:02:15,680 --> 00:02:18,290
the documentation is quite emphatic that

58
00:02:18,290 --> 00:02:20,060
it should never access your view

59
00:02:20,060 --> 00:02:22,430
hierarchy or hold a reference back to

60
00:02:22,430 --> 00:02:25,549
the activity or a fragment okay so that

61
00:02:25,549 --> 00:02:27,920
seems to be a problem if we can't hold a

62
00:02:27,920 --> 00:02:30,109
reference to the activity how can we

63
00:02:30,109 --> 00:02:32,420
call it back when data changes well the

64
00:02:32,420 --> 00:02:34,280
answer is actually in that paragraph

65
00:02:34,280 --> 00:02:36,980
that I skipped rather than the view

66
00:02:36,980 --> 00:02:38,959
model calling a function in the activity

67
00:02:38,959 --> 00:02:42,140
the activity observes the view model the

68
00:02:42,140 --> 00:02:44,810
view model exposes its data and an

69
00:02:44,810 --> 00:02:47,840
observable can monitor it the Android

70
00:02:47,840 --> 00:02:50,000
solutions are live data which we'll be

71
00:02:50,000 --> 00:02:52,010
using in the calculator app or Android

72
00:02:52,010 --> 00:02:53,450
data binding

73
00:02:53,450 --> 00:02:55,040
now when Google designed those

74
00:02:55,040 --> 00:02:57,470
architecture components they made sure

75
00:02:57,470 --> 00:02:58,549
that they would work with other

76
00:02:58,549 --> 00:03:01,879
frameworks such as rx Java so if you end

77
00:03:01,879 --> 00:03:03,980
up using a framework like that things

78
00:03:03,980 --> 00:03:05,659
will still work and I think that's good

79
00:03:05,659 --> 00:03:05,959
to know

80
00:03:05,959 --> 00:03:10,370
all right so scrolling down and we can

81
00:03:10,370 --> 00:03:11,599
see down here that the class really

82
00:03:11,599 --> 00:03:13,939
isn't huge at all it's got a single

83
00:03:13,939 --> 00:03:15,919
parameter less constructor and one

84
00:03:15,919 --> 00:03:18,590
function on cleared anything that we

85
00:03:18,590 --> 00:03:20,450
need the view model to do we write

86
00:03:20,450 --> 00:03:22,940
ourselves the basic class itself is very

87
00:03:22,940 --> 00:03:25,430
simple now the uncleared function is

88
00:03:25,430 --> 00:03:27,470
called when the view model is no longer

89
00:03:27,470 --> 00:03:29,599
required so that we can clean up any

90
00:03:29,599 --> 00:03:33,379
resources that our view models using now

91
00:03:33,379 --> 00:03:34,669
before I move on I'm just going to check

92
00:03:34,669 --> 00:03:35,959
out this link over here to the left-hand

93
00:03:35,959 --> 00:03:39,319
side for what view a model provider

94
00:03:39,319 --> 00:03:40,760
there's quite a little bit to see that

95
00:03:40,760 --> 00:03:45,560
click on view model provider so this is

96
00:03:45,560 --> 00:03:48,139
a utility class that provides view

97
00:03:48,139 --> 00:03:50,720
models for a scope so the scope that

98
00:03:50,720 --> 00:03:52,699
it's talking about is our activity or

99
00:03:52,699 --> 00:03:54,620
fragment when we come to use one of

100
00:03:54,620 --> 00:03:56,540
those so we don't create our own

101
00:03:56,540 --> 00:03:59,000
instances of a view model we use the

102
00:03:59,000 --> 00:04:01,549
view model provider to create it for us

103
00:04:01,549 --> 00:04:03,799
and we'll see that in action when we

104
00:04:03,799 --> 00:04:06,049
convert the calculator app to use a view

105
00:04:06,049 --> 00:04:07,790
model and we'll start work on that in

106
00:04:07,790 --> 00:04:10,540
the next video