The document discusses keeping Swift apps small by reducing overhead. It provides tips for optimizing app size by removing unused code and resources, using more efficient file formats, and avoiding unnecessary struct composition in Swift, which can significantly increase app size. The document also describes how Spotify tracks and enforces app size limits through continuous integration checks and size policies.

1. 1
Keeping Swift Apps Small
Why and how
Bruno Rocha - iOS Engineer @ Spotify

2. App Size
2

3. App Size
2

4. •Why do we care?
•How do we track it?
•How do we improve it?
App Size
2

5. Should I care about my
app’s size?
3

6. 4
Programming in the 90s
~300 KB
•Storage is extremely expensive
•Slow / weak hardware
•Little to no access to decent internet (if at all)
Focus: High optimization

7. 5
349 MB
Programming today
Focus: Great experiences
•Storage is cheap (Base flagship iPhones have 128GB!)
•Fast / strong hardware
•High-speed internet is widely available

8. 6
So why bother about app
size in 2023?

9. So why bother about app size in
2023?
7

10. So why bother about app size in
2023?
7
Developed World
🇩🇪🇺🇸🇸🇪🇫🇷🇬🇧, etc.

11. So why bother about app size in
2023?
7
Developed World
🇩🇪🇺🇸🇸🇪🇫🇷🇬🇧, etc.
Emerging Markets
🇧🇷🇮🇳🇨🇳🇲🇽🇹🇷, etc.

12. 8
Devices and data usage in emerging markets
🇧🇷🇮🇳🇨🇳🇲🇽🇹🇷, etc.

13. 8
Devices and data usage in emerging markets
🇧🇷🇮🇳🇨🇳🇲🇽🇹🇷, etc.
•Devices are usually older gen ones (16GB)

14. 8
Devices and data usage in emerging markets
🇧🇷🇮🇳🇨🇳🇲🇽🇹🇷, etc.
•Devices are usually older gen ones (16GB)
•Data can be prohibitively expensive and constrained

15. 8
Devices and data usage in emerging markets
🇧🇷🇮🇳🇨🇳🇲🇽🇹🇷, etc.
•Devices are usually older gen ones (16GB)
•Data can be prohibitively expensive and constrained
•Little to no access to decent internet (if at all)

16. 9

17. 9
hmmmmmmm

18. 10
in 2017:

19. 10
70% of people consider the size of
an app before downloading it
in 2017:
(Emerging markets)

20. 10
70% of people consider the size of
an app before downloading it
in 2017:
(85% of the world’s population)

21. 10
70% of people consider the size of
an app before downloading it
in 2017:
For every 6MB increase, install conversion drops by 1%
(Android)
(85% of the world’s population)
…which means

22. 11
Might be even worse for iOS!

23. 11
Might be even worse for iOS!
•Apps larger than 200mb have extra friction

24. Should I care about my
app’s size?
12

25. Should I care about my
app’s size?
12
85% of the population says yes!

26. App Size @ Spotify
13

27. 14
App Size @ Spotify

28. 14
App Size @ Spotify
Team Attribution
CI Checks
Tracking Alerting
Charts
Policies
Linters Team Pages
… …
… …

29. 15
App Size CI Check

30. 15
App Size CI Check
Push PR

31. 15
App Size CI Check
Push PR CI compiles the app

32. 15
App Size CI Check
Push PR CI compiles the app CI checks the binary

33. 15
App Size CI Check
Push PR CI compiles the app CI checks the binary

34. 15
App Size CI Check
Push PR CI compiles the app CI checks the binary

35. 16
App Size CI Check
Push PR CI compiles the app CI checks the binary

36. 16
App Size CI Check
Push PR CI compiles the app CI checks the binary
Difference is too big = PR gets blocked

37. Important sidenote!
17

38. Important sidenote!
17

39. 18
Install Size != Download Size

40. 18
Install Size != Download Size
Install Size

41. 18
Install Size != Download Size
Install Size
•Actual downloaded package may be smaller

42. 18
Install Size != Download Size
Install Size
•Actual downloaded package may be smaller
•Both can be predicted by generating an App
Thinning Size Report through Xcode

43. 18
Install Size != Download Size
Install Size
•Actual downloaded package may be smaller
•Both can be predicted by generating an App
Thinning Size Report through Xcode
•Companies usually focus on Install Size
specifically

44. 19
CI Checks
Push PR CI compiles the app CI checks the binary

45. 19
CI Checks
Push PR CI compiles the app CI checks the binary
Install size!

46. 19
CI Checks
Push PR CI compiles the app CI checks the binary
What if you can’t optimize it?

47. 20
App Size Policy

48. 20
App Size Policy

49. 20
App Size Policy
Do the benefits outweighs the impact on app size?

50. 20
App Size Policy
Do the benefits outweighs the impact on app size?
(This is checked recurrently!)

51. 21
App Size @ Spotify
Policies
Alerting
Team Attribution
CI Checks
Tracking
Charts
Linters Team Pages
… …
… …
Team Attribution
CI Checks
Tracking Alerting
Charts
Linters Team Pages
… …
… …

52. 21
App Size @ Spotify
Policies
Alerting
Team Attribution
CI Checks
Tracking
Charts
Linters Team Pages
… …
… …

53. 21
App Size @ Spotify
Policies
Alerting
Team Attribution
CI Checks
Tracking
Charts
Linters Team Pages
… …
… …
CI Checks

54. Optimizing iOS apps for
size
22

55. 23
Delete everything
Size optimization tip 1:

56. 23
Delete everything
Size optimization tip 1:
no

57. 24
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
Optimizing apps for size

58. 24
Remove overhead from the binary
Optimizing apps for size

59. 25
Optimizing apps for size

60. 25
Optimizing apps for size
Overhead
Overhead
Overhead
Overhead
Overhead
Overhead
Overhead
Overhead
Overhead
Overhead
Overhead
Overhead
Overhead
Overhead
Overhead

61. 26
App Package
Swift
iOS
Hardware
Compiler
The iPhone Stack
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
E
Optimizing apps for size

62. 26
App Package
Swift
iOS
Hardware
Compiler
The iPhone Stack
Reality: Complex problem, complex solutions
Optimizing apps for size

63. 27
App Package
Swift
iOS
Hardware
Compiler
The iPhone Stack
Group 1
Group 2
Group 3
Optimizing apps for size

64. 27
App Package
Swift
iOS
Hardware
Compiler
The iPhone Stack
Group 1
Group 2
Group 3
Objective: Overview
•What does it mean?
•Examples & Tips
•What else?
Optimizing apps for size

65. Group 1
Group 2
Group 3
28
App Package
Swift
iOS
Hardware
Compiler
The iPhone Stack
Optimizing apps for size

66. Group 1
Group 2
Group 3
28
App Package
Swift
iOS
Hardware
Compiler
The iPhone Stack
Swift Overhead
Optimizing apps for size

67. Group 1
Group 2
Group 3
28
App Package
Swift
iOS
Hardware
Compiler
The iPhone Stack
Swift Overhead
Resources Overhead
Optimizing apps for size

68. Group 1
Group 2
Group 3
28
App Package
Swift
iOS
Hardware
Compiler
The iPhone Stack
Swift Overhead
Resources Overhead
Project/OS
Overhead
Optimizing apps for size

69. 29
Resources Overhead

70. 29
•R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
Resources Overhead

71. 29
•R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
R
•U
U
U
U
U
U
U
U
U
U
U
U
U
U
U
U
U
U
U
U
U
U
U
U
•N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
N
Resources Overhead

72. 29
Resources Overhead

73. 29
Resources Overhead
Problem: Usingthewrong file formats

74. 30
Usingthewrong file formats
Resources Overhead

75. 30
Usingthewrong file formats
Resources Overhead
•Images: SVG for small icons, HEIC for everything else

76. 30
Usingthewrong file formats
Resources Overhead
•Images: SVG for small icons, HEIC for everything else
•Text content, like JSONs: Sanitize them in order to remove
things you don’t need

77. 30
Usingthewrong file formats
Resources Overhead
•Images: SVG for small icons, HEIC for everything else
•Text content, like JSONs: Sanitize them in order to remove
things you don’t need
•Lottie animation JSONs have many fields that can be safely
stripped away

78. 30
Usingthewrong file formats
Resources Overhead
•Images: SVG for small icons, HEIC for everything else
•Audio: CAF (format) + AAC (codec) for short effects (Lossy)
•Text content, like JSONs: Sanitize them in order to remove
things you don’t need
•Lottie animation JSONs have many fields that can be safely
stripped away

79. 30
Usingthewrong file formats
Resources Overhead
•Images: SVG for small icons, HEIC for everything else
•Audio: CAF (format) + AAC (codec) for short effects (Lossy)
•For regular audio: try MP3 codecs with lower bit-rates
•Text content, like JSONs: Sanitize them in order to remove
things you don’t need
•Lottie animation JSONs have many fields that can be safely
stripped away

80. 30
Usingthewrong file formats
Resources Overhead
•Images: SVG for small icons, HEIC for everything else
•Audio: CAF (format) + AAC (codec) for short effects (Lossy)
•⚠ Warning: Audio is complicated!
•For regular audio: try MP3 codecs with lower bit-rates
•Text content, like JSONs: Sanitize them in order to remove
things you don’t need
•Lottie animation JSONs have many fields that can be safely
stripped away

81. 31
Resources Overhead

82. 31
Resources Overhead
Problem: Placing resources inthe
wrong place

83. 32
Placing resources inthewrong place
Resources Overhead

84. 32
Placing resources inthewrong place
Resources Overhead
•Always use Asset Catalogs

85. 32
Placing resources inthewrong place
Resources Overhead
•Always use Asset Catalogs
•Reduces code signature overhead

86. 32
Placing resources inthewrong place
Resources Overhead
•Always use Asset Catalogs
•Reduces code signature overhead
•Makes your app compress better

87. 32
Placing resources inthewrong place
Resources Overhead
•Always use Asset Catalogs
•Use lossy compression for short-lived images
•Reduces code signature overhead
•Makes your app compress better

88. 33
•Looking for more?
Resources Overhead

89. 33
•Looking for more?
Resources Overhead

90. 33
•Looking for more?
•WWDC sessions about resources
Resources Overhead

91. 34
The iPhone Stack
App Package
Swift
iOS
Hardware
Compiler
Swift Overhead
Project/OS
Overhead
Resources Overhead
Optimizing apps for size

92. 35
The iPhone Stack
Swift Overhead
Resources Overhead
App Package
Swift
iOS
Hardware
Compiler
Project/OS
Overhead
Optimizing apps for size

93. 36
Swift Overhead

94. 36
•Certain Swift patterns can be very expensive for app size
Swift Overhead

95. 36
•Certain Swift patterns can be very expensive for app size
Swift Overhead
•Optimize for Size (-Osize)
•Whole Module Optimization
•Important: Make sure these flags are enabled in Xcode!

96. 36
•Certain Swift patterns can be very expensive for app size
Swift Overhead
•Optimize for Size (-Osize)
•Whole Module Optimization
•Important: Make sure these flags are enabled in Xcode!
•⚠ Be aware that this can affect performance

97. 37
Swift Overhead

98. 37
Swift Overhead
Problem: Improperusage ofstructs

99. 38
Swift Overhead Improperusage ofstructs
•Struct composition is expensive

100. 38
Swift Overhead Improperusage ofstructs
•Struct composition is expensive

101. 38
Swift Overhead Improperusage ofstructs
•Struct composition is expensive

102. 38
Swift Overhead Improperusage ofstructs
•Struct composition is expensive
This can be really bad!
Potential impact: Dozens of megabytes!

103. 39
Swift Overhead Improperusage ofstructs
•Other potentially expensive patterns:

104. 39
Swift Overhead Improperusage ofstructs
•Other potentially expensive patterns:
Reference types in structs Mutability in structs

105. 40
Swift Overhead Improperusage ofstructs

106. 40
Swift Overhead Improperusage ofstructs
This is a compiler issue. You should probably ignore this.

107. 40
Swift Overhead Improperusage ofstructs
This is a compiler issue. You should probably ignore this.
Optimization Tip (1):
Don’t forget about classes!
Still…

108. 40
Swift Overhead Improperusage ofstructs
This is a compiler issue. You should probably ignore this.
Optimization Tip (1):
Don’t forget about classes!
Optimization Tip (2):
Using SwiftUI? Be extra careful.
Still…

109. 41
Swift Overhead

110. 41
Problem: Unused code
Swift Overhead

111. 42
Swift Overhead Unused code

112. 42
Swift Overhead Unused code

113. 43
Swift Overhead Unused code

114. 43
Swift Overhead Unused code

115. 43
Swift Overhead Unused code
•Unnecessary Dynamic Dispatching

116. 44
Swift Overhead Unused code
•Unnecessary Dynamic Dispatching

117. 44
Swift Overhead Unused code
•Unnecessary Dynamic Dispatching
Equatable Hashable Codable
(Protocols with compiler-generated conformances)

118. 45
Swift Overhead Unused code
•Unnecessary Dynamic Dispatching

119. 45
Swift Overhead Unused code
•Unnecessary Dynamic Dispatching

120. 46
Swift Overhead Unused code
•Unnecessary Dynamic Dispatching

121. 46
Swift Overhead Unused code
•Unnecessary Dynamic Dispatching
•Potential impact: Several megabytes

122. 46
Swift Overhead Unused code
•Unnecessary Dynamic Dispatching
Optimization Tip (1):
Only add protocols if you truly need them
•Potential impact: Several megabytes

123. 46
Swift Overhead Unused code
•Unnecessary Dynamic Dispatching
Optimization Tip (1):
Only add protocols if you truly need them
•Potential impact: Several megabytes

124. 46
Swift Overhead Unused code
•Unnecessary Dynamic Dispatching
•Potential impact: Several megabytes
Optimization Tip (2):
Be familiar with Dynamic Dispatching in Swift

125. 47
Swift Overhead
•Looking for more?

126. 47
•Official Swift Forums
Swift Overhead
•Looking for more?

127. 47
•Official Swift Forums
Swift Overhead
•Looking for more?
•WWDC sessions about Swift

128. 48
The iPhone Stack
Swift Overhead
Resources Overhead
App Package
Swift
iOS
Hardware
Compiler
Project/OS
Overhead
Optimizing apps for size

129. 49
The iPhone Stack
Swift Overhead
Resources Overhead
App Package
Swift
iOS
Hardware
Compiler
Project/OS
Overhead
Optimizing apps for size

130. 50
Project/OS Overhead

131. 50
•⚠ Requires dedicated infrastructure teams
Project/OS Overhead

132. 50
•⚠ Requires dedicated infrastructure teams
•⚠ Often requires deep knowledge of Swift and iOS
Project/OS Overhead

133. 50
•⚠ Requires dedicated infrastructure teams
•⚠ Often requires deep knowledge of Swift and iOS
•⚠ Not something you would do for “individual" PRs
Project/OS Overhead

134. 50
•⚠ Requires dedicated infrastructure teams
•⚠ Often requires deep knowledge of Swift and iOS
•⚠ Not something you would do for “individual" PRs
Don’ttrythis at home!
Project/OS Overhead

135. 51
Project/OS Overhead

136. 51
Problem:Xcode’s native localization
is inefficient
Project/OS Overhead

137. 52
Project/OS Overhead Xcode’s native localization is inefficient

138. 52
// Comment for string A
“stringA" = “Button”
// Comment for string B
“stringB" = “Another Button”
// Comment for string C
“stringC" = “Cool Button”
// Comment for string D
“stringD” = “Bad Button"
en.strings
// Comment for string A
“stringA" = “Botão”
// Comment for string B
“stringB" = “Outro Button”
// Comment for string C
“stringC" = “Botão Legal”
// Comment for string D
“stringD” = “Botão Ruim”
pt-BR.strings
Project/OS Overhead Xcode’s native localization is inefficient

139. 53
en.values
[“Botão,"Outro Botão”,“Botão
Legal”,“Botão Ruim”]
pt-BR.values
["Button,"Another Button”,“Cool
Button”,“Bad Button”]
•Improve efficiency by storing them yourself!
keys
[“stringA,"stringB”,“stringC”,
“stringD”]
Project/OS Overhead Xcode’s native localization is inefficient

140. 53
en.values
[“Botão,"Outro Botão”,“Botão
Legal”,“Botão Ruim”]
pt-BR.values
["Button,"Another Button”,“Cool
Button”,“Bad Button”]
•Improve efficiency by storing them yourself!
keys
[“stringA,"stringB”,“stringC”,
“stringD”]
Improvement @ Spotify: -20MB
Project/OS Overhead Xcode’s native localization is inefficient

141. 54
Project/OS Overhead

142. 54
Project/OS Overhead
•Previously, on this talk…

143. 55
Project/OS Overhead

144. 55
Project/OS Overhead
•LLVM has optimization flags not exposed to Xcode

145. 55
Project/OS Overhead
•Machine Outlining
-mllvm —enable-machine-outliner=always -mllvm —machine-outliner-reruns=NO_OF_RERUNS
•LLVM has optimization flags not exposed to Xcode

146. 55
Project/OS Overhead
•Machine Outlining
-mllvm —enable-machine-outliner=always -mllvm —machine-outliner-reruns=NO_OF_RERUNS
•ThinLTO (Cross Module Optimization)
-
fl
to=thin
•LLVM has optimization flags not exposed to Xcode

147. 55
Project/OS Overhead
•Machine Outlining
-mllvm —enable-machine-outliner=always -mllvm —machine-outliner-reruns=NO_OF_RERUNS
•ThinLTO (Cross Module Optimization)
-
fl
to=thin
•LLVM has optimization flags not exposed to Xcode
Don’ttrythis at home!

148. 56
Swift Overhead
•Looking for more?
Project/OS Overhead

149. 56
•Official Swift Forums
Swift Overhead
•Looking for more?
Project/OS Overhead

150. 56
•Official Swift Forums
Swift Overhead
•Looking for more?
•Official LLVM Forums
Project/OS Overhead

151. 56
•Official Swift Forums
Swift Overhead
•Looking for more?
•Official LLVM Forums
Project/OS Overhead
•Large companies’ developer blogs

152. 56
•Official Swift Forums
Swift Overhead
•Looking for more?
•Official LLVM Forums
Project/OS Overhead
•Large companies’ developer blogs
•MobileNativeFoundation

153. 57

154. 57
😫

155. 58
Overwhelmed?
Start simple!

156. 58
Overwhelmed?
Start simple!
•Start by simply being aware of the problem

157. 58
Overwhelmed?
Start simple!
•Start by simply being aware of the problem
•Don’t worry about the tech until you actually need it

158. 59
@rockbruno_
https:/
/swiftrocks.com