Update Viewer docs for Android and iOS.

Change-Id: I4208221f982ce79b6c8af9aaa05644b0f1f94a5e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/268680
Reviewed-by: Tyler Denniston <tdenniston@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>

site/user/sample/viewer.md

@@ -20,38 +20,53 @@ To load resources in the desktop Viewers, use the `--resourcePath` option:
 
     <skia-path>/out/Release/viewer --resourcePath <skia-path>/resources
 
-Similarly, `--skps <skp-file-path>` will load any `.skp` files in that directory for display within the Viewer.
-
-Other useful command-line options: using `--match <pattern>` will load only SKPs or slides matching that name; using `--slide <name>` will launch at that slide; and you can start up with a particular rendering method by using `--backend`, i.e., `--backend sw`, `--backend gl`, or `--backend vk`.
-
-The desktop Viewers are controlled using the keyboard and mouse: left (&#x2190;) and right (&#x2192;) arrows to move from slide to slide; up (&#x2191;) and down (&#x2193;) arrows to zoom in and out; clicking and dragging will translate. Other display options and a slide picker can be found in the Tools UI, which can be toggled by hitting the spacebar.
-
-Key                              | Action
------------------------------|-------------
-&#x2190; &#x2192; | Move between the slides
-&#x2191; &#x2193; | Zoom in / out
-d                 | Change render methods among raster, OpenGL and Vulkan
-s                 | Display rendering times and graph
-Space             | Toggle display of Tools UI
+Similarly, `--skps <skp-file-path>` will load any `.skp` files in that directory
+for display within the Viewer.
+
+Other useful command-line options: using `--match <pattern>` will load only SKPs or slides
+matching that name; using `--slide <name>` will launch at that slide; and you can start up
+with a particular rendering method by using `--backend`, i.e., `--backend sw`, `--backend gl`,
+`--backend vk`, or `--backend mtl`.
+
+The desktop Viewers are controlled using the keyboard and mouse: left (←) and right
+(→) arrows to move from slide to slide; up (↑) and down (↓) arrows to
+zoom in and out; clicking and dragging will translate. Other display options and a slide
+picker can be found in the Tools UI, which can be toggled by hitting the spacebar.
+
+Key    | Action
+-------|-------------
+← →    | Move between the slides
+↑ ↓    | Zoom in / out
+d      | Change render methods among raster, OpenGL and Vulkan
+s      | Display rendering times and graph
+Space  | Toggle display of Tools UI
 
 Android
 -------
 
-To build Viewer as an Android App, you will need the
-[Android SDK](https://developer.android.com/studio/#command-tools) installed and your `ANDROID_HOME` environment variable set.
+To build Viewer as an Android App, first follow the
+[Android build instructions](https://skia.org/user/build#android) to set up the
+Android NDK and a ninja out directory. In addition, you will need the
+[Android SDK](https://developer.android.com/studio/#command-tools) installed and your
+`ANDROID_HOME` environment variable set.
 
     mkdir ~/android-sdk
     ( cd ~/android-sdk; unzip ~/Downloads/sdk-tools-*.zip )
     yes | ~/android-sdk/tools/bin/sdkmanager --licenses
     export ANDROID_HOME=~/android-sdk  # Or wherever you installed the Android SDK.
 
+If you are not using the NDK included with the Android SDK (at ~/android-sdk/ndk-bundle
+in this example) you'll need to set the environmental variable `ANDROID_NDK_HOME`, e.g.,
+
+    export ANDROID_NDK_HOME=/tmp/ndk
+
 The Viewer APK must be built by gradle which can be invoked on the command line
 with the following script:
 
     platform_tools/android/bin/android_build_app -C <out_dir> viewer
 
-where `<out_dir>` is the ninja out directory for android (e.g., `out/arm64`)
-that you want to use to build the app. Upon completion of the script the APK
+where `<out_dir>` is the ninja out directory (e.g., `out/arm64`)
+that you created. Upon completion of the script the APK
 can be found at `<out_dir>/viewer.apk`
 
 To load resources in the Android Viewer place them in
@@ -63,8 +78,21 @@ display options are available in the UI.
 iOS
 ---
 
-The viewer is not yet fully supported on iOS, but can be used to display
-individual slides on a device by launching via `ios-deploy` with the `--match`
-or `--slide` command-line options. The viewer will automatically bundle the
-`resources` directory in the top-level Skia directory, and will bundle an
-`skps` directory if also placed in the Skia directory.
+Viewer on iOS is built using the regular GN process, e.g.
+
+    bin/gn gen out/Release --args='target_os="ios" is_debug=false'
+    ninja -C out/Release viewer
+
+Like other iOS apps it can be deployed either by using something like
+[ios-deploy](https://github.com/ios-control/ios-deploy)
+or by building within Xcode and launching via the IDE. See the
+[iOS build instructions](https://skia.org/user/build#ios) for more information
+on managing provisioning profiles for signing and deployment.
+
+Viewer will
+automatically bundle the `resources` directory in the top-level Skia directory,
+and will bundle an `skps` directory if also placed in the Skia directory.
+
+On iOS the Viewer provides basic touch functionality: you can view slides,
+swipe between them, pinch-zoom to scale, and translate via panning. There is not
+yet support for display options or selecting from a list of slides.