Summary of Chapter 13. Bitmaps

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last updated: 2016-10

The Xamarin.Forms Image element displays a bitmap. All the Xamarin.Forms platforms support the JPEG, PNG, GIF, and BMP file formats.

Bitmaps in Xamarin.Forms come from four places:

  • Over the web as specified by a URL
  • Embedded as a resource in the common Portable Class Library
  • Embedded as a resource in the platform application projects
  • From anywhere that can be referenced by a .NET Stream object, including MemoryStream

Bitmap resources in the PCL are platform-independent, while bitmap resources in the platform projects are platform-specific.

The bitmap is specified by setting the Source property of Image to an object of type ImageSource, an abstract class with three derivatives:

  • UriImageSource for accessing a bitmap over the web based on a Uri object set to its Uri property
  • FileImageSource for accessing a bitmap stored in a platform application project based on a folder and file path set to its File property
  • StreamImageSource for loading a bitmap using a .NET Stream object specified by returning a Stream from a Func set to its Stream property

Alternatively (and more commonly) you can use the following static methods of the ImageSource class, all of which return ImageSource objects:

There is no class equivalent of the Image.FromResource methods. The UriImageSource class is useful if you need to control caching. The FileImageSource class is useful in XAML. StreamImageSource is useful for the asynchronous loading of Stream objects, whereas ImageSource.FromStream is synchronous.

Platform-independent Bitmaps

The WebBitmapCode project loads a bitmap over the web using ImageSource.FromUri. The Image element is set to the Content property of the ContentPage, so it is constrained to the size of the page. Regardless of the bitmap's size, a constrained Image element is stretched to the size of its container, and the bitmap is displayed in its maximum size within the Image element while maintaining the bitmap's aspect ratio. Areas of the Image beyond the bitmap can be colored with BackgroundColor.

The WebBitmapXaml sample is similar but simply sets the Source property to the URL. The conversion is handled by the ImageSourceConverter class.

Fit and fill

You can control how the bitmap is stretched by setting the Aspect property of the Image to one of the following members of the Aspect enumeration:

  • AspectFit: respects aspect ratio (default)
  • Fill: fills area, does not respect aspect ratio
  • AspectFill: fills area but respects aspect ratio, accomplished by cropping part of the bitmap

Embedded resources

You can add a bitmap file to a PCL, or to a folder in the PCL. Give it a Build Action of EmbeddedResource. The ResourceBitmapCode sample demonstrates how to use ImageSource.FromResource to load the file. The resource name passed to the method consists of the assembly name, followed by a dot, followed by the optional folder name and a dot, followed by the filename.

The program sets the VerticalOptions and HorizontalOptions properties of the Image to LayoutOptions.Center, which makes the Image element unconstrained. The Image and the bitmap are the same size:

  • On iOS and Android, the Image is the pixel size of the bitmap. There is a one-to-one mapping between bitmap pixels and screen pixels.
  • On the Windows Runtime platforms, the Image is the pixel size of the bitmap in device-independent units. On most devices, each bitmap pixel occupies multiple screen pixels.

The StackedBitmap sample puts an Image in a vertical StackLayout in XAML. A markup extension named ImageResourceExtension helps to reference the embedded resource in XAML. This class only loads resources from the assembly in which it's located, so it can't be placed in a library.

More on sizing

It's often desirable to size bitmaps consistently among all the platforms. Experimenting with StackedBitmap, you can set a WidthRequest on the Image element in a vertical StackLayout to make the size consistent among the platforms, but you can only reduce the size using this technique.

You can also set the HeightRequest to make the image sizes consistent on the platforms, but the constrained width of the bitmap will limit the versatility of this technique. For an image in a vertical StackLayout, HeightRequest should be avoided.

The best approach is to begin with a bitmap wider than the phone width in device-independent units and set WidthRequest to a desired width in device-independent units. This is demonstrated in the DeviceIndBitmapSize sample.

The MadTeaParty displays Chapter 7 of Lewis Carroll's Alice's Adventures in Wonderland with the original illustrations by John Tenniel:

Triple screenshot of mad tea party

Browsing and waiting

The ImageBrowser sample allows the user to browse through stock images stored on the Xamarin web site. It uses the .NET WebRequest class to download a JSON file with the list of bitmaps.

The program uses an ActivityIndicator to indicate that something's going on. As each bitmap is loading, the read-only IsLoading property of Image is true. The IsLoading property is backed by a bindable property, so a PropertyChanged event is fired when that property changes. The program attaches a handler to this event, and uses the current setting of IsLoaded to set the IsRunning property of the ActivityIndicator.

Streaming bitmaps

The ImageSource.FromStream method creates an ImageSource based on a .NET Stream. The method must be passed a Func object that returns a Stream object.

Accessing the streams

The BitmapStreams sample demonstrates how to use the ImaageSource.FromStream method to load a bitmap stored as an embedded resource, and to load a bitmap across the web.

Generating bitmaps at run time

All the Xamarin.Forms platforms support the uncompressed BMP file format, which is easy to construct in code and then store in a MemoryStream. This technique allows algorithmically creating bitmaps at runtime, as implemented in the BmpMaker class in the Xamrin.FormsBook.Toolkit library.

The "Do It Yourself" DiyGradientBitmap sample demonstrates the use of BmpMaker to create a bitmap with a gradient image.

Platform-specific bitmaps

All the Xamarin.Forms platforms allow storing bitmaps in the platform application assemblies. When retrieved by a Xamarin.Forms application, these platform bitmaps are of type FileImageSource. You use them for:

The platform assemblies already contain bitmaps for icons and splash screens:

  • In the iOS project, in the Resources folder
  • In the Android project, in subfolders of the Resources folder
  • In the Windows projects, in the Assets folder (although the Windows platforms do not restrict bitmaps to that folder)

The PlatformBitmaps sample uses code to display an icon from the platform application projects.

Bitmap resolutions

All the platforms allow storing multiple versions of bitmap images for different device resolutions. At runtime, the proper version is loaded based on the device resolution of the screen.

On iOS, these bitmaps are differentiated by a suffix on the filename:

  • No suffix for 160 DPI devices (1 pixel to the device-independent unit)
  • '@2x' suffix for 320 DPI devices (2 pixels to the DIU)
  • '@3x' suffix for 480 DPI devices (3 pixels to the DIU)

A bitmap intended to be displayed as one-inch square would exist in three versions:

  • MyImage.jpg at 160 pixels square
  • MyImage@2x.jpg at 320 pixels square
  • MyImage@3x.jpg at 480 pixels square

The program would refer to this bitmap as MyImage.jpg, but the proper version is retrieved at runtime based on the resolution of the screen. When unconstrained, the bitmap will always render at 160 device-independent units.

For Android, bitmaps are stored in various subfolders of the Resources folder:

  • drawable-ldpi (low DPI) for 120 DPI devices (0.75 pixels to the DIU)
  • drawable-mdpi (medium) for 160 DPI devices (1 pixel to the DIU)
  • drawable-hdpi (high) for 240 DPI devices (1.5 pixels to the DIU)
  • drawable-xhdpi (extra high) for 320 DPI devices (2 pixels to the DIU)
  • drawable-xxhdpi (extra extra high) for 480 DPI devices (3 pixels to the DIU)
  • drawable-xxxhdpi (three extra highs) for 640 DPI devices (4 pixels to the DIU)

For a bitmap intended to be rendered at one square inch, the various versions of the bitmap will have the same name but a different size, and be stored in these folders:

  • drawable-ldpi/MyImage.jpg at 120 pixels square
  • drawable-mdpi/MyImage.jpg at 160 pixels square
  • drawable-hdpi/MyImage.jpg at 240 pixels square
  • drawable-xhdpi/MyImage.jpg at 320 pixels square
  • drawable-xxhdpi/MyImage.jpg at 480 pixels square
  • drawable-xxxhdpi/MyImage.jpg at 640 pixels square

The bitmap will always render at 160 device-independent units. (The standard Xamarin.Forms solution template only includes the hdpi, xhdpi, and xxhdpi folders.)

The Windows Runtime projects support a bitmap naming scheme that consists of a scaling factor in pixels per device-independent unit as a percentage, for example:

  • MyImage.scale-200.jpg at 320 pixels square

However, only some percentages are valid.

When adding bitmaps to the platform projects, the Build Action should be:

  • iOS: BundleResource
  • Android: AndroidResource
  • Windows Runtime: Content

The ImageTap sample creates two button-like objects consisting of Image elements with a TapGestureRecognizer installed. It is intended that the objects be one-inch square. The Source property of Image is set using an OnPlatform object to reference potentially different filenames on the platforms. The bitmap images include numbers indicating their pixel size, so you can see which size bitmap is retrieved and rendered.

Toolbars and their icons

One of the primary uses of platform-specific bitmaps is the Xamarin.Forms toolbar, which is constructed by adding ToolbarItem objects to the ToolbarItems collection defined by Page. ToobarItem derives from MenuItem from which it inherits some properties.

The most important ToolbarItem properties are:

The number of Primary items should be limited to three or four. You should include a Text setting for all items. For most platforms, only the Primary items require an Icon but Windows 8.1 requires an Icon for all items. The icons should be 32 device-independent units square. The FileImageSource type indicates that they are platform-specific.

The ToolbarItem fires a Clicked event when tapped, much like a Button. ToolbarItem also supports Command and CommandParameter properties often used in connection with MVVM. (See Chapter 18, MVVM).

Both iOS and Android require that a page that displays a toolbar be a NavigationPage or a page navigated to by a NavigationPage. The ToolbarDemo program sets the MainPage property of its App class to the NavigationPage constructor with a ContentPage argument, and demonstrates the construction and event handler of a toolbar.

Button images

You can also use platform-specific bitmaps to set the Image property of Button to a bitmap of 32 device-independent units square, as demonstrated by the ButtonImage sample.

Xamarin Workbook

If it's not already installed, install the Xamarin Workbooks app first. The workbook file should download automatically, but if it doesn't, just click to start the workbook download manually.