Android jni get screen size

Tek Eye

For efficient bitmap handling or dynamic View creation in an app, the area that a screen item or layout is using needs to be known. If no fixed sizes are allocated at design time the size of a View may not be known until an app is executed. This is because of the wide range of display sizes that Android supports. Just look on GSMArena to see the huge range of Android devices produced over the years, and to see the wide variation in screen sizes and pixel densities. The example code in this article shows how to read the screen size and the size of Views as the app runs.

(Note: All Android screen items are derived from Views. A screen component, e.g. a TextView , is derived from the View class. Such screen components are also known as widgets. Layouts are ViewGroups and are also derived from Views.)

Determining the Size of an Android View or Screen at Run Time

To run the example code in this article first create a new Android project. Those new to Android programming can read the article Your First Android Hello World Java Program to see how. For this article the app is called View Size.

Use a simple layout for activity_main.xml (the layout file may have another name). Add a TextView with id labXY and set the Text attribute to X,Y. Next to it add another TextView called textXY with Text set to ? (actually \? to make it valid in the XML). Here is the layout used for this example:

Add this code to the bottom of the onCreate method in MainActivity.java (or whatever the class was called). Add the required an imports for TextView and DisplayMetrics when prompted with the Alt-Enter:

This is the code running on an Android Virtual Device (AVD) with a 320×480 screen:

Finding the Size of an Android View in Code

Drop an ImageView onto the layout, here using the ic_launcher.png icon file, or other images can be used. The size of a View can be retrieved using the getWidth and getHeight methods. Change the code in the onCreate to set the TextView to the ImageView’s width and height (an import for View is required, again usually prompted for and added with Alt-Enter):

Mmmmm! The code is showing 0,0 for the ImageView size, even though we can see that it is not 0,0:

This is because in onCreate the screen has not yet been laid out so the size of the ImageView has not been determined hence the getWidth() and getHeight() methods are returning zero. In fact they will likely return zero in onStart() and onResume(). What is needed is to override onWindowFocusChanged() to get the ImageView sizes:

Finding the Size of an Android Layout in Code

The same code can be used to get the size of the View (the layout, i.e. ViewGroup) in which the screen components sit. Notice that in the screen XML the RelativeLayout was given an id (@+id/screen), which means the base View’s width and height can be grabbed (change R.id.imageView to R.id.screen in the code):

Notice that the layout height is less than the screen height because of the notification bar.

Finding the Size of an Android View During Screen Construction

To get the the size of a View as soon as it is known (rather than waiting for the onWindowFocusChanged event) attach a listener to its ViewTreeObserver . Do this by writing a class that implements ViewTreeObserver.OnGlobalLayoutListener in the Activity’s class. This new class will have an onGlobalLayout method that gets the View dimensions that can then be stored for later use (here they are displayed as before). Here is the example source code for the entire MainActivity.java file to show this way of getting the ImageView’s width and height:

Download some example code in view-size.zip from this article, ready for importing into Android Studio. See the instructions in the zip file, alternatively the code can also be accessed via the Android Example Projects page.

See Also

• Using Immersive Full-Screen Mode on Android Developers
• See the Android Example Projects page for lots of Android sample projects with source code.
• For a full list of all the articles in Tek Eye see the full site alphabetical Index.

Archived Comments

Kestrel on December 15, 2014 at 4:20 am said: Hey fantastic article, can you also talk about the fitSystemWindows and how things are affected when its set or not set by default. Thanks in advance.

Author: Daniel S. Fowler Published: 2013-06-19 Updated: 2017-12-17

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Support different pixel densities

Not only do Android devices come in different screen sizes (handsets, tablets, TVs, and so on), but their screens also have different pixel sizes. That is, while one device has 160 pixels per square inch, another device fits 480 pixels in the same space. If you don’t consider these variations in pixel density, the system might scale your images (resulting in blurry images) or the images might appear at the completely wrong size.

This page shows you how you can design your app to support different pixel densities by using resolution-independent units of measurements and providing alternative bitmap resources for each pixel density.

Watch the video below for an overview of these techniques.

For more information about designing the actual icons assets, see the material design icon guidelines.

Use density-independent pixels

The first pitfall you must avoid is using pixels to define distances or sizes. Defining dimensions with pixels is a problem because different screens have different pixel densities, so the same number of pixels may correspond to different physical sizes on different devices.

Figure 1. Two screens of the same size may have a different number of pixels

To preserve the visible size of your UI on screens with different densities, you must design your UI using density-independent pixels (dp) as your unit of measurement. One dp is a virtual pixel unit that’s roughly equal to one pixel on a medium-density screen (160dpi; the «baseline» density). Android translates this value to the appropriate number of real pixels for each other density.

For example, consider the two devices in figure 1. If you were to define a view to be «100px» wide, it will appear much larger on the device on the left. So you must instead use «100dp» to ensure it appears the same size on both screens.

When defining text sizes, however, you should instead use scalable pixels (sp) as your units (but never use sp for layout sizes). The sp unit is the same size as dp, by default, but it resizes based on the user’s preferred text size.

For example, when you specify spacing between two views, use dp :

When specifying text size, always use sp :

Convert dp units to pixel units

In some cases, you will need to express dimensions in dp and then convert them to pixels. The conversion of dp units to screen pixels is simple:

Imagine an app in which a scroll or fling gesture is recognized after the user’s finger has moved by at least 16 pixels. On a baseline screen, a user’s must move by 16 pixels / 160 dpi , which equals 1/10th of an inch (or 2.5 mm) before the gesture is recognized. On a device with a high-density display (240dpi), the user’s must move by 16 pixels / 240 dpi , which equals 1/15th of an inch (or 1.7 mm). The distance is much shorter and the app thus appears more sensitive to the user.

To fix this issue, the gesture threshold must be expressed in code in dp and then converted to actual pixels. For example:

Kotlin

The DisplayMetrics.density field specifies the scale factor you must use to convert dp units to pixels, according to the current pixel density. On a medium-density screen, DisplayMetrics.density equals 1.0; on a high-density screen it equals 1.5; on an extra-high-density screen, it equals 2.0; and on a low-density screen, it equals 0.75. This figure is the factor by which you should multiply the dp units in order to get the actual pixel count for the current screen.

Use pre-scaled configuration values

You can use the ViewConfiguration class to access common distances, speeds, and times used by the Android system. For instance, the distance in pixels used by the framework as the scroll threshold can be obtained with getScaledTouchSlop() :

Kotlin

Methods in ViewConfiguration starting with the getScaled prefix are guaranteed to return a value in pixels that will display properly regardless of the current pixel density.

Provide alternative bitmaps

To provide good graphical qualities on devices with different pixel densities, you should provide multiple versions of each bitmap in your app—one for each density bucket, at a corresponding resolution. Otherwise, Android must scale your bitmap so it occupies the same visible space on each screen, resulting in scaling artifacts such as blurring.

Figure 2. Relative sizes for bitmaps at different density sizes

There are several density buckets available for use in your apps. Table 1 describes the different configuration qualifiers available and what screen types they apply to.

Table 1. Configuration qualifiers for different pixel densities.

Density qualifier	Description
ldpi	Resources for low-density (ldpi) screens ( 120dpi).
mdpi	Resources for medium-density (mdpi) screens ( 160dpi). (This is the baseline density.)
hdpi	Resources for high-density (hdpi) screens ( 240dpi).
xhdpi	Resources for extra-high-density (xhdpi) screens ( 320dpi).
xxhdpi	Resources for extra-extra-high-density (xxhdpi) screens ( 480dpi).
xxxhdpi	Resources for extra-extra-extra-high-density (xxxhdpi) uses ( 640dpi).
nodpi	Resources for all densities. These are density-independent resources. The system does not scale resources tagged with this qualifier, regardless of the current screen’s density.
tvdpi	Resources for screens somewhere between mdpi and hdpi; approximately 213dpi. This is not considered a «primary» density group. It is mostly intended for televisions and most apps shouldn’t need it—providing mdpi and hdpi resources is sufficient for most apps and the system will scale them as appropriate. If you find it necessary to provide tvdpi resources, you should size them at a factor of 1.33*mdpi. For example, a 100px x 100px image for mdpi screens should be 133px x 133px for tvdpi.

To create alternative bitmap drawables for different densities, you should follow the 3:4:6:8:12:16 scaling ratio between the six primary densities. For example, if you have a bitmap drawable that’s 48×48 pixels for medium-density screens, all the different sizes should be:

• 36×36 (0.75x) for low-density (ldpi)
• 48×48 (1.0x baseline) for medium-density (mdpi)
• 72×72 (1.5x) for high-density (hdpi)
• 96×96 (2.0x) for extra-high-density (xhdpi)
• 144×144 (3.0x) for extra-extra-high-density (xxhdpi)
• 192×192 (4.0x) for extra-extra-extra-high-density (xxxhdpi)

Then, place the generated image files in the appropriate subdirectory under res/ and the system will pick the correct one automatically based on the pixel density of the device your app is running on:

Then, any time you reference @drawable/awesomeimage , the system selects the appropriate bitmap based on the screen’s dpi. If you don’t provide a density-specific resource for that density, the system picks the next best match and scales it to fit the screen.

Tip: If you have some drawable resources that the system should never scale (perhaps because you perform some adjustments to the image yourself at runtime), you should place them in a directory with the nodpi configuration qualifier. Resources with this qualifier are considered density-agnostic and the system will not scale them.

For more information about other configuration qualifiers and how Android selects the appropriate resources for the current screen configuration, see Providing Resources.

Put app icons in mipmap directories

Like all other bitmap assets, you need to provide density-specific versions of your app icon. However, some app launchers display your app icon as much as 25% larger than what’s called for by the device’s density bucket.

For example, if a device’s density bucket is xxhdpi and the largest app icon you provide is in drawable-xxhdpi , the launcher app scales up this icon, and that makes it appear less crisp. So you should provide an even higher density launcher icon in the mipmap-xxxhdpi directory. Now the launcher can use the xxxhdpi asset instead.

Because your app icon might be scaled up like this, you should put all your app icons in mipmap directories instead of drawable directories. Unlike the drawable directory, all mipmap directories are retained in the APK even if you build density-specific APKs. This allows launcher apps to pick the best resolution icon to display on the home screen.

For icon design guidelines, see the material guide for icons.

Use vector graphics instead

An alternative to creating multiple density-specific versions of an image is to create just one vector graphic. Vector graphics create an image using XML to define paths and colors, instead of using pixel bitmaps. As such, vector graphics can scale to any size without scaling artifacts, though they’re usually best for illustrations such as icons, not photographs.

Vector graphics are often provided as an SVG (Scalable Vector Graphics) file, but Android does not support this format so you must convert SVGs files to Android’s vector drawable format.

You can easily convert an SVG to a vector drawable from Android Studio using Vector Asset Studio as follows:

1. In the Project window, right-click on the res directory and select New > Vector Asset.
2. Select Local file (SVG, PSD).

Locate the file you want to import and make any adjustments.

Figure 3. Importing an SVG with Android Studio

You might notice some errors appear in the Asset Studio window, indicating some properties of the file that vector drawables do not support. But this will not prevent you from importing—the unsupported properties are simply ignored.

Click Next.

On the next screen, confirm the source set where you want the file in your project and click Finish.

Because one vector drawable can be used on all pixel densities, this file goes in your default drawables directory (you don’t need to use density-specific directories):

For more information about creating vector graphics, read the Vector Drawable documentation.

Advice for uncommon density issues

This section describes more about how Android performs scaling for bitmaps on different pixel densities and how you can further control how bitmaps are drawn on different densities. Unless your app manipulates graphics or you have encountered problems when running on different pixel densities, you can ignore this section.

To better understand how you can support multiple densities when manipulating graphics at runtime, you should understand that the system helps ensure the proper scale for bitmaps in the following ways:

Pre-scaling of resources (such as bitmap drawables)

Based on the density of the current screen, the system uses any density-specific resources from your app. If resources are not available in the correct density, the system loads the default resources and scales them up or down as needed. The system assumes that default resources (those from a directory without configuration qualifiers) are designed for the baseline pixel density (mdpi) and will resize those bitmaps to the appropriate size for the current pixel density.

If you request the dimensions of a pre-scaled resource, the system returns values representing the dimensions after scaling. For example, a bitmap designed at 50×50 pixels for an mdpi screen is scaled to 75×75 pixels on an hdpi screen (if there is no alternative resource for hdpi) and the system reports the size as such.

There are some situations in which you might not want Android to pre-scale a resource. The easiest way to avoid pre-scaling is to put the resource in a resource directory with the nodpi configuration qualifier. For example:

When the system uses the icon.png bitmap from this folder, it does not scale it based on the current device density.

Auto-scaling of pixel dimensions and coordinates

You can disable pre-scaling dimensions and images by setting android:anyDensity to «false» in the manifest or programmatically for a Bitmap by setting inScaled to «false» . In this case, the system auto-scales any absolute pixel coordinates and pixel dimension values at draw time. It does this to ensure that pixel-defined screen elements are still displayed at approximately the same physical size as they would be at the baseline pixel density (mdpi). The system handles this scaling transparently to the app and reports the scaled pixel dimensions to the app, rather than physical pixel dimensions.

For instance, suppose a device has a WVGA high-density screen, which is 480×800 and about the same size as a traditional HVGA screen, but it’s running an app that has disabled pre-scaling. In this case, the system will «lie» to the app when it queries for screen dimensions, and report 320×533 (the approximate mdpi translation for the pixel density). Then, when the app does drawing operations, such as invalidating the rectangle from (10,10) to (100, 100), the system transforms the coordinates by scaling them the appropriate amount, and actually invalidate the region (15,15) to (150, 150). This discrepancy may cause unexpected behavior if your app directly manipulates the scaled bitmap, but this is considered a reasonable trade-off to keep the performance of apps as good as possible. If you encounter this situation, read Converting dp units to pixel units.

Usually, you should not disable pre-scaling. The best way to support multiple screens is to follow the basic techniques described in this document.

If your app manipulates bitmaps or directly interacts with pixels on the screen in some other way, you might need to take additional steps to support different pixel densities. For example, if you respond to touch gestures by counting the number of pixels that a finger crosses, you need to use the appropriate density- independent pixel values, instead of actual pixels, but you can easily convert between dp and px values.

Test on all pixel densities

It’s important to test your app on multiple devices with different pixel densities so you can ensure your UI scales correctly. Testing on a physical device is easy but you can also use the Android Emulator if you don’t have access to physical devices for all the different pixel densities.

If you would rather test on a physical device, but don’t want to buy the devices, you can use Firebase Test Lab to access devices in a Google data center.

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