Supporting Different Densities

Use Density-independent Pixels
One common pitfall you must avoid when designing your layouts is using absolute pixels to define distances or sizes. Defining layout 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. Therefore, when specifying dimensions, always use either dp or sp units. A dp is a density-independent pixel that corresponds to the physical size of a pixel at 160 dpi. An sp is the same base unit, but is scaled by the user's preferred text size (it’s a scale-independent pixel), so you should use this measurement unit when defining text size (but never for layout sizes).

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

<Button android:layout_width="wrap_content"
    android:layout_marginTop="20dp" />
When specifying text size, always use sp:

<TextView android:layout_width="match_parent"
    android:textSize="20sp" />
Provide Alternative Bitmaps
Since Android runs in devices with a wide variety of screen densities, you should always provide your bitmap resources tailored to each of the generalized density buckets: low, medium, high and extra-high density. This will help you achieve good graphical quality and performance on all screen densities.

To generate these images, you should start with your raw resource in vector format and generate the images for each density using the following size scale:

xhdpi: 2.0
hdpi: 1.5
mdpi: 1.0 (baseline)
ldpi: 0.75

This means that if you generate a 200×200 image for xhdpi devices, you should generate the same resource in 150×150 for hdpi, 100×100 for mdpi and finally a 75×75 image for ldpi devices.

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

Then, any time you reference @drawable/awesomeimage, the system selects the appropriate bitmap based on the screen's dpi.



發佈留言必須填寫的電子郵件地址不會公開。 必填欄位標示為 *