Supporting Different Densities 支持各种屏幕密度

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 eitherdp or sp units. A dp is a density-independent pixel that corresponds to the physical size of a pixel at 160 dpi. Ansp 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 thanpx:

<Button android:layout_width="wrap_content" 
    android:layout_height="wrap_content" 
    android:text="@string/clickme"
    android:layout_marginTop="20dp" />

When specifying text size, always use sp:

<TextView android:layout_width="match_parent" 
    android:layout_height="wrap_content" 
    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 200x200 image for xhdpi devices, you should generate the same resource in 150x150 forhdpi, 100x100 for mdpi and finally a 75x75 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:

MyProject/
  res/
    drawable-xhdpi/
        awesomeimage.png
    drawable-hdpi/
        awesomeimage.png
    drawable-mdpi/
        awesomeimage.png
    drawable-ldpi/
        awesomeimage.png

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