Android N Display Size

Android N新特性-屏幕缩放

Android 7.0 支持用户设置显示尺寸，以放大或缩小屏幕上的所有元素，从而提升设备对视力不佳用户的可访问性。用户无法将屏幕缩放至低于最小屏幕宽度 sw320dp，该宽度是 Nexus 4 的宽度，也是常规中等大小手机的宽度。

当设备密度发生更改时，系统会以如下方式通知正在运行的应用：
如果是面向 API 级别 23 或更低版本系统的应用，系统会自动终止其所有后台进程。这意味着如果用户切换离开此类应用，转而打开 Settings 屏幕并更改 Display size 设置，则系统会像处理内存不足的情况一样终止该应用。如果应用具有任何前台进程，则系统会如处理运行时更改中所述将配置变更通知给这些进程，就像对待设备屏幕方向变更一样。
如果是面向 Android 7.0 的应用，则其所有进程（前台和后台）都会收到有关配置变更的通知，如处理运行时更改中所述。
大多数应用并不需要进行任何更改即可支持此功能，不过前提是这些应用遵循 Android 最佳做法。具体要检查的事项：

• 在屏幕宽度为 sw320dp 的设备上测试您的应用，并确保其充分运行。
• 当设备配置发生变更时，更新任何与密度相关的缓存信息，例如缓存位图或从网络加载的资源。当应用从暂停状态恢复运行时，检查配置变更。
  注：如果您要缓存与配置相关的数据，则最好也包括相关元数据，例如该数据对应的屏幕尺寸或像素密度。保存这些元数据便于您在配置变更后决定是否需要刷新缓存数据。
• 避免用像素单位指定尺寸，因为像素不会随屏幕密度缩放。应改为使用与密度无关像素 (dp) 单位指定尺寸
  

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Dpi,dp,ppi,sp,in&px参考

Android屏幕适配全攻略

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Display Size分析

Display Size 对应手机的Settings->Display->Display size。用来缩放手机屏幕。
流程分析
1. screen_zoom_activity.xml
Display Size调整设置项对应的布局文件,通过seekbar来显示缩放等级，通过按钮来调整等级
2. CustomPreviewSeekBarPreferenceFragment.java

final int max = Math.max(1, mEntries.length - 1);
final LabeledSeekBar seekBar = (LabeledSeekBar) content.findViewById(R.id.seek_bar);
seekBar.setLabels(mEntries);
seekBar.setMax(max);
seekBar.setProgress(mInitialIndex);

mEntries数组长度决定了缩放调整的级数，数组内容为每个等级的标签，mInitialIndex决定了当前所在的级数，第一次获取的则为默认等级。
3. ScreenZoomSettings.java
该类继承自PreviewSeekBarPreferenceFragment.java，该类确定了mEntries和mInitialIndex的值。这两个值都是由DisplayDensityUtils对象获得。
4. DisplayDensityUtils.java
mEntries和mInitialIndex可以看到是通过DisplayDensityUtils的getEntries()和getCurrentIndex()方法获得。而该方法中返回值搜索代码即可看到是在该类的构造方法中确定。
源码如下

public class DisplayDensityUtils {
    /** Minimum increment between density scales. */
    private static final float MIN_SCALE_INTERVAL = 0.09f;
    /** Minimum density scale. This is available on all devices. */
    private static final float MIN_SCALE = 0.85f;
    /** Maximum density scale. The actual scale used depends on the device. */
    private static final float MAX_SCALE = 1.50f;
    /** Summary used for "default" scale. */
    public static final int SUMMARY_DEFAULT = R.string.screen_zoom_summary_default;
    /** Summary used for "custom" scale. */
    private static final int SUMMARY_CUSTOM = R.string.screen_zoom_summary_custom;
    /**
     * Summaries for scales smaller than "default" in order of smallest to
     * largest.
     */
    private static final int[] SUMMARIES_SMALLER = new int[] {
            R.string.screen_zoom_summary_small
    };
    /**
     * Summaries for scales larger than "default" in order of smallest to
     * largest.
     */
    private static final int[] SUMMARIES_LARGER = new int[] {
            R.string.screen_zoom_summary_large,
            R.string.screen_zoom_summary_very_large,
            R.string.screen_zoom_summary_extremely_large,
    };
    /**
     * Minimum allowed screen dimension, corresponds to resource qualifiers
     * "small" or "sw320dp". This value must be at least the minimum screen
     * size required by the CDD so that we meet developer expectations.
     */
    private static final int MIN_DIMENSION_DP = 320;

    private final String[] mEntries;
    private final int[] mValues;

    private final int mDefaultDensity;
    private final int mCurrentIndex;

    public DisplayDensityUtils(Context context) {
        final int defaultDensity = DisplayDensityUtils.getDefaultDisplayDensity(
                Display.DEFAULT_DISPLAY);
        if (defaultDensity <= 0) {
            mEntries = null;
            mValues = null;
            mDefaultDensity = 0;
            mCurrentIndex = -1;
            return;
        }

        final Resources res = context.getResources();
        final DisplayMetrics metrics = res.getDisplayMetrics();
        final int currentDensity = metrics.densityDpi;
        int currentDensityIndex = -1;

        // Compute number of "larger" and "smaller" scales for this display.
        final int minDimensionPx = Math.min(metrics.widthPixels, metrics.heightPixels);
        final int maxDensity = DisplayMetrics.DENSITY_MEDIUM * minDimensionPx / MIN_DIMENSION_DP;
        final float maxScale = Math.min(MAX_SCALE, maxDensity / (float) defaultDensity);
        final float minScale = MIN_SCALE;
        final int numLarger = (int) MathUtils.constrain((maxScale - 1) / MIN_SCALE_INTERVAL,
                0, SUMMARIES_LARGER.length);
        final int numSmaller = (int) MathUtils.constrain((1 - minScale) / MIN_SCALE_INTERVAL,
                0, SUMMARIES_SMALLER.length);
        String[] entries = new String[1 + numSmaller + numLarger];
        int[] values = new int[entries.length];
        int curIndex = 0;

        if (numSmaller > 0) {
            final float interval = (1 - minScale) / numSmaller;
            for (int i = numSmaller - 1; i >= 0; i--) {
                // Round down to a multiple of 2 by truncating the low bit.
                final int density = ((int) (defaultDensity * (1 - (i + 1) * interval))) & ~1;
                if (currentDensity == density) {
                    currentDensityIndex = curIndex;
                }
                entries[curIndex] = res.getString(SUMMARIES_SMALLER[i]);
                values[curIndex] = density;
                curIndex++;
            }
        }

        if (currentDensity == defaultDensity) {
            currentDensityIndex = curIndex;
        }
        values[curIndex] = defaultDensity;
        entries[curIndex] = res.getString(SUMMARY_DEFAULT);
        curIndex++;

        if (numLarger > 0) {
            final float interval = (maxScale - 1) / numLarger;
            for (int i = 0; i < numLarger; i++) {
                // Round down to a multiple of 2 by truncating the low bit.
                final int density = ((int) (defaultDensity * (1 + (i + 1) * interval))) & ~1;
                if (currentDensity == density) {
                    currentDensityIndex = curIndex;
                }
                values[curIndex] = density;
                entries[curIndex] = res.getString(SUMMARIES_LARGER[i]);
                curIndex++;
            }
        }

        final int displayIndex;
        if (currentDensityIndex >= 0) {
            displayIndex = currentDensityIndex;
        } else {
            // We don't understand the current density. Must have been set by
            // someone else. Make room for another entry...
            int newLength = values.length + 1;
            values = Arrays.copyOf(values, newLength);
            values[curIndex] = currentDensity;

            entries = Arrays.copyOf(entries, newLength);
            entries[curIndex] = res.getString(SUMMARY_CUSTOM, currentDensity);

            displayIndex = curIndex;
        }

        mDefaultDensity = defaultDensity;
        mCurrentIndex = displayIndex;
        mEntries = entries;
        mValues = values;
    }

    public String[] getEntries() {
        return mEntries;
    }
    public int getCurrentIndex() {
        return mCurrentIndex;
    }
}

构造函数首先根据默认DPI来计算可缩放的等级数->填充smaller的mEntries标签内容->设置默认密度等级的下标和填充标签->填充larger的mEntries的标签内容

计算等级数

final int minDimensionPx = Math.min(metrics.widthPixels, metrics.heightPixels);
final int maxDensity = DisplayMetrics.DENSITY_MEDIUM * minDimensionPx / MIN_DIMENSION_DP;
final float maxScale = Math.min(MAX_SCALE, maxDensity / (float) defaultDensity);
final float minScale = MIN_SCALE;
final int numLarger = (int) MathUtils.constrain((maxScale - 1) / MIN_SCALE_INTERVAL,
        0, SUMMARIES_LARGER.length);
final int numSmaller = (int) MathUtils.constrain((1 - minScale) / MIN_SCALE_INTERVAL,
        0, SUMMARIES_SMALLER.length);
String[] entries = new String[1 + numSmaller + numLarger];

defaultDensity是通过 DisplayDensityUtils.getDefaultDisplayDensity(Display.DEFAULT_DISPLAY)获取，该默认密度对应代码中ro.sf.lcd_density
numSmaller为最小为0，最大为1，此处值为1
numLarger则为0到3之间，具体取决于maxScale，它又取决于 maxDensity / (float) defaultDensity

此处以测试机5015为例，其ro.sf.lcd_density默认为240dpi,width为480px,height为854px
则defaultDensity为240  
maxDensity = DisplayMetrics.DENSITY_MEDIUM * minDimensionPx / MIN_DIMENSION_DP 
 ==>160*480/320 =240
则maxScale = 240/240 = 1
则numLarger = 0

修改Android DPI
代码路径:alps/device/sagetel/sr6580_we_n/system.prop,ro.sf.lcd_density修改该值则修改了手机的默认密度

填充smaller

设置默认密度

if (currentDensity == defaultDensity) {
    currentDensityIndex = curIndex;//设置默认下标
}
values[curIndex] = defaultDensity;
entries[curIndex] = res.getString(SUMMARY_DEFAULT);填充默认等级的描述

填充larger

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等级客制化

可通过修改ro.sf.lcd_density值来改变缩放等级数，density越小，等级数越多，但最大5级。为了界面显示正常，我们需调整合适的density，density测试可通过

修改屏幕Dpi
adb shell wm density 240    //修改屏幕密度为240dpi
adb shell wm density reset //重置屏幕密度为默认密度