系统默认的字体缩放大小

// Configuration.java
public float fontScale;

public Configuration() {
    setToDefaults();
}

public void setToDefaults() {
    fontScale = 1; // 默认为1
    ...
}

系统提供字体缩放的设置项Settings.System.FONT_SCALE，可以通过修改这个设置项，使系统字体改变，默认不配置此值。

// Settings.java
public static final class System extends NameValueTable {
    private static final float DEFAULT_FONT_SCALE = 1.0f;
    ...    
    public static final String FONT_SCALE = "font_scale";
}

设置应用的无障碍功能中，可以通过"放大或缩小文字"修改此设置项，并定义了一组数值，作为参考选项

<string-array name="entryvalues_font_size" translatable="false">
    <item>0.85</item>
    <item>1.0</item>
    <item>1.15</item>
    <item>1.30</item>
</string-array>

ActivityManagerService中监测这个设置项的改变，这个值的改变会更新系统配置

private final class FontScaleSettingObserver extends ContentObserver {
    private final Uri mFontScaleUri = Settings.System.getUriFor(FONT_SCALE);

    public FontScaleSettingObserver() {
        super(mHandler);
        ContentResolver resolver = mContext.getContentResolver();
        resolver.registerContentObserver(mFontScaleUri, false, this, UserHandle.USER_ALL);
    }

    @Override
    public void onChange(boolean selfChange, Uri uri, @UserIdInt int userId) {
        if (mFontScaleUri.equals(uri)) {
            updateFontScaleIfNeeded(userId);
        }
    }
}

private void updateFontScaleIfNeeded(@UserIdInt int userId) {
    final float scaleFactor = Settings.System.getFloatForUser(mContext.getContentResolver(),
            FONT_SCALE, 1.0f, userId);
    if (mConfiguration.fontScale != scaleFactor) {
        final Configuration configuration = mWindowManager.computeNewConfiguration();
        configuration.fontScale = scaleFactor;
        synchronized (this) {
            updatePersistentConfigurationLocked(configuration, userId);
        }
    }
}

private void updatePersistentConfigurationLocked(Configuration values, @UserIdInt int userId) {
    final long origId = Binder.clearCallingIdentity();
    try {
        // 更新到当前配置mConfiguration中
        updateConfigurationLocked(values, null, false, true, userId, false /* deferResume */);
    } finally {
        Binder.restoreCallingIdentity(origId);
    }
}

使用adb测试系统屏幕字体大小的变化

adb shell settings put system font_scale 1.30

adb获取font_scale的值

adb shell settings get system font_scale