Android中正确获取屏幕高度的方式

在 Android开发中经常会遇到获取屏幕高度做布局适配的情况，那么今天就来讲讲我在项目中遇到的一个坑。

问题的发现

在Android 10 全面屏手机的横屏全屏中，项目中的视频播放时，在底部留了一段虚拟导航栏的高度的空白，起初我以为是全屏代码是不是有问题导致，全屏代码如下：

public static void hideSystemUI(Activity activity) {
    if (activity == null) {
        return;
    }
    View decorView = activity.getWindow().getDecorView();
    // Set the content to appear under the system bars so that the
    // content doesn't resize when the system bars hide and show.
    int visibility = View.SYSTEM_UI_FLAG_LAYOUT_STABLE
            | View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
            | View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
            // Hide the nav bar and status bar
            | View.SYSTEM_UI_FLAG_HIDE_NAVIGATION
            | View.SYSTEM_UI_FLAG_FULLSCREEN;
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
        visibility |= View.SYSTEM_UI_FLAG_IMMERSIVE_STICKY;
    }
    decorView.setSystemUiVisibility(visibility);
}

这段代码是谷歌官方文档里提取出来的，我用新建一个项目的方式运行，发现没有问题，深入业务逻辑中发现播放器的 View 是自动计算大小赋值的。

public static int getScreenHeight(Context context) {
        DisplayMetrics dm = new DisplayMetrics();
        WindowManager localWindowManager =
                (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
        localWindowManager.getDefaultDisplay().getMetrics(dm);
        return dm.heightPixels;
}        

上面的代码乍看没什么问题，很多同学就是用这段代码获取高度的，深入 getMetrics() 方法，看到文档的注释心里有数了。

/**
 * Gets display metrics that describe the size and density of this display.
 * The size returned by this method does not necessarily represent the
 * actual raw size (native resolution) of the display.
 * <p>
 * 1. The returned size may be adjusted to exclude certain system decor elements
 * that are always visible.
 * </p><p>
 * 2. It may be scaled to provide compatibility with older applications that
 * were originally designed for smaller displays.
 * </p><p>
 * 3. It can be different depending on the WindowManager to which the display belongs.
 * </p><p>
 * - If requested from non-Activity context (e.g. Application context via
 * {@code (WindowManager) getApplicationContext().getSystemService(Context.WINDOW_SERVICE)})
 * metrics will report the size of the entire display based on current rotation and with
 * subtracted system decoration areas.
 * </p><p>
 * - If requested from activity (either using {@code getWindowManager()} or
 * {@code (WindowManager) getSystemService(Context.WINDOW_SERVICE)}) resulting metrics will
 * correspond to current app window metrics. In this case the size can be smaller than physical
 * size in multi-window mode.
 * </p>
 *
 * @param outMetrics A {@link DisplayMetrics} object to receive the metrics.
 */
 public void getMetrics(DisplayMetrics outMetrics) {...}
 
// 和谐翻译
// 此方法获取的显示大小和密度指标不一定表示实际大小的指标。
// 1. 大小可能已经调整过，排除了一些系统永远可见的元素
// 2. 为了适配老的应用，可能已经被缩放了
// 3. 依赖不同的 WindowManager 可能是不同的值

看到这里聪明的你已经知道了，那留的那一块就是这个方法导致的。

解决问题

根据同级 api 找到了以下的方法

/**
 * Gets display metrics based on the real size of this display.
 * <p>
 * The size is adjusted based on the current rotation of the display.
 * </p><p>
 * The real size may be smaller than the physical size of the screen when the
 * window manager is emulating a smaller display (using adb shell wm size).
 * </p>
 *
 * @param outMetrics A {@link DisplayMetrics} object to receive the metrics.
 */
public void getRealMetrics(DisplayMetrics outMetrics) {...}

// 和谐翻译
// 获取真实尺寸的显示指标
// 尺寸会根据横竖屏调整
// 当 window manager模拟了一个小尺寸时，返回值可能会比物理尺寸小

显然这个方法更适合全屏隐藏状态栏及导航栏的情况, 更改代码后解决问题:

public static int getScreenHeight(Context context) {
    DisplayMetrics dm = null;
    try {
        dm = new DisplayMetrics();
        WindowManager localWindowManager =
                (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);
        if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.JELLY_BEAN_MR1) {
            localWindowManager.getDefaultDisplay().getRealMetrics(dm);
        } else {
            localWindowManager.getDefaultDisplay().getMetrics(dm);
        }
        return dm.heightPixels;
    } catch (Exception e) {
        e.printStackTrace();
    }
    return 0;
}

总结

使用系统方法时一定要读一下 api 文档说明，以免踩了不必要的坑。

代码搬运时先理解，后搬运，切忌。