【经验总结】- 框架中哪些鲜为人知的处理

简介

在分析优秀的第三方框架或者Android系统源码的时候，有时会发现一些鲜为人知的一些特殊处理，而这些处理平时也很少被开发者注意到。虽然测试的时候很难出现问题，但是一旦使用场景复杂后，难免会出问题。优秀的代码不仅是能够实现漂亮的功能，同时也得有很好的稳键性。这篇文章主要记录自己在分析第三方框架或者系统源码时候，遇到的一些不错的，但很少被人注意的处理。内容会持续更新。

realm数据库

Context.getFilesDir()返回null

在调用 Realm.init(context)初始化数据库的时候，会检查存储数据库文件的路径是否存在和有效。

  /**
   * In some cases, Context.getFilesDir() is not available when the app launches the first time.
   * This should never happen according to the official Android documentation, but the race condition wasn't fixed
   * until Android 4.4.
   * <p>
   * This method attempts to fix that situation. If this doesn't work an {@link IllegalStateException} will be
   * thrown.
   * <p>
   * See these links for further details:
   * https://issuetracker.google.com/issues/36918154
   * https://github.com/realm/realm-java/issues/4493#issuecomment-295349044
   */
  private static void checkFilesDirAvailable(Context context) {
      File filesDir = context.getFilesDir();
      if (filesDir != null) {
          if (filesDir.exists()) {
              return;
          } else {
              try {
                  filesDir.mkdirs();
              } catch (SecurityException ignored) {
              }
          }
      }
      if (filesDir == null || !filesDir.exists()) {
          // Wait a "reasonable" amount of time before quitting.
          // In this case we define reasonable as 200 ms (~12 dropped frames) before giving up (which most likely
          // will result in the app crashing). This lag would only be seen in worst case scenarios, and then, only
          // when the app is started the first time.
          long[] timeoutsMs = new long[]{1, 2, 5, 10, 16}; // Exponential waits, capped at 16 ms;
          long maxTotalWaitMs = 200;
          long currentTotalWaitMs = 0;
          int waitIndex = -1;
          while (context.getFilesDir() == null || !context.getFilesDir().exists()) {
              long waitMs = timeoutsMs[Math.min(++waitIndex, timeoutsMs.length - 1)];
              SystemClock.sleep(waitMs);
              currentTotalWaitMs += waitMs;
              if (currentTotalWaitMs > maxTotalWaitMs) {
                  break;
              }
          }
      }
      // One final check before giving up
      if (context.getFilesDir() == null || !context.getFilesDir().exists()) {
          throw new IllegalStateException("Context.getFilesDir() returns " + context.getFilesDir() + " which is not an existing directory. See https://issuetracker.google.com/issues/36918154");
      }
  }   

方法上面的注释也告诉了可能出现的情况，注释里面的两个地址，都是关于Context.getFilesDir()返回null提给Google和realm的issue。