去大厂面试，结果没想到一个Handler还有中高级几种问法，我慌了...

class Thread {
  //内部持有ThreadLocalMap
  ThreadLocal.ThreadLocalMap 
    threadLocals;
}
class ThreadLocal<T>{
  public T get() {
    //首先获取线程持有的
    //ThreadLocalMap
    ThreadLocalMap map =
      Thread.currentThread()
        .threadLocals;
    //在ThreadLocalMap中
    //查找变量
    Entry e = 
      map.getEntry(this);
    return e.value;  
  }
  static class ThreadLocalMap{
    //内部是数组而不是Map
    Entry[] table;
    //根据ThreadLocal查找Entry
    Entry getEntry(ThreadLocal key){
      //省略查找逻辑
    }
    //Entry定义
    static class Entry extends
    WeakReference<ThreadLocal>{
      Object value;
    }
  }
}

ExecutorService es;
ThreadLocal tl;
es.execute(()->{
  //ThreadLocal增加变量
  tl.set(obj);
  try {
    // 省略业务逻辑代码
  }finally {
    //手动清理ThreadLocal 
    tl.remove();
  }
});

//ViewRootImpl.java
//view是DecorView
public void setView(View view, WindowManager.LayoutParams attrs, View panelParentView) {
    synchronized (this) {
        if (mView == null) {
            mView = view;
                        ···
            // Schedule the first layout -before- adding to the window
            // manager, to make sure we do the relayout before receiving
            // any other events from the system.
            requestLayout(); //发起布局请求
                        ···
            view.assignParent(this); //将当前ViewRootImpl对象this作为参数调用了DecorView的                   assignParent
            ···
        }
    }
}

//View.java
/*
 * Caller is responsible for calling requestLayout if necessary.
 * (This allows addViewInLayout to not request a new layout.)
 */
@UnsupportedAppUsage
void assignParent(ViewParent parent) {
    if (mParent == null) {
        mParent = parent;
    } else if (parent == null) {
        mParent = null;
    } else {
        throw new RuntimeException("view " + this + " being added, but"
                + " it already has a parent");
    }
}

@Override
public void requestLayout() {
    if (!mHandlingLayoutInLayoutRequest) {
        checkThread();
        mLayoutRequested = true;
        scheduleTraversals();
    }
}

public class FPSFrameCallback implements Choreographer.FrameCallback {
    private static final String TAG = "FPS_TEST";
    private long mLastFrameTimeNanos;
    private long mFrameIntervalNanos;

    public FPSFrameCallback(long lastFrameTimeNanos) {
        mLastFrameTimeNanos = lastFrameTimeNanos;
        //每一帧渲染时间 多少纳秒
        mFrameIntervalNanos = (long) (1000000000 / 60.0);
    }

    @Override
    public void doFrame(long frameTimeNanos) { //Vsync信号到来的时间frameTimeNanos
        //初始化时间
        if (mLastFrameTimeNanos == 0) {
            //上一帧的渲染时间
            mLastFrameTimeNanos = frameTimeNanos;
        }
        final long jitterNanos = frameTimeNanos - mLastFrameTimeNanos;
        if (jitterNanos >= mFrameIntervalNanos) {
            final long skippedFrames = jitterNanos / mFrameIntervalNanos;
            if (skippedFrames > 5) {
                Log.d(TAG, "Skipped " + skippedFrames + " frames!  "
                    + "The application may be doing too much work on its main thread.");
            }
        }
        mLastFrameTimeNanos = frameTimeNanos;
        //注册下一帧回调
        Choreographer.getInstance().postFrameCallback(this);
    }
}

Choreographer.getInstance().postFrameCallback(FPSFrameCallback(System.nanoTime()))

boolean enqueueMessage(Message msg, long when) {
    if (msg.target == null) {
        throw new IllegalArgumentException("Message must have a target.");
    }
    if (msg.isInUse()) {
        throw new IllegalStateException(msg + " This message is already in use.");
    }
        //加锁保证安全
    synchronized (this) {
        ···
    }    
}

public final boolean runWithScissors(@NonNull Runnable r, long timeout) {
    if (r == null) {
        throw new IllegalArgumentException("runnable must not be null");
    }
    if (timeout < 0) {
        throw new IllegalArgumentException("timeout must be non-negative");
    }

    if (Looper.myLooper() == mLooper) {
        r.run();
        return true;
    }

    BlockingRunnable br = new BlockingRunnable(r);
    return br.postAndWait(this, timeout);
}

public Looper getLooper() {
        if (!isAlive()) {
            return null;
        }

        // If the thread has been started, wait until the looper has been created.
        synchronized (this) {
            while (isAlive() && mLooper == null) {
                try {
                    wait();
                } catch (InterruptedException e) {
                }
            }
        }
        return mLooper;
    }

public abstract class IntentService extends Service {
    private volatile Looper mServiceLooper;
    @UnsupportedAppUsage
    private volatile ServiceHandler mServiceHandler;

private RequestManagerFragment getRequestManagerFragment(
    @NonNull final android.app.FragmentManager fm,
    @Nullable android.app.Fragment parentHint,
    boolean isParentVisible) {
  //1.通过FragmentManager获取 RequestManagerFragment，如果已添加到FragmentManager则返回实例，否则为空
  RequestManagerFragment current = (RequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
  if (current == null) {
    //2.如果fm里面没有则从map缓存里取
    current = pendingRequestManagerFragments.get(fm);
    if (current == null) {
      //3.第1和2步都没有，说明确实没创建，则走创建的流程
      current = new RequestManagerFragment();
      current.setParentFragmentHint(parentHint);
      if (isParentVisible) {
        current.getGlideLifecycle().onStart();
      }
      //4.将新创建的fragment 保存到map容器
      pendingRequestManagerFragments.put(fm, current);
      //5.发送添加fragment事务事件
      fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
      //6.发送remove 本地缓存事件
      handler.obtainMessage(ID_REMOVE_FRAGMENT_MANAGER, fm).sendToTarget();
    }
  }
  return current;
}

//跟上面那个方法唯一的区别是 这个是Fragment 的FragmentManager，上面的是Acitivity 的FragmentManager
private SupportRequestManagerFragment getSupportRequestManagerFragment(
    @NonNull final FragmentManager fm, @Nullable Fragment parentHint, boolean isParentVisible) {
  SupportRequestManagerFragment current =
      (SupportRequestManagerFragment) fm.findFragmentByTag(FRAGMENT_TAG);
  if (current == null) {
    current = pendingSupportRequestManagerFragments.get(fm);
    if (current == null) {
      current = new SupportRequestManagerFragment();
      current.setParentFragmentHint(parentHint);
      if (isParentVisible) {
        current.getGlideLifecycle().onStart();
      }
      pendingSupportRequestManagerFragments.put(fm, current);
      fm.beginTransaction().add(current, FRAGMENT_TAG).commitAllowingStateLoss();
      handler.obtainMessage(ID_REMOVE_SUPPORT_FRAGMENT_MANAGER, fm).sendToTarget();
    }
  }
  return current;
}

@Override
public boolean handleMessage(Message message) {
  boolean handled = true;
  Object removed = null;
  Object key = null;
  switch (message.what) {
    case ID_REMOVE_FRAGMENT_MANAGER:
      //7.移除缓存
      android.app.FragmentManager fm = (android.app.FragmentManager) message.obj;
      key = fm;
      removed = pendingRequestManagerFragments.remove(fm);
      break;
        //省略代码...
  }
  //省略代码...
  return handled;
}

//FragmentManagerImpl.java
void scheduleCommit() {
    synchronized (this) {
        boolean postponeReady =
                mPostponedTransactions != null && !mPostponedTransactions.isEmpty();
        boolean pendingReady = mPendingActions != null && mPendingActions.size() == 1;
        if (postponeReady || pendingReady) {
            mHost.getHandler().removeCallbacks(mExecCommit);
            mHost.getHandler().post(mExecCommit);
            updateOnBackPressedCallbackEnabled();
        }
    }
}