Android日常开发中，总是离不开Activity；说起Activity那就避不开Activity启动模式(launchMode)。

Activity的四种启动模式:

1. standard:
2. singleTop:
3. singleTask:
4. singleInstance:

我们都知道2，3，4这几种情况，如果Activity栈中有相应的实例，则直接重用该实例而不需要new。重用时会调用该实例的onNewIntent()，让该实例回到栈顶，并将其上面的其他实例退出栈。

我们从源码角度来看一下onNewIntent():

  /**
     * This is called for activities that set launchMode to "singleTop" in
     * their package, or if a client used the {@link Intent#FLAG_ACTIVITY_SINGLE_TOP}
     * flag when calling {@link #startActivity}.  In either case, when the
     * activity is re-launched while at the top of the activity stack instead
     * of a new instance of the activity being started, onNewIntent() will be
     * called on the existing instance with the Intent that was used to
     * re-launch it.
     *
     * <p>An activity will always be paused before receiving a new intent, so
     * you can count on {@link #onResume} being called after this method.
     *
     * <p>Note that {@link #getIntent} still returns the original Intent.  You
     * can use {@link #setIntent} to update it to this new Intent.
     *
     * @param intent The new intent that was started for the activity.
     *
     * @see #getIntent
     * @see #setIntent
     * @see #onResume
     */
    protected void onNewIntent(Intent intent) {
    }

It's clear that 该方法的注释是关键，初级英语你可以的...

下面来谈一下过程：

1. 如果Activity第一次启动，则执行序列：

onCreate() ---> onStart() --->onResume() ....and so on...

2. 如果Activity栈有该实例(启动模式2，3，4)，则执行序列：

onNewIntent() ---> onRestart() ---> onStart() ---> onResume...

3. 如果android系统由于内存不足把已存在Activity释放掉，则只能重新启动Activity，即执行：

onCreate() ---> onStart() ---> onResume() ... and so on...

注意事项

1. 当我们把当前Activity置于后台(不启动其他Activity)，再切换至前台，其执行序列为：

onRestart() ---> onStart() ---> onResume()...

2. 当我们在当前Activity，弹出对话框orToast，将不会对Activity的生命周期产生任何影响。下面是源码中AlertDialog的show()的实现：

   /**
     * Start the dialog and display it on screen.  The window is placed in the
     * application layer and opaque.  Note that you should not override this
     * method to do initialization when the dialog is shown, instead implement
     * that in {@link #onStart}.
     */
    public void show() {
        if (mShowing) {
            if (mDecor != null) {
                if (mWindow.hasFeature(Window.FEATURE_ACTION_BAR)) {
                    mWindow.invalidatePanelMenu(Window.FEATURE_ACTION_BAR);
                }
                mDecor.setVisibility(View.VISIBLE);
            }
            return;
        }

        mCanceled = false;

        if (!mCreated) {
            dispatchOnCreate(null);
        } else {
            // Fill the DecorView in on any configuration changes that
            // may have occured while it was removed from the WindowManager.
            final Configuration config = mContext.getResources().getConfiguration();
            mWindow.getDecorView().dispatchConfigurationChanged(config);
        }

        onStart();
        /**mWindowManager = (WindowManager) context.getSystemService(Context.WINDOW_SERVICE);*/
       mDecor = mWindow.getDecorView();

        if (mActionBar == null && mWindow.hasFeature(Window.FEATURE_ACTION_BAR)) {
            final ApplicationInfo info = mContext.getApplicationInfo();
            mWindow.setDefaultIcon(info.icon);
            mWindow.setDefaultLogo(info.logo);
            mActionBar = new WindowDecorActionBar(this);
        }

        WindowManager.LayoutParams l = mWindow.getAttributes();
        if ((l.softInputMode
                & WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION) == 0) {
            WindowManager.LayoutParams nl = new WindowManager.LayoutParams();
            nl.copyFrom(l);
            nl.softInputMode |=
                    WindowManager.LayoutParams.SOFT_INPUT_IS_FORWARD_NAVIGATION;
            l = nl;
        }
       /**这句话是关键，AlertDialog的本质就是：
        * windowManager.addView(mWindow.getDecorView()，mWindow.getAttribute())
        */
        mWindowManager.addView(mDecor, l);
        mShowing = true;
        /**通过发送Message，通知Handler来做相应处理**/
        sendShowMessage();
    }

通过上述的Source Code，We will be find that AlertDialog的本质就是通过WindowManager.addView()来实现的，所以对Activity的生命周期有毛的影响...
如果你此时对Activity Lifecycle产生了质疑，Please click here,我还是怕没人点啊。

onPause() --- Called when the system is about to start resuming a previous activity(当系统将要开始显示另一个Activity时调用).

立帖为证,回头写个Dialog的源码分析...

总结

1. 除了startand模式外，无论采用什么模式，只有activity时同一个实例，且Intent发生了变化，即启动了其他Activity，才会触发onNewIntent()方法。
2. 使用onNewIntent()方法时，需要在onNewIntent()中使用setIntent(intent)赋值给Activity的Intent,否则，我们只能得到以前的Intent.(详见onNewIntent()源码的注释)

onNewItent()的坑

欢迎留言...期待指导...