Activity中OnDestory()回调慢的原因及解决办法

前言：近日在测试时，发现bug，聊天页面的数据，有时候有，有时候没有。经过查找，原来是这个页面销毁后，很长时间才回调OnDestroy()方法！如果在回调OnDestroy()方法之前，又重新点击这个页面，就导致这个页面没有数据。

导致原因：

在OnDestroy()中进行了数据存储等操作！

原因分析：

先附上源码中Activity的OnDestroy()的官方解释：

* Perform any final cleanup before an activity is destroyed.  This can
* happen either because the activity is finishing (someone called
* {@link #finish} on it, or because the system is temporarily destroying
* this instance of the activity to save space.  You can distinguish
* between these two scenarios with the {@link #isFinishing} method.
*
* <p><em>Note: do not count on this method being called as a place for
* saving data! For example, if an activity is editing data in a content
* provider, those edits should be committed in either {@link #onPause} or
* {@link #onSaveInstanceState}, not here.</em> This method is usually implemented to
* free resources like threads that are associated with an activity, so
* that a destroyed activity does not leave such things around while the
* rest of its application is still running.  There are situations where
* the system will simply kill the activity's hosting process without
* calling this method (or any others) in it, so it should not be used to
* do things that are intended to remain around after the process goes
* away.
*
* <p><em>Derived classes must call through to the super class's
* implementation of this method.  If they do not, an exception will be
* thrown.</em></p>

从官方文档中我们就可以找到解决办法了，就是在生命周期OnPause()执行时，判断当前的activity是否isFinishing()，如果isFinishing则可以提前进行一些数据操作等。

解决办法：

在OnPause()方法中通过判断当前的activity是否isFinishing提前就行数据操作，释放资源等操作。
如下：

@Override
    protected void onPause() {
        super.onPause();
        if(this.isFinishing()) {
            //进行数据操作及释放资源等操作
        }
    }

同时根据官方文档，如果要进行状态维护的话，最好在onSaveInstanceState方法中进行。
但是上面不能完美解决资源回收的问题。如果一个activity的启动模式为singleTask，先后被启动多次的话，前面几次执行onPause时不会执行回收资源的代码，故还需要在onDestroy()中进一步检测。
下面就直接引用博文Activity onDestroy() 回调缓慢问题分析及完美解决方案中的方案：

    private boolean isDestroyed = false;
    private void destroy()  {
        if (isDestroyed) {
            return;
        }
        // 回收资源
　　　　 isDestroyed = true;
    }

    @Override
    protected void onPause() {
        super.onPause();
        if (isFinishing()) {
            destroy();
        }
    }

    @Override
    public void onDestroy() {
        destroy();//需要在onDestroy方法中进一步检测是否回收资源等。
    }

参考博文：

Activity onDestroy() 回调缓慢问题分析及完美解决方案
Activity onDestroy() 调用研究