我们在写一个自己的Activity的时候需要注意：
1、都必须extends Activity（或者其子类）；
2、必须在方法onCreate中存在一条这样的语句：super.onCreate（也就是调用Activity的onCreate方法），对于super.onCreate方法放的位置要求不是很高，只要被调用了就可以了，super.onCreate主要是加载一些组件。
如下面：
import android.app.Activity;
import android.os.Bundle;
public class TargetActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView...
}
}
需要引起注意的是：
1、参数Bundle savedInstanceState来自何处？
2、为何super.onCreate(savedInstanceState)语句的缺失会带给应用致命要害。
也许熟悉源码的会发现，其实在Instrumentation类中的方法callActivityOnCreate方法，前面的调用过程略过，主要是在ActivityThread的方法performLaunchActivity中调用了Instrumentation类中的方法callActivityOnCreate方法，继而调用了TargetActivity中的onCreate方法。
步骤：
1、在ActivityThread中的方法performLaunchActivity调用了mInstrumentation.callActivityOnCreate(activity,
r.state);
2、Instrumentation类中的方法callActivityOnCreate方法，源码如下：
/**
* Perform calling of an activity's {@link Activity#onCreate} method. The
* default implementation simply calls through to that method.
*
* @param activity
*The activity being created.
* @param icicle
*The previously frozen state (or null) to pass through to
*onCreate().
*/
public void callActivityOnCreate(Activity activity, Bundle icicle) {
if (mWaitingActivities != null) {
synchronized (mSync) {
final int N = mWaitingActivities.size();
for (int i = 0; i
final ActivityWaiter aw = mWaitingActivities.get(i);
final Intent intent = aw.intent;
if (intent.filterEquals(activity.getIntent())) {
aw.activity = activity;
mMessageQueue.addIdleHandler(new ActivityGoing(aw));
}
}
}
}
//?????activity引用是TargetActivity对象
activity.performCreate(icicle);
if (mActivityMonitors != null) {
synchronized (mSync) {
final int N = mActivityMonitors.size();
for (int i = 0; i
final ActivityMonitor am = mActivityMonitors.get(i);
am.match(activity, activity, activity.getIntent());
}
}
}
}
再次看方法activity.performCreate(icicle)，其中的方法是通过activity，这个activity，形如：Activity activity
= 子Activity的对象（其实也就是Java中的一种多态形式）。
在Activity类中的方法performCreate(icicle)，源码如下：
final void performCreate(Bundle icicle) {
onCreate(icicle);
mVisibleFromClient = !mWindow.getWindowStyle().getBoolean(
com.android.internal.R.styleable.Window_windowNoDisplay, false);
mFragments.dispatchActivityCreated();
}
注意：在performCreate方法中调用的onCreate方法是TargetActivity中的onCreate方法，那么到此TargetActivity中的方法onCreate方法中的参数Bundle
savedInstanceState也就知道来源了，此时，TargetActivity中的方法也就被调用了。
再次看TargetActivity中的方法onCreate:
public class TargetActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
// TODO Auto-generated method stub
super.onCreate(savedInstanceState);
setContentView...方法
}
}
第一件事情便是super.onCreate(savedInstanceState)，其实这条语句放在子类中的onCreate方法中的任何位置都可，问题只是super.onCreate(savedInstanceState)必须要被执行，所以，最好也就是放在第一行，看起来比较明确。还有就是调用了setContentView方法。在Activity中的setContentView方法，源码如下：
public
void setContentView(int layoutResID) {
getWindow().setContentView(layoutResID);
initActionBar();
}
与WindowManager、PhoneWindow等等进行联系。
回到super.onCreate(savedInstanceState)，来到Activity的onCreate(savedInstanceState)方法，源码如下：
/**
* Called when the activity is starting. This is where most initialization
* should go: calling {@link #setContentView(int)} to inflate the activity's
* UI, using {@link #findViewById} to programmatically interact with widgets
* in the UI, calling
* {@link #managedQuery(android.net.Uri , String[], String, String[], String)}
* to retrieve cursors for data being displayed, etc.
*
*
* You can call {@link #finish} from within this function, in which case
* onDestroy() will be immediately called without any of the rest of the
* activity lifecycle ({@link #onStart}, {@link #onResume}, {@link #onPause}
* , etc) executing.
*
*
* Derived classes must call through to the super class's
* implementation of this method.If they do not, an exception will be
* thrown.
*
*
* @param savedInstanceState
*If the activity is being re-initialized after previously being
*shut down then this Bundle contains the data it most recently
*supplied in {@link #onSaveInstanceState}. Note:
*Otherwise it is null.
*
* @see #onStart
* @see #onSaveInstanceState
* @see #onRestoreInstanceState
* @see #onPostCreate
*/
protected void onCreate(Bundle savedInstanceState) {
if (DEBUG_LIFECYCLE)
Slog.v(TAG, "onCreate " + this + ": " + savedInstanceState);
if (mLastNonConfigurationInstances != null) {
mAllLoaderManagers = mLastNonConfigurationInstances.loaders;
}
if (mActivityInfo.parentActivityName != null) {
if (mActionBar == null) {
mEnableDefaultActionBarUp = true;
} else {
mActionBar.setDefaultDisplayHomeAsUpEnabled(true);
}
}
if (savedInstanceState != null) {
Parcelable p = savedInstanceState.getParcelable(FRAGMENTS_TAG);
mFragments
.restoreAllState(
p,
mLastNonConfigurationInstances != null ? mLastNonConfigurationInstances.fragments
: null);
}
mFragments.dispatchCreate();
getApplication().dispatchActivityCreated(this, savedInstanceState);
mCalled = true;
}
在TargetActivity中重写了onCreate方法，而在Activity中的onCreate方法中已经对一些基本的基础进行了操作，如果子类没有super.onCraete方法的调用，会导致子类中的onCreate方法功能不全，从而出现错误。
额外问题：onCreate中的savedInstanceState有何具体作用？具体例子？
在activity的生命周期中，只要离开了可见阶段，或者说失去了焦点，activity就很可能被进程终止了！，被KILL掉了，，这时候，就需要有种机制，能保存当时的状态，这就是savedInstanceState的作用。
当一个Activity在PAUSE时，被kill之前，它可以调用onSaveInstanceState()来保存当前activity的状态信息（在paused状态时，要被KILLED的时候）。用来保存状态信息的Bundle会同时传给两个method,即onRestoreInstanceState()
and onCreate().
示例代码如下：
package com.myandroid.test;
import android.app.Activity;
import android.os.Bundle;
import android.util.Log;
public class AndroidTest extends Activity {
private static final String TAG = "MyNewLog";
/** Called when the activity is first created. */
@Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// If an instance of this activity had previously stopped, we can
// get the original text it started with.
if(null != savedInstanceState)
{
int IntTest = savedInstanceState.getInt("IntTest");
String StrTest = savedInstanceState.getString("StrTest");
Log.e(TAG, "onCreate get the savedInstanceState+IntTest="+IntTest+"+StrTest="+StrTest);
}
setContentView(R.layout.main);
Log.e(TAG, "onCreate");
}
@Override
public void onSaveInstanceState(Bundle savedInstanceState) {
// Save away the original text, so we still have it if the activity
// needs to be killed while paused.
savedInstanceState.putInt("IntTest", 0);
savedInstanceState.putString("StrTest", "savedInstanceState test");
super.onSaveInstanceState(savedInstanceState);
Log.e(TAG, "onSaveInstanceState");
}
@Override
public void onRestoreInstanceState(Bundle savedInstanceState) {
super.onRestoreInstanceState(savedInstanceState);
int IntTest = savedInstanceState.getInt("IntTest");
String StrTest = savedInstanceState.getString("StrTest");
Log.e(TAG, "onRestoreInstanceState+IntTest="+IntTest+"+StrTest="+StrTest);
}
}
