解析Activity中的onCreate方法.doc

解析Activity中的onCreate方法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 Overrideprotected void onCreate(Bundle savedInstanceState) / TODO Auto-generated method stubsuper.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 activitys 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 N; i+) final ActivityWaiter aw = mWaitingActivities.get(i);final Intent intent = ent;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 N; 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(ernal.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 Overrideprotected void onCreate(Bundle savedInstanceState) / TODO Auto-generated method stubsuper.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 activitys* UI, using link #findViewById to programmatically interact with widgets* in the UI, calling* link #managedQuery(.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 classs* 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 classAndroidTest extends Activity private static final String TAG = MyNewLog;/* Called when the activity is first created. */Overridepublic 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