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update facade
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@hehonghui
hehonghui committed Mar 13, 2015
1 parent 58d8784 commit 71c1088
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247 changes: 191 additions & 56 deletions facade/elsdnwn/readme.md
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Expand Up @@ -39,7 +39,7 @@ Android设计模式源码解析之外观模式(Facade)
### 实现源码
TvController.java

```
```java
public class TvController {
private PowerSystem mPowerSystem = new PowerSystem();
private VoiceSystem mVoiceSystem = new VoiceSystem();
Expand Down Expand Up @@ -168,85 +168,220 @@ public class TvController {
上面的TvController封装了对电源、声音、频道切换的操作，为用户提供了一个统一的接口。使得用户控制电视机更加的方便、更易于使用。

## Android源码中的模式实现
在Android源码中，ContextImpl这个类封装了很多模块（子系统），比如startActivity()、sendBroadcast()等，分别给用户一个统一的操作入口，简单实例如下：
在开发过程中，Context是最重要的一个类型。它封装了很多重要的操作，比如startActivity()、sendBroadcast()等，几乎是开发者对应用操作的统一入口。Context是一个抽象类，它只是定义了抽象接口，真正的实现在ContextImpl类中。它就是今天我们要分析的外观类。

在应用启动时，首先会fork一个子进程，并且调用ActivityThread.main方法启动该进程。ActivityThread又会构建Application对象，然后和Activity、ContextImpl关联起来，然后再调用Activity的onCreate、onStart、onResume函数使Activity运行起来。我们看看下面的相关代码:

```
package com.elsdnwn.Facade;
private final void handleLaunchActivity(ActivityClientRecord r, Intent customIntent) {
// 代码省略
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
// 1、创建并且加载Activity，调用其onCreate函数
Activity a = performLaunchActivity(r, customIntent);
public class MainActivity extends Activity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if (a != null) {
r.createdConfig = new Configuration(mConfiguration);
Bundle oldState = r.state;
// 2、调用Activity的onResume方法，使Activity变得可见
handleResumeActivity(r.token, false, r.isForward);
Intent intent = new Intent();
intent.setClass(this, MainActivity2.class);
startActivity(intent);
}
}
}
}
```
private final Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
// System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");
// 代码省略
通过上面的代码可以看到，启动startActivity需要传递一个Context上下文对象，为什么要传递呢？下面我们看一下ContextImpl相关源码：
Activity activity = null;
try {
java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
// 1、创建Activity
activity = mInstrumentation.newActivity(
cl, component.getClassName(), r.intent);
r.intent.setExtrasClassLoader(cl);
if (r.state != null) {
r.state.setClassLoader(cl);
}
} catch (Exception e) {
if (!mInstrumentation.onException(activity, e)) {
throw new RuntimeException(
"Unable to instantiate activity " + component
+ ": " + e.toString(), e);
}
}
```
try {
// 2、创建Application
Application app = r.packageInfo.makeApplication(false, mInstrumentation);
if (activity != null) {
// ***** 构建ContextImpl ******
ContextImpl appContext = new ContextImpl();
appContext.init(r.packageInfo, r.token, this);
appContext.setOuterContext(activity);
// 获取Activity的title
CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
Configuration config = new Configuration(mConfiguration);
// 3、Activity与context, Application关联起来
activity.attach(appContext, this, getInstrumentation(), r.token,
r.ident, app, r.intent, r.activityInfo, title, r.parent,
r.embeddedID, r.lastNonConfigurationInstance,
r.lastNonConfigurationChildInstances, config);
// 代码省略
// 4、回调Activity的onCreate方法
mInstrumentation.callActivityOnCreate(activity, r.state);
// 代码省略
}
r.paused = true;
mActivities.put(r.token, r);
} catch (SuperNotCalledException e) {
throw e;
} catch (Exception e) {
}
return activity;
}
final void handleResumeActivity(IBinder token, boolean clearHide, boolean isForward) {
unscheduleGcIdler();
// 1、最终调用Activity的onResume方法
ActivityClientRecord r = performResumeActivity(token, clearHide);
// 代码省略
// 2、这里是重点，在这里使DecorView变得可见
if (r.window == null && !a.mFinished && willBeVisible) {
// 获取Window，即PhoneWindow类型
r.window = r.activity.getWindow();
// 3、获取Window的顶级视图，并且使它可见
View decor = r.window.getDecorView();
decor.setVisibility(View.INVISIBLE);
// 4、获取WindowManager
ViewManager wm = a.getWindowManager();
// 5、构建LayoutParams参数
WindowManager.LayoutParams l = r.window.getAttributes();
a.mDecor = decor;
l.type = WindowManager.LayoutParams.TYPE_BASE_APPLICATION;
l.softInputMode |= forwardBit;
if (a.mVisibleFromClient) {
a.mWindowAdded = true;
// 6、将DecorView添加到WindowManager中，最终的操作是通过WindowManagerService的addView来操作
wm.addView(decor, l);
}
} else if (!willBeVisible) {
if (localLOGV) Slog.v(
TAG, "Launch " + r + " mStartedActivity set");
r.hideForNow = true;
}
// 代码省略
}
@Override
public final ActivityClientRecord performResumeActivity(IBinder token,
boolean clearHide) {
ActivityClientRecord r = mActivities.get(token);
if (r != null && !r.activity.mFinished) {
try {
// 代码省略
// 执行onResume
r.activity.performResume();
// 代码省略
} catch (Exception e) {
}
}
return r;
}
```

Activity启动之后，Android给我们提供了操作系统服务的统一入口，也就是Activity本身。这些工作并不是Activity自己实现的，而是将操作委托给Activity父类ContextThemeWrapper的mBase对象，这个对象的实现类就是ContextImpl ( 也就是performLaunchActivity方法中构建的ContextImpl )。


```java
class ContextImpl extends Context {
private final static String TAG = "ApplicationContext";
private final static boolean DEBUG = false;
private final static boolean DEBUG_ICONS = false;

private static final Object sSync = new Object();
private static AlarmManager sAlarmManager;
private static PowerManager sPowerManager;
private static ConnectivityManager sConnectivityManager;
private AudioManager mAudioManager;
LoadedApk mPackageInfo;
private Resources mResources;
private PackageManager mPackageManager;
private NotificationManager mNotificationManager = null;
private ActivityManager mActivityManager = null;

// 代码省略

@Override
public void sendBroadcast(Intent intent) {
String resolvedType = intent.resolveTypeIfNeeded(getContentResolver());
try {
ActivityManagerNative.getDefault().broadcastIntent(
mMainThread.getApplicationThread(), intent, resolvedType, null,
Activity.RESULT_OK, null, null, null, false, false);
} catch (RemoteException e) {
}
}


@Override
public void startActivity(Intent intent) {
if ((intent.getFlags()&Intent.FLAG_ACTIVITY_NEW_TASK) == 0) {
throw new AndroidRuntimeException(
"Calling startActivity() from outside of an Activity "
+ " context requires the FLAG_ACTIVITY_NEW_TASK flag."
+ " Is this really what you want?");
}
// 其实是用的getInstrumentation().execStartActivity();
mMainThread.getInstrumentation().execStartActivity(
getOuterContext(), mMainThread.getApplicationThread(), null,
(Activity)null, intent, -1);
getOuterContext(), mMainThread.getApplicationThread(), null, null, intent, -1);
}
```

我们可以看出启动Activity其实是用的Instrumentation()类里的getInstrumentation().execStartActivity(); 继续查看Instrumentation类里的execStartActivity()的源码：

```
/**
* 需要接受一个Context上下文对象来确定activity的开始
*/
public ActivityResult execStartActivity(
Context who, IBinder contextThread, IBinder token, Activity target,
Intent intent, int requestCode, Bundle options) {
IApplicationThread whoThread = (IApplicationThread) contextThread;
if (mActivityMonitors != null) {
synchronized (mSync) {
final int N = mActivityMonitors.size();
for (int i=0; i<N; i++) {
final ActivityMonitor am = mActivityMonitors.get(i);
if (am.match(who, null, intent)) {
am.mHits++;
if (am.isBlocking()) {
return requestCode >= 0 ? am.getResult() : null;
}
break;
}
}


@Override
public ComponentName startService(Intent service) {
try {
ComponentName cn = ActivityManagerNative.getDefault().startService(
mMainThread.getApplicationThread(), service,
service.resolveTypeIfNeeded(getContentResolver()));
if (cn != null && cn.getPackageName().equals("!")) {
throw new SecurityException(
"Not allowed to start service " + service
+ " without permission " + cn.getClassName());
}
return cn;
} catch (RemoteException e) {
return null;
}
// 代码略...
return null;
}

@Override
public String getPackageName() {
if (mPackageInfo != null) {
return mPackageInfo.getPackageName();
}
throw new RuntimeException("Not supported in system context");
}
}
```
可以看到，ContextImpl内部有很多xxxManager类的对象，也就是我们上文所说的各种子系统的角色。ContextImpl内部封装了一些系统级别的操作，有的子系统功能虽然没有实现，但是也提供了访问该子系统的接口，比如获取ActivityManager的getActivityManager方法。

比如我们要启动一个Activity的时候，我们调用的是startActivity方法，这个功能的内部实现实际上是Instrumentation完成的。ContextImpl封装了这个功能，使得用户根本不需要知晓Instrumentation相关的信息，直接使用startActivity即可完成相应的工作。其他的子系统功能也是类似的实现，比如启动Service和发送广播内部使用的是ActivityManagerNative等。ContextImpl的结构图如下 :
![context](images/contextimpl.png)

通过上面的代码可以看出启动Activity需要传递一个上下文Context才可以，通过外观模式我们不需要知道startActivity是怎么启动的，只需要传递上下文Context，这样就大大降低了客户端与子系统的耦合。
外观模式非常的简单，只是封装了子系统的操作，并且暴露接口让用户使用，避免了用户需要与多个子系统进行交互，降低了系统的耦合度、复杂度。


## 4. 杂谈
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