Android应用开发之Acitivy创建Context的过程(一)-职坐标

Android应用开发之Acitivy创建Context的过程(一)

白羽 2018-08-10 来源：网络阅读 1251 评论 0

摘要：本文将带你了解Android应用开发之Acitivy创建Context的过程(一)，希望本文对大家学Android有所帮助

本文将带你了解Android应用开发之Acitivy创建Context的过程(一)，希望本文对大家学Android有所帮助

page1
从本篇文章开始,我们分析一下Activity创建Context的过程.
Context是在ActivityThread的performLaunchActivity函数中创建的, 因此我们就从performLaunchActivity函数作为入口开始分析:
1     private Activity performLaunchActivity(ActivityClientRecord r, Intent customIntent) {
2         // System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");
3
4         ActivityInfo aInfo = r.activityInfo;
5         if (r.packageInfo == null) {
6             r.packageInfo = getPackageInfo(aInfo.applicationInfo, r.compatInfo,
7                     Context.CONTEXT_INCLUDE_CODE);
8         }
9
10         ComponentName component = r.intent.getComponent();
11         if (component == null) {
12             component = r.intent.resolveActivity(
13                 mInitialApplication.getPackageManager());
14             r.intent.setComponent(component);
15         }
16
17         if (r.activityInfo.targetActivity != null) {
18             component = new ComponentName(r.activityInfo.packageName,
19                     r.activityInfo.targetActivity);
20         }
21
22         Activity activity = null;
23         try {
24             java.lang.ClassLoader cl = r.packageInfo.getClassLoader();
25             activity = mInstrumentation.newActivity(
26                     cl, component.getClassName(), r.intent);
27             StrictMode.incrementExpectedActivityCount(activity.getClass());
28             r.intent.setExtrasClassLoader(cl);
29             if (r.state != null) {
30                 r.state.setClassLoader(cl);
31             }
32         } catch (Exception e) {
33             if (!mInstrumentation.onException(activity, e)) {
34                 throw new RuntimeException(
35                     "Unable to instantiate activity " + component
36                     + ": " + e.toString(), e);
37             }
38         }
39
40         try {
41             Application app = r.packageInfo.makeApplication(false, mInstrumentation);
42
43             if (localLOGV) Slog.v(TAG, "Performing launch of " + r);
44             if (localLOGV) Slog.v(
45                     TAG, r + ": app=" + app
46                     + ", appName=" + app.getPackageName()
47                     + ", pkg=" + r.packageInfo.getPackageName()
48                     + ", comp=" + r.intent.getComponent().toShortString()
49                     + ", dir=" + r.packageInfo.getAppDir());
50
51             if (activity != null) {
52                 Context appContext = createBaseContextForActivity(r, activity);
53                 CharSequence title = r.activityInfo.loadLabel(appContext.getPackageManager());
54                 Configuration config = new Configuration(mCompatConfiguration);
55                 if (DEBUG_CONFIGURATION) Slog.v(TAG, "Launching activity "
56                         + r.activityInfo.name + " with config " + config);
57                 activity.attach(appContext, this, getInstrumentation(), r.token,
58                         r.ident, app, r.intent, r.activityInfo, title, r.parent,
59                         r.embeddedID, r.lastNonConfigurationInstances, config);
60
61                 if (customIntent != null) {
62                     activity.mIntent = customIntent;
63                 }
64                 r.lastNonConfigurationInstances = null;
65                 activity.mStartedActivity = false;
66                 int theme = r.activityInfo.getThemeResource();
67                 if (theme != 0) {
68                     activity.setTheme(theme);
69                 }
70
71                 activity.mCalled = false;
72                 mInstrumentation.callActivityOnCreate(activity, r.state);
73                 if (!activity.mCalled) {
74                     throw new SuperNotCalledException(
75                         "Activity " + r.intent.getComponent().toShortString() +
76                         " did not call through to super.onCreate()");
77                 }
78                 r.activity = activity;
79                 r.stopped = true;
80                 if (!r.activity.mFinished) {
81                     activity.performStart();
82                     r.stopped = false;
83                 }
84                 if (!r.activity.mFinished) {
85                     if (r.state != null) {
86                         mInstrumentation.callActivityOnRestoreInstanceState(activity, r.state);
87                     }
88                 }
89                 if (!r.activity.mFinished) {
90                     activity.mCalled = false;
91                     mInstrumentation.callActivityOnPostCreate(activity, r.state);
92                     if (!activity.mCalled) {
93                         throw new SuperNotCalledException(
94                             "Activity " + r.intent.getComponent().toShortString() +
95                             " did not call through to super.onPostCreate()");
96                     }
97                 }
98             }
99             r.paused = true;
100
101             mActivities.put(r.token, r);
102
103         } catch (SuperNotCalledException e) {
104             throw e;
105
106         } catch (Exception e) {
107             if (!mInstrumentation.onException(activity, e)) {
108                 throw new RuntimeException(
109                     "Unable to start activity " + component
110                     + ": " + e.toString(), e);
111             }
112         }
113
114         return activity;
115     }
第52行(ActivityThread->performLaunchActivity)调用createBaseContextForActivity函数为刚刚创建的activity对象创建Context对象, 关于performLaunchActivity函数的详细分析可以参考page2文件.
第57-59行(ActivityThread->performLaunchActivity)调用Activity的attach函数, 关于attach函数的详细分析可以参考page5文件.
page2
ActivityThread的createBaseContextForActivity函数定义如下:
1     private Context createBaseContextForActivity(ActivityClientRecord r,
2             final Activity activity) {
3         ContextImpl appContext = new ContextImpl();
4         appContext.init(r.packageInfo, r.token, this);
5         appContext.setOuterContext(activity);
6
7         // For debugging purposes, if the activity's package name contains the value of
8         // the "debug.use-second-display" system property as a substring, then show
9         // its content on a secondary display if there is one.
10         Context baseContext = appContext;
11         String pkgName = SystemProperties.get("debug.second-display.pkg");
12         if (pkgName != null && !pkgName.isEmpty()
13                 && r.packageInfo.mPackageName.contains(pkgName)) {
14             DisplayManagerGlobal dm = DisplayManagerGlobal.getInstance();
15             for (int displayId : dm.getDisplayIds()) {
16                 if (displayId != Display.DEFAULT_DISPLAY) {
17                     Display display = dm.getRealDisplay(displayId);
18                     baseContext = appContext.createDisplayContext(display);
19                     break;
20                 }
21             }
22         }
23         return baseContext;
24     }
第3行(ActivityThread->createBaseContextForActivity)会new一个ContextImpl对象. ContextImpl的构造函数的详细分析可以参考page3文件.
第4行(ActivityThread->createBaseContextForActivity)会调用ContextImpl的init函数, 关于init函数的详细分析可以参考page4文件.
第5行(ActivityThread->createBaseContextForActivity)调用ContextImpl的setOuterContext函数, ContextImpl的setOuterContext函数定义如下:
final void setOuterContext(Context context) {
         mOuterContext = context;
     }
这样, ContextImpl也会拿着activity对象.

第10-22行(ActivityThread->createBaseContextForActivity)是干什么呢?不知道.
page3
在这里我们分析一下ContextImpl类的构造过程.我们先来看一下ContextImpl类的继承体系, ContextImpl类的定义如下:
class ContextImpl extends Context {
public abstract class Context {

ContextImpl类的构造函数如下所示
ContextImpl() {
         mOuterContext = this;
     }
在ContextImpl的构造函数中, 只是初始化了成员变量mOuterContext, 使之指向该ContextImpl对象.
成员变量mOuterContext的定义如下:
private Context mOuterContext;