resolveActivity解析

resolveActivity是Activity创建过程中非常重要的一个函数，会在ActivityStack的startActivityMayWait中调用。这个函数的主要作用就是根据intent去收集需要启动的activity的信息，看下函数的原型：

   ActivityInfo resolveActivity(Intent intent, String resolvedType, boolean debug,           String profileFile, ParcelFileDescriptor profileFd, boolean autoStopProfiler) {       // Collect information about the target of the Intent.       ActivityInfo aInfo;       try {           ResolveInfo rInfo =               AppGlobals.getPackageManager().resolveIntent(                       intent, resolvedType,                       PackageManager.MATCH_DEFAULT_ONLY                       | ActivityManagerService.STOCK_PM_FLAGS);           aInfo = rInfo != null ? rInfo.activityInfo : null;       } catch (RemoteException e) {           aInfo = null;       }   if (aInfo != null) {       // Store the found target back into the intent, because now that       // we have it we never want to do this again.  For example, if the       // user navigates back to this point in the history, we should       // always restart the exact same activity.       intent.setComponent(new ComponentName(               aInfo.applicationInfo.packageName, aInfo.name));       // Don't debug things in the system process       if (debug) {           if (!aInfo.processName.equals("system")) {               mService.setDebugApp(aInfo.processName, true, false);           }       }       if (profileFile != null) {           if (!aInfo.processName.equals("system")) {               mService.setProfileApp(aInfo.applicationInfo, aInfo.processName,                       profileFile, profileFd, autoStopProfiler);           }       }   }   return aInfo;

}
为了不影响对resolveActivity的分析，需要把Activity启动过程中传到此函数的参数说明一下，当我们点击launcher界面启动一个Activity的时候，intent就是我们正常启动启动Activity时的intent;其余的boolean类型的值为false,非boolean类型的值为null。
函数的关键在AppGlobals.getPackageManager().resolveIntent(intent, resolvedType, PackageManager.MATCH_DEFAULT_ONLY | ActivityManagerService.STOCK_PM_FLAGS)。AppGlobals这个变量提供了一个静态方法，方便应用可以获取系统的函数调用接口。最重要的部分应该就是getPackageManager().resolveInten了，ok，到PackageManagerService看下resolveIntent这个方法。

   public ResolveInfo resolveIntent(Intent intent, String resolvedType,           int flags) {       List query = queryIntentActivities(intent, resolvedType, flags);       return chooseBestActivity(intent, resolvedType, flags, query);   }

resolveIntent经过了两个步骤，首先根据传递的信息选出符合条件的集合，然后从集合中找出最合适的信息。第一步queryIntentActivities根据intent, resolveType,flag信息选出可能存在的Activity的信息;chooseBestActivity会从第一步的信息的集合中选择最符合条件的Activity,如果chooseBestActivity选出的符合条件的Activity不止一个，则会让用户选择具体需要启动哪个Activity。可以看下chooseBestActivity的实现：

   private ResolveInfo chooseBestActivity(Intent intent, String resolvedType,                                          int flags, List query) {       if (query != null) {           final int N = query.size();           /**如果根据条件只找到了一个Activity，则直接返回*/           if (N == 1) {               return query.get(0);           } else if (N > 1) {               /**根据条件找到了多个Activity*/               // If there is more than one activity with the same priority,               // then let the user decide between them.               ResolveInfo r0 = query.get(0);               ResolveInfo r1 = query.get(1);               if (DEBUG_INTENT_MATCHING) {                   Log.d(TAG, r0.activityInfo.name + "=" + r0.priority + " vs "                           + r1.activityInfo.name + "=" + r1.priority);               }               /**如果第一个Activity有更高的优先级，或者default设置不相同，则返回第一个Activity*/               // If the first activity has a higher priority, or a different               // default, then it is always desireable to pick it.               if (r0.priority != r1.priority                       || r0.preferredOrder != r1.preferredOrder                       || r0.isDefault != r1.isDefault) {                   return query.get(0);               }               // If we have saved a preference for a preferred activity for               // this Intent, use that.               ResolveInfo ri = findPreferredActivity(intent, resolvedType,                       flags, query, r0.priority);               if (ri != null) {                   return ri;               }               return mResolveInfo;           }       }       return null;   }

多有多个Activity符合查询条件时，最终会进入findPreferredActivity找到最合适的Activity。findPreferredActivity最终会调用findPersistentPreferredActivityLP这个函数。由于正在分析Activity的启动流程，这个函数我还没有去分析，个人猜想可能是这样一个流程，类似于我们在android上打开一个网页，如果你有多个浏览器可以去打开网页，那么android弹出对话框，让用户选择使用哪个浏览器去打开网页;如果用户选择了一个浏览器并且勾选了总是，那么下次再打开网页的时候就会启动已经默认的浏览器，如果用户没有勾选总是，则每次打开网页的时候就会弹出对话框让用户选择。

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