mContext.startActivity(intent)失敗原因分析

    在android中經常要啟動Activity，顯示啟動或者隐式啟動。啟動Activity都要調用mContext.startActivity(intent)方法。.startActivity(intent)為Context類的的抽象方法。

    當在Activity中調用this..startActivity(intent)來啟動Activity時，this為該activity的一個執行個體，在Activity中實作了該方法，是以會直接調用Activity..startActivity(intent),該實作方法最終調用startActivityForResult(Intent intent, int requestCode, Bundle options)方法，源碼如下：

public void startActivityForResult(Intent intent, int requestCode, Bundle options) {

        if (mParent == null) {

            Instrumentation.ActivityResult ar =

                mInstrumentation.execStartActivity(

                    this, mMainThread.getApplicationThread(), mToken, this,

                    intent, requestCode, options);

            if (ar != null) {

                mMainThread.sendActivityResult(

                    mToken, mEmbeddedID, requestCode, ar.getResultCode(),

                    ar.getResultData());

            }

            if (requestCode >= 0) {

                // If this start is requesting a result, we can avoid making

                // the activity visible until the result is received.  Setting

                // this code during onCreate(Bundle savedInstanceState) or onResume() will keep the

                // activity hidden during this time, to avoid flickering.

                // This can only be done when a result is requested because

                // that guarantees we will get information back when the

                // activity is finished, no matter what happens to it.

                mStartedActivity = true;

            }

        } else {

            if (options != null) {

                mParent.startActivityFromChild(this, intent, requestCode, options);

            } else {

                // Note we want to go through this method for compatibility with

                // existing applications that may have overridden it.

                mParent.startActivityFromChild(this, intent, requestCode);

            }

        }

    }

如果mContext不是Activity的執行個體（如service,application等），則會調用其父類ContextWrapper的startActivity(intent)方法，方法如下：

@Override

    public void startActivity(Intent intent) {

        mBase.startActivity(intent);

    }

方法中mBase為抽象類Context的實作類ContextImpl的執行個體，該執行個體實作startActivity(intent)如下：

@Override

    public void startActivity(Intent intent, Bundle options) {

        warnIfCallingFromSystemProcess();

        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?");

        }

        mMainThread.getInstrumentation().execStartActivity(

            getOuterContext(), mMainThread.getApplicationThread(), null,

            (Activity)null, intent, -1, options);

    }

    從if語句可以看出，如果該intent的Flag不是Intent.FLAG_ACTIVITY_NEW_TASK(intent.getIFlags()&Intent.FLAG_ACTIVITY_NEW_TASK)，就會抛出異常，并不啟動Activity。

 結論：(1)從Activity中啟動新的Activity時可以直接mContext.startActivity(intent)就好，

(2)如果從其他Context中啟動Activity則必須給intent設定Flag:

intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK) ; 

mContext.startActivity(intent);