StartActivity的內部代碼實現原理

1.  首先調用Activity的startActivity，-1表示 target Activity finish的時候不通知original Activity

  public void startActivity(Intent intent) {
        startActivityForResult(intent, -1);
    }

2 . 調用Activityd的startActivityForResult方法, Instrumentation class 裡面有一個ActivityMonitor 的內部類，用來監控view和system的交互

  public void startActivityForResult(Intent intent, int requestCode) {
        if (mParent == null) {
            Instrumentation.ActivityResult ar =
                mInstrumentation.execStartActivity(
                    this, mMainThread.getApplicationThread(), mToken, this,
                    intent, requestCode);
            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 {
            mParent.startActivityFromChild(this, intent, requestCode);
        }
    }

3. 調用Instrumentation class的 ExecStartActivity：

 public ActivityResult execStartActivity(
        Context who, IBinder contextThread, IBinder token, Activity target,
        Intent intent, int requestCode) {
        IApplicationThread whoThread = (IApplicationThread) contextThread;

      // 如果找到匹配的Activity，am.mHits++
        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;
                    }
                }
            }
        }
        try {

     //調用ActivityManagerProxy的startActivity方法
            int result = ActivityManagerNative.getDefault()
                .startActivity(whoThread, intent,
                        intent.resolveTypeIfNeeded(who.getContentResolver()),
                        null, 0, token, target != null ? target.mEmbeddedID : null,
                        requestCode, false, false);
            checkStartActivityResult(result, intent);
        } catch (RemoteException e) {
        }
        return null;
    }

4. ActivityManagerProxy 的startActivity的具體實現：

public int startActivity(IApplicationThread caller, Intent intent,
            String resolvedType, Uri[] grantedUriPermissions, int grantedMode,
            IBinder resultTo, String resultWho,
            int requestCode, boolean onlyIfNeeded,
            boolean debug) throws RemoteException {

     // Parcel 本質上把它當成一個Serialize就可以了，只是它是在內存中完成的序列化和反序列化，利用的是連續的內存空間(這裡不要作為重點)
        Parcel data = Parcel.obtain();
        Parcel reply = Parcel.obtain();
        data.writeInterfaceToken(IActivityManager.descriptor);
        data.writeStrongBinder(caller != null ? caller.asBinder() : null);
        intent.writeToParcel(data, 0);
        data.writeString(resolvedType);
        data.writeTypedArray(grantedUriPermissions, 0);
        data.writeInt(grantedMode);
        data.writeStrongBinder(resultTo);
        data.writeString(resultWho);
        data.writeInt(requestCode);
        data.writeInt(onlyIfNeeded ? 1 : 0);
        data.writeInt(debug ? 1 : 0);

//具體請參照後面的代碼   （這個函數只看到這裡就好了！！！）
        mRemote.transact(START_ACTIVITY_TRANSACTION, data, reply, 0);
        reply.readException();
        int result = reply.readInt();
        reply.recycle();
        data.recycle();
        return result;
    }

//清除sOwnedPool中parcel對象，返回一個新的parcel

public static Parcel obtain() {

        final Parcel[] pool = sOwnedPool;

        synchronized (pool) {

            Parcel p;

            for (int i=0; i<POOL_SIZE; i++) {

                p = pool[i];

                if (p != null) {

                    pool[i] = null;

                    if (DEBUG_RECYCLE) {

                        p.mStack = new RuntimeException();

                    }

                    return p;

                }

            }

        }

        return new Parcel(0);

}

 

// 清除pool中存儲parcel,通過jni free剛才分配的內存

  public final void recycle() {

        if (DEBUG_RECYCLE) mStack = null;

        freeBuffer();

        final Parcel[] pool = mOwnObject != 0 ? sOwnedPool : sHolderPool;

        synchronized (pool) {

            for (int i=0; i<POOL_SIZE; i++) {

                if (pool[i] == null) {

                    pool[i] = this;

                    return;

                }

            }

        }

    }

//

private native void freeBuffer();

 

 

// mRemote 實際通過ActivityManagerNative.getDefault進行注冊的，實際就是ActivityManagerService
static public IActivityManager getDefault()

    {

        if (gDefault != null) {

            //if (Config.LOGV) Log.v(

            //    "ActivityManager", "returning cur default = " + gDefault);

            return gDefault;

        }

        IBinder b = ServiceManager.getService("activity");

        if (Config.LOGV) Log.v(

            "ActivityManager", "default service binder = " + b);

        gDefault = asInterface(b);

        if (Config.LOGV) Log.v(

            "ActivityManager", "default service = " + gDefault);

        return gDefault;

}

5. 實際調用proxy中的Ontransact方法

   public boolean onTransact(int code, Parcel data, Parcel reply, int flags)

            throws RemoteException {

        switch (code) {

        case START_ACTIVITY_TRANSACTION:

        {

            data.enforceInterface(IActivityManager.descriptor);

            IBinder b = data.readStrongBinder();

            IApplicationThread app = ApplicationThreadNative.asInterface(b);

            Intent intent = Intent.CREATOR.createFromParcel(data);

            String resolvedType = data.readString();

            Uri[] grantedUriPermissions = data.createTypedArray(Uri.CREATOR);

            int grantedMode = data.readInt();

            IBinder resultTo = data.readStrongBinder();

            String resultWho = data.readString();  

            int requestCode = data.readInt();

            boolean onlyIfNeeded = data.readInt() != 0;

            boolean debug = data.readInt() != 0;

            int result = startActivity(app, intent, resolvedType,

                    grantedUriPermissions, grantedMode, resultTo, resultWho,

                    requestCode, onlyIfNeeded, debug);

            reply.writeNoException();

            reply.writeInt(result);

            return true;

        }

6 . startActivity 實際調用ActivityManagerService中的startActivity方法

由於StartActivity太長，拿關鍵部分分享

synchronized(this) {

            final long origId = Binder.clearCallingIdentity();

            int res = startActivityLocked(caller, intent, resolvedType,

                    grantedUriPermissions, grantedMode, aInfo,

                    resultTo, resultWho, requestCode, -1, -1,

                    onlyIfNeeded, componentSpecified);

            Binder.restoreCallingIdentity(origId);

            return res;

        }

7. startActivityLocked實際調用ApplicationThread中scheduleLaunchActivity

public final void scheduleLaunchActivity(Intent intent, IBinder token,

                ActivityInfo info, Bundle state, List<ResultInfo> pendingResults,

                List<Intent> pendingNewIntents, boolean notResumed, boolean isForward) {

            ActivityRecord r = new ActivityRecord();（histroyRicord）

 

            r.token = token;

            r.intent = intent;

            r.activityInfo = info;

            r.state = state;

 

            r.pendingResults = pendingResults;

            r.pendingIntents = pendingNewIntents;

 

            r.startsNotResumed = notResumed;

            r.isForward = isForward; www.2cto.com

 

            queueOrSendMessage(H.LAUNCH_ACTIVITY, r);

        }

8. queueOrSendMessage 然後發消息給ActivityThread 的Handler

  case LAUNCH_ACTIVITY: {

                    ActivityRecord r = (ActivityRecord)msg.obj;

 

                    r.packageInfo = getPackageInfoNoCheck(

                            r.activityInfo.applicationInfo);

                    handleLaunchActivity(r);

                } break;

 

具體調用

    private final Activity performLaunchActivity(ActivityRecord r) {

        // System.out.println("##### [" + System.currentTimeMillis() + "] ActivityThread.performLaunchActivity(" + r + ")");

 

                ***

                mInstrumentation.callActivityOnCreate(activity, r.state);

                ***

        } catch (SuperNotCalledException e) {

            throw e;

 

        } catch (Exception e) {

            if (!mInstrumentation.onException(activity, e)) {

                throw new RuntimeException(

                    "Unable to start activity " + component

                    + ": " + e.toString(), e);

            }

        }

        return activity;

}

作者：康春晖