activity间数据传递--startActivityForResult()&setResult()

一个activity启动另一个activity最简单的方式是使用startActivity()方法，activity调用startActivity()方法时，实际上是把调用请求发送给操作系统的ActivityManager。ActivityManager负责创建Intent中指定activity的实例并调用其onCreate()方法，应用内部交互图

activity间传递数据则要调用startActivityForResult(…)方法，

FirstActivity:

Intent intent=new Intent(this,Second.class);
intent.putExtra("index",true);
startActivityForResult(intent,0);

@Override
    public void startActivityForResult(Intent intent, int requestCode) {
        super.startActivityForResult(intent, requestCode);
    }

SecondActivity接收数据并调用setResult(…)把处理结果回传给FirstActivity

public class SecondActivity extends AppCompatActivity implements View.OnClickListener{
    private TextView answer_tv;
    private Button show_answer_tn;
    private Boolean answer_cheat;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_cheat);
        answer_tv= (TextView) findViewById(R.id.answer_textview);
        show_answer_tn= (Button) findViewById(R.id.show_answer_btn);    answer_cheat=getIntent().getBooleanExtra("index",false);
    }

    @Override
    public void onClick(View v) {
        switch (v.getId()){
            case R.id.show_answer_btn:             answer_tv.setText(answer_cheat.toString().trim());
                setAnswerShownResult(true);
                break;
        }
    }

    /*setResult()方法应在finish()方法前调用，点击后退键就是调用finish()方法，
      最好就是在onCreate()方法中调用,否则会出现还没调用setResult()方法就返回前一个activity
      点击后退键默认回传的resultCode为RESULT_CANCELED,表示什么都没做，操作取消
    */
    private void setAnswerShownResult(boolean isAnswerShown){
        Intent data=new Intent();
        data.putExtra("answer_shown",isAnswerShown);
        setResult(RESULT_OK,data);
    }
}

调用setResult()的时机，查看源码

public final void setResult(int resultCode, Intent data) {
        synchronized (this) {
            mResultCode = resultCode;
            mResultData = data;
        }
    }

public void finish() {
        finish(DONT_FINISH_TASK_WITH_ACTIVITY);
    }

private void finish(int finishTask) {
        if (mParent == null) {
            int resultCode;
            Intent resultData;
            synchronized (this) {
                resultCode = mResultCode;
                resultData = mResultData;
            }
            if (false) Log.v(TAG, "Finishing self: token=" + mToken);
            try {
                if (resultData != null) {
                    resultData.prepareToLeaveProcess(this);
                }
                if (ActivityManagerNative.getDefault()
                        .finishActivity(mToken, resultCode, resultData, finishTask)) {
                    mFinished = true;
                }
            } catch (RemoteException e) {
                // Empty
            }
        } else {
            mParent.finishFromChild(this);
        }
    }

可以看出activity返回result是在被finish的时候，也就是说调用setResult()方法必须在finish()之前。
那么如果在如下方法中调用setResult()也有可能不会返回成功： onPause()， onStop()， onDestroy(),
因为这些方法调用不一定是在finish之前的，当然在onCreate()就调用setResult肯定是在finish之前的

按BACK键从一个Activity退出来的，一按BACK，android就会自动调用Activity的finish()方法，然后设置resultCode为RESULT_CANCELED，也就不会返回任何数据了 .

@Override
    public void onBackPressed() {
        setResult(...);
        super.onBackPressed();
    }

也可通过重写onBackPressed()方法捕获用户点击后退键的动作，要注意的是setResult(…)要在super.onBackPressed()之前调用

FirstActivity要接收来自SecondActivity返回的数据，重写onActivityResult()方法

    //此方法是接收来自其他activity中setResult()传回来的resultCode和intent
    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (requestCode==0&&resultCode==RESULT_OK){
            if (data!=null){
                isCheated=data.getBooleanExtra("answer_shown",false);
            }
        }
    }