startActivityForResult的用法

startActivityForResult的用法

startActivity()，应该是安卓开发初学者最先接触的一个方法。从一个Activity：A启动跳转到另一个Activity：B，借助启动的Intent能够携带一些参数，数据流向A->B：

val intent = Intent()
    //init
    resultData.putExtra(FROM, "Quibbler")
    
    startActivity(intent)

1、startActivityForResult方法

通常还会有一些场景，启动完B，返回到A界面。需要返回结果，这时候就要用到startActivityForResult()方法：

startActivityForResult(intent, REQUEST_CODE)

其实startActivity()内部也是调用startActivityForResult()方法，传入的requestCode参数默认是-1，即不需要接收所启动Activity设置的返回结果（即使有的话）。

@Override
    public void startActivity(Intent intent, @Nullable Bundle options) {
        getAutofillClientController().onStartActivity(intent, mIntent);
        if (options != null) {
            startActivityForResult(intent, -1, options);
        } else {
            // Note we want to go through this call for compatibility with
            // applications that may have overridden the method.
            startActivityForResult(intent, -1);
        }
    }

2、setResult设置返回结果

与此同时，被启动的Activity：B要在返回之前调用setResult(int resultCode, Intent data)方法，设置需要传给启动Activity：A的数据，数据载体也是Intent，requestCode参数值不能小于0。

val resultData = Intent() //put some data in this intent
    
    setResult(RESULT_CODE, resultData)

需要注意的是，只有被启动的Activity：B销毁时数据才会返回。且setResult(int resultCode, Intent data)方法的调用要早于finish()：

override fun finish() {
        val resultData = Intent() //put some data in this intent
        
        //should call before finish()
        setResult(RESULT_CODE, resultData)
        
        super.finish()
    }

因为在finish(int finishTask)方法中判断当前Activity是否有设置resultData，同时做准备工作，将resultData传递给前一个Activity：A。

/**
     * Finishes the current activity and specifies whether to remove the task associated with this
     * activity.
     */
    @UnsupportedAppUsage(maxTargetSdk = Build.VERSION_CODES.R, trackingBug = 170729553)
    private void finish(int finishTask) {
        ...
            int resultCode;
            Intent resultData;
            synchronized (this) {
                resultCode = mResultCode;
                resultData = mResultData;
            }
            if (false) Log.v(TAG, "Finishing self: token=" + mToken);
            if (resultData != null) {
                resultData.prepareToLeaveProcess(this);
            }
            if (ActivityClient.getInstance().finishActivity(mToken, resultCode, resultData,
                    finishTask)) {
                mFinished = true;
        ...
    }

3、onActivityResult返回数据

回到上一个Activity：A中，重写onActivityResult(requestCode: Int, resultCode: Int, data: Intent?)方法。

override fun onActivityResult(requestCode: Int, resultCode: Int, data: Intent?) {
        super.onActivityResult(requestCode, resultCode, data)
        when (requestCode) {
            REQUEST_CODE -> {
                Log.d(TAG, "resultCode = $resultCode")
                
                //TODO(what to do)
            }
            ...
        }
    }

onActivityResult()方法回调早于onResume()，可在该回调方法中获取返回的结果，执行具体业务逻辑。

本篇回顾了Android开发中的一个经典方法：startActivityForResult()，这也是安卓开发中最早接触的一个方法。原理值得深入了解，详见后续《startActivityForResult的原理》。

该方法在AndroidX的ComponentActivity类中已经被废弃，推荐使用registerForActivityResult(ActivityResultContract, ActivityResultCallback)注册来获取启动Activity返回的结果，目的是为了将回调结果业务和Activity解耦。详细用法详见《startActivityForResult的替代方法：registerForActivityResult》。

/**
     * {@inheritDoc}
     *
     * @deprecated This method has been deprecated in favor of using the Activity Result API
     * which brings increased type safety via an {@link ActivityResultContract} and the prebuilt
     * contracts for common intents available in
     * {@link androidx.activity.result.contract.ActivityResultContracts}, provides hooks for
     * testing, and allow receiving results in separate, testable classes independent from your
     * activity. Use
     * {@link #registerForActivityResult(ActivityResultContract, ActivityResultCallback)}
     * passing in a {@link StartActivityForResult} object for the {@link ActivityResultContract}.
     */
    @Override
    @Deprecated
    public void startActivityForResult(@NonNull Intent intent,
            int requestCode) {
        super.startActivityForResult(intent, requestCode);
    }