第四周——关于startActivityForResult和on

一使用场景

当我们需要从跳转的Activity中返回时候执行相应的操作。启动的时候就使用startActivityForResult(@RequiresPermission Intent intent, int requestCode)来启动。返回的时候就会触发onActivityResult(int requestCode, int resultCode, Intent data)方法

二requestCode和resultCode的作用（onActivityResult）

requestCode:用于标识一个请求，在onActivityResult区分这个请求是哪个页面返回的
resultCode:用于表示返回值，一般使用（RESULT_OK、RESULT_FIRST_USER、RESULT_CANCELED），我一般都是区别是不是RESULT_OK,是的话就执行返回requestCode对应的操作，否则不执行，这里没有什么具体的限定，随心。
eg：

                Intent intent=new Intent(MainActivity.this,Main3Activity.class);
                startActivityForResult(intent,1);
                Intent intent2=new Intent(MainActivity.this,Main2Activity.class);
                startActivityForResult(intent2,2);

 @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
        if (resultCode == RESULT_OK) {
            switch (requestCode) {
                case 1:
                    break;
                case 2:
                    break;
                default:
            }
        }
    }

如上，逻辑简单的话，resultCode 用来判断是否要执行操作，requestCode来执行那个操作。复杂的话可以这样

   @Override
   protected void onActivityResult(int requestCode, int resultCode, Intent data) {
       super.onActivityResult(requestCode, resultCode, data);
       switch (requestCode) {
           case 1:
               switch (requestCode) {
                   case 1:
                       break;
                   case 2:
                       break;
                   default:

               }
               break;
           case 2:
               break;
           default:
       }
   }

三setResult

setResult有两个方法

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

   /**
     * Call this to set the result that your activity will return to its
     * caller.
     *
     * @param resultCode The result code to propagate back to the originating
     *                   activity, often RESULT_CANCELED or RESULT_OK
     *
     * @see #RESULT_CANCELED
     * @see #RESULT_OK
     * @see #RESULT_FIRST_USER
     * @see #setResult(int, Intent)
     */
    public final void setResult(int resultCode) {
        synchronized (this) {
            mResultCode = resultCode;
            mResultData = null;
        }
    }

通过源码可知就是赋值，而且是给成员变量赋值。这两个成员变量在Activity finish的时候会用到。下面我们看下finish源码

finish源码

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

        // Activity was launched when user tapped a link in the Autofill Save UI - Save UI must
        // be restored now.
        if (mIntent != null && mIntent.hasExtra(AutofillManager.EXTRA_RESTORE_SESSION_TOKEN)) {
            getAutofillManager().onPendingSaveUi(AutofillManager.PENDING_UI_OPERATION_RESTORE,
                    mIntent.getIBinderExtra(AutofillManager.EXTRA_RESTORE_SESSION_TOKEN));
        }
    }

finish取了mResultCode和mResultData这两个成员变量的值，结合setResult源码，所以setResult方法必须在finish之前调用，否则resultCode会是默认值0，mResultData会为NULL。