第七课：返回子活动结果（基于AndroidStudio3.2）

        在上一节中，我们发起了一个子活动，我们传递了一些数据。 在这我们将看一下如何从子活动中返回数据。下图显示了如何解决这个问题的事件顺序。

        我们将从MainActivity启动子活动。 这可以通过创建一个来管理显式的intent对象并调用startActivityForResult。 活动可能会启动多个其他活动，每个活动都可以返回一些结果。 当这些结果回来时，它们都将在onActivityResult方法中，所以我们需要知道从哪个活动每个结果都来自; REQUEST_CODE将帮助我们解决这个问题。

Intent intent = new Intent(MainActivity.this, SubActivity.class);
startActivityForResult(intent, REQUEST_CODE)

        当运行时解析intent时，SubActivitygets将被创建并将成为可见。 到那时，它可以通过用户输入创建自己的数据。 如果它想
将数据返回给MainActivity，它需要创建一个intent对象将数据发送回去MainActivity通过捎带意图对象。

String data = "Data to send back";
intent.putExtra("key", data);
setResult(Activity.RESULT_OK, intent);
finish();

        当SubActivity调用finish方法时，它将被销毁，并且MainActivity然后将返回到活动堆栈的顶部。 运行时将调用MainActivity’s 
onActivityResult;这是我们可以提取SubActivitysent的数据的地方。

一、建立演示的工程和UI

1、创建新项目

名称为GetResultsSubActivity

建立子活动SecondActivity

2、添加相应控件

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginTop="80dp"
        android:layout_marginEnd="16dp"
        android:layout_marginBottom="40dp"
        android:gravity="center"
        android:text="TextView"
        app:layout_constraintBottom_toTopOf="@+id/button"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginEnd="16dp"
        android:layout_marginBottom="294dp"
        android:text="Launch 2nd Activity"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />
</android.support.constraint.ConstraintLayout>

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".SecondActivity">

    <EditText
        android:id="@+id/editText"
        android:layout_width="215dp"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginTop="90dp"
        android:layout_marginEnd="16dp"
        android:layout_marginBottom="27dp"
        android:ems="10"
        android:hint="Type something"
        android:inputType="text"
        android:text="Name"
        app:layout_constraintBottom_toTopOf="@+id/button2"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent" />

    <Button
        android:id="@+id/button2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="16dp"
        android:layout_marginEnd="16dp"
        android:layout_marginBottom="301dp"
        android:text="Close"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent" />
</android.support.constraint.ConstraintLayout>

先简单测试ok

二、代码实现

package com.example.administrator.getresultssubactivity;

import android.app.Activity;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity implements View.OnClickListener{

//The request code can be any value; we just need to mark each activity that will send data back to us
    private static final int REQUEST_CODE = 1000;
    Button b;
    TextView t;

    // This will contain all our initialization codes
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        b = (Button) findViewById(R.id.button);
        t = (TextView) findViewById(R.id.textView);
        b.setOnClickListener(this);
    }

    //When the button is clicked, we will launch SecondActivity
    @Override
    public void onClick(View v) {
        //This creates just the usual explicit intent object
        Intent intent = new Intent(this, SecondActivity.class);
        /*
        The startActivity method will simply launch another screen; we are using the
startActivityForResult method because we expect the target activity to return some data to
us. The request code will serve as a marker that we can use later when the result comes back
to us, and we can use that value to route program logic, in case our application starts several
activities
        */
        startActivityForResult(intent, REQUEST_CODE);
    }

    @Override
    protected void onActivityResult(int requestCode, int resultCode, Intent data) {
        super.onActivityResult(requestCode, resultCode, data);
/*
We check if the request code is from an activity we’re interested in. Also, we need to check if the
result code is RESULT_OK, which means the operation has succeeded. Otherwise, you might
need to add, at the very least, some logging codes here
*/
        if((requestCode == REQUEST_CODE) && (resultCode == Activity.RESULT_OK)) {
            //We pull the data using the key “secondactivity”; this is the same key that we used back in
            //SecondActivity, when we injected the data into the intent
            t.setText(data.getStringExtra("secondactivity"));
        }
    }
}

package com.example.administrator.getresultssubactivity;

import android.app.Activity;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;

public class SecondActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_second);

        Button b = (Button) findViewById(R.id.button2);
        b.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Intent intent = new Intent();
                EditText e = (EditText) findViewById(R.id.editText);
                String data = e.getText().toString();
                //We inject the contents of the data variable into the intent object using the putExtra method. We’re
                //assigning it a key of “secondactivity”, and we need to use the same key when we retrieve it later
                intent.putExtra("secondactivity", data);
                //This will prepare the data we are about to return
                setResult(Activity.RESULT_OK, intent);
                //This will kill SecondActivity. As soon as it dies, the runtime will call MainActivity’s onActivityResult
                finish();
            }
        });
    }
}

 

运行ok