# 安卓APP

## 目的

1. 实现页面按钮跳转页面
2. 点击按钮后，页面显示来自JNI的字符串

## 实验步骤

### 1. 准备开发环境

- Android Studio
- JDK
- NDK（创建项目时会自动部署）
- CMake（创建项目时会自动部署）

### 2. 创建一个原生C++安卓项目

![image-20241217213119132](images/image-20241217213119132.png)

> 部署原生C++安卓项目时，路径中不能有空格
>
> 使用了API 28: Android 9.0

### 3. 主页面添加按钮

在`MainActivity.java`中添加一个按钮，点击后跳转到`SecondActivity`

```
package com.example.jnidemoapp;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // 页面跳转按钮
        Button button = findViewById(R.id.button_go_to_second);
        button.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(MainActivity.this, SecondActivity.class);
                startActivity(intent);
            }
        });
    }
}
```

### 4. 创建SecondActivity.java

新建`SecondActivity.java`文件，显示来自JNI的字符串

```
package com.example.jnidemoapp;

import android.os.Bundle;
import android.widget.TextView;
import androidx.appcompat.app.AppCompatActivity;

public class SecondActivity extends AppCompatActivity {

    // 加载本地库
    static {
        System.loadLibrary("jnidemoapp");
    }

    // 声明一个本地方法
    public native String stringFromJNI();

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

        // 显示 JNI 返回的字符串
        TextView textView = findViewById(R.id.text_jni_string);
        textView.setText(stringFromJNI());
    }
}
```

### 5. 调整界面布局

- 在`activity_main.xml`中添加一个按钮：
  ```
  <?xml version="1.0" encoding="utf-8"?>
  <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
      android:layout_width="match_parent"
      android:layout_height="match_parent">
  
      <Button
          android:id="@+id/button_go_to_second"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:text="跳转"
          android:layout_centerInParent="true"/>
  </RelativeLayout>
  ```

- 在 `activity_second.xml` 添加一个 `TextView`：
  ```
  <?xml version="1.0" encoding="utf-8"?>
  <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
      android:layout_width="match_parent"
      android:layout_height="match_parent">
  
      <TextView
          android:id="@+id/text_jni_string"
          android:layout_width="wrap_content"
          android:layout_height="wrap_content"
          android:text="等待JNI数据..."
          android:textSize="18sp"
          android:layout_centerInParent="true"/>
  </RelativeLayout>
  ```

### 6. 添加JNI字符串

- 配置`CMakeLists.txt`文件

  ```
  # For more information about using CMake with Android Studio, read the
  # documentation: https://d.android.com/studio/projects/add-native-code.html
  
  # Sets the minimum version of CMake required to build the native library.
  
  cmake_minimum_required(VERSION 3.22.1)
  
  # Declares and names the project.
  
  project("jnidemoapp")
  
  # Creates and names a library, sets it as either STATIC
  # or SHARED, and provides the relative paths to its source code.
  # You can define multiple libraries, and CMake builds them for you.
  # Gradle automatically packages shared libraries with your APK.
  
  add_library( # Sets the name of the library.
          jnidemoapp
  
          # Sets the library as a shared library.
          SHARED
  
          # Provides a relative path to your source file(s).
          native-lib.cpp)
  
  # Searches for a specified prebuilt library and stores the path as a
  # variable. Because CMake includes system libraries in the search path by
  # default, you only need to specify the name of the public NDK library
  # you want to add. CMake verifies that the library exists before
  # completing its build.
  
  find_library( # Sets the name of the path variable.
          log-lib
  
          # Specifies the name of the NDK library that
          # you want CMake to locate.
          log)
  
  # Specifies libraries CMake should link to your target library. You
  # can link multiple libraries, such as libraries you define in this
  # build script, prebuilt third-party libraries, or system libraries.
  
  target_link_libraries( # Specifies the target library.
          jnidemoapp
  
          # Links the target library to the log library
          # included in the NDK.
          ${log-lib})
  ```

  > 这里的库名称需要和SecondActivity.java中的System.loadLibrary函数中的库名称对应

### 7. 编译运行模拟器，并测试

- 起始页面：

  ![image-20241217214158354](images/image-20241217214158354.png)

- 点击按钮后：

  ![image-20241217214245435](images/image-20241217214245435.png)