第三章笔记(2)

从本地代码引用java对象：所有的java对象在jni中有相同的表示→jobject

    在这部分，我们关注的是如何存储一个对象在native端，和如何从native端取回一个对象送往java端；案列将定义一个Color类

Saving a reference to an object in the Store---------------------------------------------

1.定义一个新的类：Color；它包含一个了表示color的Integer，这个Integer将被转换成String（列如“#FF0000”）;

package cn.StoreActivity;
public class Color {
    private int mColor;
    public Color(String color) {
        super();
        mColor = android.graphics.Color.parseColor(color);
    }
    public String toString(){
        return String.format("#%06X",mColor);
    }
}

2.在StoreType的枚举类型中增加一个Color类型

package cn.StoreActivity;
public enum StoreType {
  Integer,String,Color;
}

3.Store.java在原来的基础上增加两个native方法来取回和设置Color对象

public class Store {
    static{
        System.loadLibrary("store");
    }
.............................
    public native Color getColor(String key);
    public native void setColor(String key,Color color);

}

4.在jni/Store.h中；增加一个新的color类型到StoreType中和添加一个新成员到StoreValue union;

......
typedef enum {
    StoreType_Integer, StoreType_String, StoreType_Color
} StoreType;


typedef union {
    int32_t  mInteger;
    char* mString;
    jobject  mColor;
} StoreValue;

5.重新用javah编译store.java,得到jni/cn_StoreActivity_Store.h,并在StoreActivity中实现两个新方法；

........
JNIEXPORT void JNICALL Java_cn_StoreActivity_Store_setColor
  (JNIEnv* pEnv, jobject pThis, jstring pKey, jobject pColor) {
    // The Java Color is going to be stored on the native side.
    // Need to keep a global reference to avoid a potential
    // garbage collection after method returns.
    jobject lColor = (*pEnv)->NewGlobalRef(pEnv, pColor);
    if (lColor == NULL) {
        return;
    }

    // Save the Color reference in the store.
    StoreEntry* lEntry = allocateEntry(pEnv, &mStore, pKey);
    if (lEntry != NULL) {
        lEntry->mType = StoreType_Color;
        lEntry->mValue.mColor = lColor;
    } else {
        (*pEnv)->DeleteGlobalRef(pEnv, lColor);
    }
}

6.使用NewGlobalRef()之后就必须对应的使用DeleteGlobalRef()。在我们的例子中当entry被替换的时候就必须删除全局引用。在Store.c里面修改releaseEntryValue();

void releaseEntryValue(JNIEnv* pEnv, StoreEntry* pEntry) {
    switch (pEntry->mType) {
        case StoreType_String:
            free(pEntry->mValue.mString);
            break;
        case StoreType_Color:
            // Unreferences the Id object for garbage collection.
            (*pEnv)->DeleteGlobalRef(pEnv, pEntry->mValue.mColor);
            break;
    }
}

7.使用ndk-build重新编译生成新的so库后，接下来就可以编写Activity端的代码了

package cn.StoreActivity;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Spinner;
import android.widget.Toast;

public class StoreActivity extends Activity {
    /** Called when the activity is first created. */
    private EditText et_key,et_value;
    private Button bt_getValue,bt_setValue;
    /*A view that displays one child at a time and lets the user pick
    among them. The items in the Spinner come from the Adapter
    associated with this view.*/
    private Spinner spinner;
    private Store store;

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        et_key = (EditText) this.findViewById(R.id.et_Key);
        et_value = (EditText) this.findViewById(R.id.et_Value);

        ArrayAdapter<StoreType> adapter = new ArrayAdapter<StoreType>(this,
                android.R.layout.simple_spinner_item, StoreType.values());
        //Sets the layout resource to create the drop down views.
        adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);

        spinner = (Spinner) this.findViewById(R.id.sp_Type);
        spinner.setAdapter(adapter);
        bt_getValue = (Button) this.findViewById(R.id.bt_GetValue);
        bt_getValue.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                onGetValue();

            }
        });

        bt_setValue = (Button)this.findViewById(R.id.bt_SetValue);
        bt_setValue.setOnClickListener(new OnClickListener() {

            @Override
            public void onClick(View v) {
                onSetValue();

            }
        });
        store = new Store();//不要忘了初始化！
    }
    public void onGetValue(){
        String key = et_key.getText().toString().trim();
        StoreType type = (StoreType)spinner.getSelectedItem();
        switch (type) {
        case Integer:
            et_value.setText(Integer.toString(store.getInteger(key)));
            break;
        case String:
            et_value.setText(store.getString(key));
            break;
        case Color:
            et_value.setText(store.getColor(key).toString());
            break;
        }

    }

    public void onSetValue(){
        String key = et_key.getText().toString().trim();
        String value = et_value.getText().toString().trim();
        StoreType type = (StoreType)spinner.getSelectedItem();
        try {
            switch (type) {
            case Integer:
                store.setInteger(key, Integer.parseInt(value));
                break;
            case String:
                store.setString(key,value);
                break;
            case Color:
                store.setColor(key, new Color(value));
                break;
            }
        } catch (Exception e) {
            // TODO Auto-generated catch block
            Toast.makeText(getApplicationContext(), "incorrect value", 0).show();
        }
    }
}

8.运行程序，在Value中输入#FF0000