android-Android自定義控件中為Button設置自定義監聽器出現空指針。

Android自定義控件中為Button設置自定義監聽器出現空指針。

Android自定義TopBar中給ImageButton設置自定義點擊監聽器，當點擊ImageButton時在listener處出現NullPointerException。

會出錯的地方我在後面寫了//TODO　。

請問為什麼會出現listener沒有被實例化的情況？

還有為什麼在MainActivity中使用findViewById實例化TopBar也會出現不能實例化控件的情況？

IDE : Android Studio ;

Compile Sdk Version : API 23 ;

Build Tool Version : 23.0.1 ;

Min Sdk Version : API 15 ;

Target Sdk Version : API 19 ;

部分代碼：

#MainActivity.java

public class MainActivity extends AppCompatActivity {

    private TopBar topBar;

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

//        topBar = (TopBar) findViewById(R.id.topBar);        //TODO 這樣無法實例化topBar  ???
        topBar = new TopBar(getApplicationContext(), null);

        if (topBar == null) {
            Toast.makeText(MainActivity.this, "NULL", Toast.LENGTH_SHORT).show();
        }

        topBar.setOnTopBarClickListener(new TopBar.OnClickListener() {
            @Override
            public void onLeftButtonClick() {
                Toast.makeText(MainActivity.this, "Left", Toast.LENGTH_SHORT).show();
            }

            @Override
            public void onRightButtonClick() {
                Toast.makeText(MainActivity.this, "Right", Toast.LENGTH_SHORT).show();
            }
        });
    }
}

#TopBar.java

public class TopBar extends RelativeLayout {

    private ImageButton ibtLeft;
    private ImageButton ibtRight;
    private TextView tvTitle;

    private OnClickListener listener;

    public interface OnClickListener {

        void onLeftButtonClick();

        void onRightButtonClick();
    }

    public void setOnTopBarClickListener(OnClickListener listener) {
        this.listener = listener;
    }

    public TopBar(Context context, AttributeSet attrs) {
        super(context);
        LayoutInflater.from(context).inflate(R.layout.layout_top_bar, this);

        ibtLeft = (ImageButton) findViewById(R.id.ibt_left);
        ibtRight = (ImageButton) findViewById(R.id.ibt_right);
        tvTitle = (TextView) findViewById(R.id.tv_title);

        ibtLeft.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Toast.makeText(getContext(), "leftButton", Toast.LENGTH_SHORT).show();      //Toast可以顯示
//                listener.onLeftButtonClick();       //TODO NullPointerException  為什麼沒有實例化listener？
            }
        });

        ibtRight.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                listener.onRightButtonClick();       //TODO NullPointerException
            }
        });

    }
}

#logcat輸出

10-14 13:42:14.442 1366-1366/com.vder.puzzlegame E/AndroidRuntime: FATAL EXCEPTION: main
10-14 13:42:14.442 1366-1366/com.vder.puzzlegame E/AndroidRuntime: java.lang.NullPointerException
10-14 13:42:14.442 1366-1366/com.vder.puzzlegame E/AndroidRuntime:     at com.vder.puzzlegame.ui.TopBar$2.onClick(TopBar.java:55)
10-14 13:42:14.442 1366-1366/com.vder.puzzlegame E/AndroidRuntime:     at android.view.View.performClick(View.java:4204)
10-14 13:42:14.442 1366-1366/com.vder.puzzlegame E/AndroidRuntime:     at android.view.View$PerformClick.run(View.java:17355)
10-14 13:42:14.442 1366-1366/com.vder.puzzlegame E/AndroidRuntime:     at android.os.Handler.handleCallback(Handler.java:725)
10-14 13:42:14.442 1366-1366/com.vder.puzzlegame E/AndroidRuntime:     at android.os.Handler.dispatchMessage(Handler.java:92)
10-14 13:42:14.442 1366-1366/com.vder.puzzlegame E/AndroidRuntime:     at android.os.Looper.loop(Looper.java:137)
10-14 13:42:14.442 1366-1366/com.vder.puzzlegame E/AndroidRuntime:     at android.app.ActivityThread.main(ActivityThread.java:5041)
10-14 13:42:14.442 1366-1366/com.vder.puzzlegame E/AndroidRuntime:     at java.lang.reflect.Method.invokeNative(Native Method)
10-14 13:42:14.442 1366-1366/com.vder.puzzlegame E/AndroidRuntime:     at java.lang.reflect.Method.invoke(Method.java:511)
10-14 13:42:14.442 1366-1366/com.vder.puzzlegame E/AndroidRuntime:     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
10-14 13:42:14.442 1366-1366/com.vder.puzzlegame E/AndroidRuntime:     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
10-14 13:42:14.442 1366-1366/com.vder.puzzlegame E/AndroidRuntime:     at dalvik.system.NativeStart.main(Native Method)

最佳回答：

構造方法不對，像這樣，把三個全上才可以在Xml文件中使用

public MyView(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
    // TODO Auto-generated constructor stub
}

public MyView(Context context, AttributeSet attrs) {
    super(context, attrs);
    // TODO Auto-generated constructor stub
}

public MyView(Context context) {
    super(context);
    // TODO Auto-generated constructor stub
}