view - 处理程序未获取数据以查看

Question

我正在尝试使用处理程序将消息传递给View类；但不知道我哪里出错了。请纠正我。从 Handler 获取值后，我从未使用画布进行绘制。

提前致谢！

运行此程序后，我的日志猫显示NullPointerException在onDraw方法上。因此无法绘制线。

public class List extends Activity{

Handler handler=new Handler();
Message msg=new Message();
Bundle bundle=new Bundle();

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(new TestView(this));
    bundle.putFloat("x1", 10);
    bundle.putFloat("y1", 10);
    bundle.putFloat("x2", 100);
    bundle.putFloat("y2", 100);
    msg.setData(bundle);
    handler.sendMessage(msg);

}


}

 class TestView extends View {

Paint p;
float x1;
float y1;
float x2;
float y2;

public TestView(Context context) {
    super(context);
    Paint p=new Paint();
    p.setColor(Color.BLUE);
    Handler handler=new Handler(){

        @Override
        public void handleMessage(Message msg) {
            Looper.prepare();
            Bundle bundle=msg.getData();
            x1=bundle.getFloat("x1");
            y1=bundle.getFloat("y1");
            x2=bundle.getFloat("x2");
            y2=bundle.getFloat("y2");
            Looper.loop();
        }

    };

}



boolean isDrawing=true;

@Override
protected void onDraw(Canvas canvas) {

      canvas.drawLine(x1, y1, x2, y2, p);

    invalidate();
}


}

Answer 1

处理程序始终只附加到一个线程，除非并且直到我们没有线程，否则它无法工作。即在主线程和后台线程之间发送或接收数据。

View 类的正确程序应该如下：

public class List extends Activity{


@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(new TestView(this));


}


}

 class TestView extends View {


Bitmap bitmap=Bitmap.createBitmap(1000, 1000, Config.ARGB_8888);
Canvas canvas=new Canvas(bitmap);
Paint p=new Paint();


Handler handler=new Handler(){

    @Override
    public void handleMessage(Message msg) {

        p.setColor(Color.BLUE);
        Bundle bundle=msg.getData();
        String name=bundle.getString("Name");
        Toast.makeText(getContext(), name, Toast.LENGTH_LONG).show();



    }

};

public TestView(Context context) {
    super(context);
    Paint p=new Paint();
    p.setColor(Color.BLUE);
    new Thread(){
        public void run(){

            Message msg=new Message();
            Bundle bundle=new Bundle();
            Bundle bundle1=new Bundle();
            Looper.prepare();
            bundle.putString("Name", "Nishant");
            bundle1.putString("Name", "Neha");
            msg.setData(bundle);
            handler.sendMessage(msg);
            Message m1=new Message();
            m1.setData(bundle1);
            handler.sendMessage(m1);




        }
    }.start();


}


@Override
protected void onDraw(Canvas canvas) {


}





}