Android OnDraw和onTouchEvent无法一起使用

Question

I am new to android development as well as the Java programming language and would definitely appreciate some help with this issue.

Currently I have some code which creates a canvas and draws a bitmap picture onto it, this appears to work. I also have another bit of code which detects where the users finger is on the devices screen and this also works. when I try to combine the two the bitmap will display but as soon as I click the screen the app crashes.

I have a feeling that this issue is with the canvas/bitmap side of this as i have also noticed that any widgets that have added to layout vanish when the canvas/bitmap feature is running.

This is the code for the canvas/bitmap:

public class myView extends View{

Bitmap image;
DisplayMetrics metrics;
public myView(Context context) {
    super(context);
    metrics = context.getResources().getDisplayMetrics();
    // TODO Auto-generated constructor stub
    image = BitmapFactory.decodeResource(getResources(), R.drawable.bhead);

}
@Override
protected void onDraw(Canvas canvas) {
    // TODO Auto-generated method stub
    super.onDraw(canvas);

    int width = metrics.widthPixels;
    int height = metrics.heightPixels;

    Matrix matrix = new Matrix();
    float angle = 90;
    float imageCenterX = 50;
    float imageCenterY = 50;
    matrix.setRotate(angle, imageCenterX, imageCenterY);
    Bitmap temp;
    temp = BitmapFactory.decodeResource(getResources(), R.drawable.bhead);
    image = Bitmap.createScaledBitmap(temp, 100, 100,true);
    matrix.postTranslate(((width/2)-50), ((height/2)-100));
    canvas.drawBitmap(image, matrix, null);

}

}

and this is the code for the finger location on the screen:

public class MainActivity extends ActionBarActivity {

myView mview;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    mview = new myView(this);
    setContentView(mview);
}

@Override
public boolean onTouchEvent(MotionEvent event) {
    int x = (int)event.getX();
    int y = (int)event.getY();

    switch (event.getAction()) {
        case MotionEvent.ACTION_DOWN:
        case MotionEvent.ACTION_MOVE:
        case MotionEvent.ACTION_UP:
    }

    EditText txt = (EditText)findViewById(R.id.editText1);
    txt.setText("X = "+ x +" Y = " + y);

    return false;
}




@Override
public boolean onCreateOptionsMenu(Menu menu) {
    // Inflate the menu; this adds items to the action bar if it is present.
    getMenuInflater().inflate(R.menu.main, menu);
    return true;
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {
    // Handle action bar item clicks here. The action bar will
    // automatically handle clicks on the Home/Up button, so long
    // as you specify a parent activity in AndroidManifest.xml.
    int id = item.getItemId();
    if (id == R.id.action_settings) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}

}

Answer 1

Your view hierarchy does not have an EditText with the id editText1 . This is because you have set the content view to be a single instance of your class MyView (class names should start with uppercase) with this code in onCreate() :

mview = new myView(this);
setContentView(mview);

As a result, you get a NullPointerException on the line txt.setText("X = "+ x +" Y = " + y); because the previous line cannot find the EditText.

You need to include both views in the layout. You can use MyView in XML by writing its fully qualified name. For example:

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical" >

    <com.your.package.name.MyView
        android:id="@+id/my_view"
        android:layout_width="match_parent"
        android:layout_height="0dp"
        android:layout_weight="1" />

    <EditText
        android:id="@+id/editText1"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        ... />
</LinearLayout>