Android：如何制作可滚动的画布？

Question

I am writing an app. One aspect of it is lines being drawn over an image. Here's the practice code I've been working with:

public class DrawView extends View { Paint paint = new Paint();

private void init() {
    paint.setColor(Color.BLACK);
}

public DrawView(Context context) {
    super(context);
    init();
}

public DrawView(Context context, AttributeSet attrs) {
    super(context, attrs);
    init();
}

public DrawView(Context context, AttributeSet attrs, int defStyle) {
    super(context, attrs, defStyle);
    init();
}

@Override
public void onDraw(Canvas canvas) {
    paint.setStrokeWidth(20);
    canvas.drawLine(100, 100, 20, 20, paint);
    canvas.drawLine(20, 0, 0, 20, paint);

}

}

I want the Canvas to be fairly large, so I need people to be able to scroll in both directions (left/right and up/down). How do I accomplish this? I'm unfamiliar with the Canvas class so any help will be appreciated.

Answer 1

If you want to draw on canvas with your custom height and width you have to call setContentView(android.view.View yourView , android.view.Viewgroup.LayoutParam yourLayout) in your activity class.Because by default setContentView(View view) method use full width and height.So you have to use its overloaded method with two parameter along with your desired. See documentation for more info.And don`t use only LayoutParams() constructor to create its object. Use it by writing its full path like android.view.ViewGroup.LayoutParams. Because there are some other classes with same name in Android SDK.

MyView customView;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    customView = new MyView(getApplicationContext());
    android.view.ViewGroup.LayoutParams lp = new android.view.ViewGroup.LayoutParams(100,200);//100 is width and 200 is height
    setContentView(customView, lp);
    customView.setOnClickListener(this);

}`