Develop Reference

Android. Button does not work in Canvas

Button does not work in Canvas. What you need to change / add the caller to a message? The button is created but does not respond when you press. Is it possible that the canvas is over the button?
public class ButtonInCanvas extends AppCompatActivity implements View.OnClickListener {
Button button;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
button = new Button(ButtonInCanvas.this);
button.setOnClickListener(this);
button.setText("OK!");
setContentView(new BtInCanvas(ButtonInCanvas.this));
}
public class BtInCanvas extends View {
public BtInCanvas(Context context) {
super(context);
}
public void onDraw(Canvas canvas){
button.layout(50,50,300,300);
button.draw(canvas);
}
}
#Override
public void onClick(View v) {
Toast.makeText(this,"OK!",Toast.LENGTH_LONG).show();
}
}

Let refer to the link Android drawing button to canvas with custom view?
"You cannot insert a button into canvas. Canvas is an interface for bitmap or a bitmap buffer for a view. You can only draw other bitmap or pixels in it, not insert an object or a widget.
There are some solutions:
as Nikolay suggested, use a FrameLayout and create two layers (views), first your custom view and the second LinerView or RelativeView, which will come on top, where you can have buttons etc
draw an image of a buttun on Canvas then use onTouchEvent in your custom view and test for the coordinates of the touch, then do something... an example for onTouchEvent here: Make certain area of bitmap transparent on touch"

How can I use a drawn rectangle by myself as a button

You know that if we want to use a button in android, we use the following code:
button.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
Intent intent = new Intent(MainActivity.this, B.class);
startActivity(intent);
}
});
Now my question is if I draw a rectangle by myself, can I use the MotionEvent class for this purpose? If it is possible so how is it? I wrote the following code to draw a rectangle. Now I want that the rectangle behaviors like the button in the above code.
public class B extends View {
Paint paint;
B(Context context) {
super(context);
paint = new Paint();
}
#Override
protected void onDraw(Canvas canvas) {
paint.setColor(Color.RED);
canvas.drawRect(10,20,40,100,paint);
}
#Override
public boolean onTouchEvent(MotionEvent event){
return true; // I am a little confused in this section in spite of searching in internet.
}
}

You can assign on OnClickListener to any View, not just Buttons. You don't need to override onTouchEvent().
Alternatively, you can use a regular Button (or just a View even) and just give a solid rectangle as a background drawable.

How to use a SurfaceView from an Adapter (Android)

I have a GridView. I would like each item to be an object derived form SurfaceView. I have created a class which extends SurfaceView, and an Adapter whose getView() returns an instance of this.
All I get is a little black square for each grid item. The SurfaceView's draw() is never called (shown by adding trace) and neither is its surfaceCreated().
How do I do this in a way which allows me to draw stuff on the SurfaceView please? I think the crux of the problem is that surfaceCreated() is never called. Why not?
This is my Adapter:
public class LevelAdapter extends BaseAdapter
{
...
#Override
public View getView(int position, View convertView, ViewGroup parent)
{
LevelNumberView number = new LevelNumberView(m_context);
number.setLayoutParams(new GridView.LayoutParams(70, 70));
number.setPadding(1, 1, 1, 1);
number.masDraw(); // Experiment - but it doesn't work
return number;
}
}
Then my SurfaceView class:
public class LevelNumberView extends SurfaceView
implements SurfaceHolder.Callback
{
...
#Override
public void surfaceCreated(SurfaceHolder holder)
{
m_surfaceHolder = holder;
}
#Override
public void draw(Canvas canvas)
{
Log.i("LevelNumberView","Draw");
doDraw(canvas, Color.YELLOW);
}
public void masDraw()
{
Log.i("LevelNumberView","masDraw");
if (null != m_surfaceHolder)
{
Canvas canvas = m_surfaceHolder.lockCanvas();
if (null != canvas)
{
doDraw(canvas, Color.GREEN);
m_surfaceHolder.unlockCanvasAndPost(canvas);
}
}
}
private void doDraw(Canvas canvas, int colour)
{
int w = canvas.getWidth();
int h = canvas.getHeight();
int min = Math.min(w,h);
Paint paint = new Paint(Paint.ANTI_ALIAS_FLAG);
paint.setColor(colour);
paint.setStyle(Paint.Style.FILL);
canvas.drawCircle(w/2, h/2, min, paint);
}
}
Thanks!
-Mark

Overriding draw() probably isn't what you want. If you want to draw on the View, override onDraw(). If you want to draw on the Surface, that's usually done from a separate thread. (The SurfaceView has two parts; the surface part is a completely separate layer that is drawn behind the layer with the View-based UI.)
If you haven't called SurfaceHolder().addCallback(), no object will be receiving callbacks -- the SurfaceView doesn't send callbacks to itself, so subclassing SurfaceView doesn't really do much for you.
Perhaps what you want is simply a custom View?

Drawing over a view and all it's children

I'm trying to apply a visual effect to a viewgroup. My idea is to grab a bitmap of the viewgroup, shrink it down, expand it back up, and draw it over the viewgroup to give it a blocky, low quality effect.
I've got most of the way there using this code:
public class Blocker {
private static final float RESAMPLE_QUALITY = 0.66f; // less than 1, lower = worse quality
public static void block(Canvas canvas, Bitmap bitmap_old) {
block(canvas, bitmap_old, RESAMPLE_QUALITY);
}
public static void block(Canvas canvas, Bitmap bitmap_old, float quality) {
Bitmap bitmap_new = Bitmap.createScaledBitmap(bitmap_old, Math.round(bitmap_old.getWidth() * RESAMPLE_QUALITY), Math.round(bitmap_old.getHeight() * RESAMPLE_QUALITY), true);
Rect from = new Rect(0, 0, bitmap_new.getWidth(), bitmap_new.getHeight());
RectF to = new RectF(0, 0, bitmap_old.getWidth(), bitmap_old.getHeight());
canvas.drawBitmap(bitmap_new, from, to, null);
}
}
I simply pass in the canvas to draw on and a bitmap of what needs to be scaled down+up and it works well.
public class BlockedLinearLayout extends LinearLayout {
private static final String TAG = BlockedLinearLayout.class.getSimpleName();
public BlockedLinearLayout(Context context, AttributeSet attrs) {
super(context, attrs);
applyCustomAttributes(context, attrs);
setup();
}
public BlockedLinearLayout(Context context) {
super(context);
setup();
}
private void setup() {
this.setDrawingCacheEnabled(true);
}
#Override
public void draw(Canvas canvas) {
super.draw(canvas);
// block(canvas); If I call this here, it works but no updates
}
#Override
public void onDraw(Canvas canvas) {
// block(canvas); If I call this here, draws behind children, still no updates
}
private void block(Canvas canvas) {
Blocker.block(canvas, this.getDrawingCache());
}
}
The problem I'm having is in my viewgroup. If I run the block method in the viewgroup's draw, it draws over everything but doesn't ever update when child views change. I've traced function calls with Log, and the draw method seems to be running, but nothing changes.
I've also tried implementing this in onDraw. This draws the bitmap behind all the children views, and again they aren't updating.
Can anyone explain how I would go about fixing this?

Try this:
#Override
protected void dispatchDraw(Canvas canvas) {
// call block() here if you want to draw behind children
super.dispatchDraw(canvas);
// call block() here if you want to draw over children
}
And call destroyDrawingCache() and then, buildDrawingCache() each time you change a child.

Draw() method will work well for you.
I'm now trying to make a count time view in a circle shape, when time is passing, the view will reducing his angle. It's used to cover profile photo(a circle shape photo).

Starting with Android API 23, you can use onDrawForeground(Canvas) to draw on top of child views: https://developer.android.com/reference/android/view/View#onDrawForeground(android.graphics.Canvas)
Unlike onDraw() though, be sure to call through to the super class:
#Override
public void onDrawForeground(final Canvas canvas) {
super.onDrawForeground(canvas);
// Your code here
}

Drawing Bitmap in Android?

My project is about image processing in android.I have a bitmap that I have loaded from a resource file (an PNG image). I want to draw it. But I couldn't.
Here my code snippet:
mB = BitmapFactory.decodeResource(getResources(), R.drawable.picture);
Canvas c = new Canvas(mB);
Paint p = new Paint();
c.drawBitmap(mB,0,0,p);
it didn't work. Is the code true? .Is there any thing more that I must do?

You should use an ImageView instead and load it by
imageView.setImageResource(R.drawable.picture);
If you want to manually draw it with a Canvas, you have to use a canvas that is passed into a draw() method and implement a Custom View.
Update to add example CustomView:
public class CustomView extends View {
private Paint mPaint;
private Drawable mDrawable;
public CustomView(Context context, AttributeSet attrs) {
super(context, attrs);
mPaint = new Paint();
mDrawable = context.getResources().getDrawable(R.drawable.some_drawable);
}
#Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
mDrawable.draw(canvas);
}
}

There are a couple of things you are missing.
First, I think you're misunderstanding the Canvas(Bitmap b) constructor. The Bitmap passed in there is one that the Canvas will draw into. This could be just a new Bitmap that you constructed.
Second, it is recommended that you use the Canvas that is passed to you in your View's onDraw method. Presumably that View is one from your Activity, either fetched from your XML layout via findViewById or constructed and passed to setContentView in the Activity's onCreate() method.
So, you are going to have to subclass View and override the onDraw method to get your drawing done. Something like:
public class MyView extends View {
#Override
public void onDraw (Canvas c) {
Bitmap mB = BitmapFactory.decodeResource(this.getContext().getResources(), R.drawable.picture);
c.drawBitmap(mB, 0, 0, null);
}
}
Then, in your Activity, you'll need to create an instance of your new View and pass it to the Activity via setContentView:
public class MyActivity extends Activity {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
mv = new MyView(this);
setContentView(mv);
}
You can instead call the setContentView(View v, ViewGroup.LayoutParameters lp) overload if you want to set up the LayoutParameters.
I haven't tested any of this, but it should at least get you on the right path.