绘制/布局期间的对象分配？

Question

I get 3 warning for object allocation during draw/layout

super.onDraw(canvas);
canvas.drawColor(Color.WHITE);
Paint textPaint = new Paint();
textPaint.setARGB(50,100,100,250);
textPaint.setTextAlign(Align.CENTER);
textPaint.setTextSize(50);
textPaint.setTypeface(font);
canvas.drawText("Logan is awesom",canvas.getWidth()/2,200,textPaint);
canvas.drawBitmap(pBall, (canvas.getWidth()/2), changingY, null);
if (changingY <canvas.getHeight()){
changingY += 10;
}else{
changingY=0;
}
Rect middleRect = new Rect();
middleRect.set(0, 400, canvas.getWidth(), 550);
Paint ourBlue = new Paint();
ourBlue.setColor(Color.BLUE);
canvas.drawRect(middleRect, ourBlue);

I get an error on new Rect(); and on both new Paint();

The exact error is avoid object allocation during draw/layout operations (prelocate and reuse instead)

Answer 1

Well, your 'error' points to exact problem. onDraw() method is called by OS many-many times, thus allocating something within this function is extremely bad idea. You need to allocate your Rect and Paint beforehand and just use them within onDraw

class YourClass extends View
{
    Rect middleRect;
    Paint ourBlue;
    Paint textPaint;

    public YourClass()
    {
         //constructor
         init();
    }

    private void init()
    {
        middleRect = new Rect();
        ourBlue; = new Paint();
        textPaint = new Paint();

        ourBlue.setColor(Color.BLUE);
        textPaint.setARGB(50,100,100,250);
        textPaint.setTextAlign(Align.CENTER);
        textPaint.setTextSize(50);
        textPaint.setTypeface(font);
    }

    @Override
    protected void onDraw(Canvas canvas) {
        super.onDraw(canvas);
        canvas.drawColor(Color.WHITE);

        canvas.drawText("Logan is awesom",canvas.getWidth()/2,200,textPaint);
        canvas.drawBitmap(pBall, (canvas.getWidth()/2), changingY, null);
        if (changingY <canvas.getHeight()){
            changingY += 10;
        }else{
            changingY=0;
        }

        //if canvas size doesn't change - this can be moved to init() as well
        middleRect.set(0, 400, canvas.getWidth(), 550);

        canvas.drawRect(middleRect, ourBlue);
    }
}

Answer 2

For me, I have got this error in layout especially when using display metrices.

I have used this imageview :

   <com.steve.thirdparty.ScalableImageView
        android:id="@+id/iv_posted_img_home"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:scaleType="fitXY"
        android:background="@drawable/golive_load_image"
        android:layout_below="@+id/tv_user_posted_msg_post_items_home"
        android:contentDescription="@string/cont_desc"/>

ScalableImageView.java:

import static com.steve.TabHomeActivity.displaymetrics;

public class ScalableImageView extends ImageView {


        ((Activity) getContext()).getWindowManager().getDefaultDisplay().getMetrics(displaymetrics);

}

I solved this issue by adding the static DisplayMetrices in onCreate() method in Activity.

TabHomeActivity.java:

public static DisplayMetrics displaymetrics;

*inside onCreate()*

 displaymetrics = new DisplayMetrics();

Answer 3

avoid object allocation during draw/layout operations (prelocate and reuse instead)

The message give You a solution on plate, so I don't understand what is Your question.

You have to move creation of new objects to onCreate method, so they are created just once and use them later in onDraw method.