How to get when an ImageView is completely loaded in Android

Question

I'm developing an App which draws lines over a bunch of Images. To choose these images, I have a radio group and, whenever the user clicks in a radio button, the image is load with all its own drawings.

In my radio listenner I have the following code:

bitmap = BitmapUtils.decodeSampledBitmapFromResource(root + DefinesAndroid.CAMINHO_SHOPPINGS_SDCARD + nomeImagemAtual, size.x, size.y);
mImage.setImageBitmap(bitmap);

mImage.setDrawLines(true);
mImage.setImageBitmap(loadBitmapFromView(mImage));

the decodeSampledBitmapFromResource method I got from this link on android developers (it loads bitmaps more effitiently) http://developer.android.com/training/displaying-bitmaps/load-bitmap.html

And here's the method I call to get a Bitmap of a View

    public static Bitmap loadBitmapFromView(View v) {
        Bitmap b = Bitmap.createBitmap( v.getLayoutParams().width, v.getLayoutParams().height, Bitmap.Config.ARGB_8888);                
        Canvas c = new Canvas(b);
        v.layout(0, 0, v.getLayoutParams().width, v.getLayoutParams().height);
        v.draw(c);
        return b;
}

I'm setting the image Bitmap of mImage because I'm using ImageViewTouch library (which enables pinch zooming over an ImageView) and if I don't do it, all the canvas drawing is deleted with any interaction over the image (like zooming in/out).

The error log is the following

07-11 21:13:41.567: E/AndroidRuntime(20056): java.lang.IllegalArgumentException: width and height must be > 0
07-11 21:13:41.567: E/AndroidRuntime(20056):    at android.graphics.Bitmap.createBitmap(Bitmap.java:638)
07-11 21:13:41.567: E/AndroidRuntime(20056):    at android.graphics.Bitmap.createBitmap(Bitmap.java:620)

I'm almost sure that this error is occuring cause the image bitmap is not completely loaded when I call getBitmapFromView method.

How can I know when the view is loaded completely?

Answer 1

There is actually another, more reliable way to do that by using ViewTreeObserver.OnPreDrawListener. And here is an example:

mImage.getViewTreeObserver().addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener() {
    @Override
    public boolean onPreDraw() {
        try {
            loadBitmapFromView(mImage);            
             // Note that returning "true" is important,
             // since you don't want the drawing pass to be canceled
            return true;    
        } finally {
            // Remove listener as further notifications are not needed
            mImage.getViewTreeObserver().removeOnPreDrawListener(this);
        }
    }
});

Using OnPreDrawListener guarantees that View was measured and layouted, while View#post(Runnable) just executes your Runnable when all Views are already most likely measured and layouted.

Answer 2

Below is a working NotifyImageView class that incorporates Dmitry's method above and adds code to notify only after the ImageView is truly usable.

I hope you find it useful.

`

public interface NotifyImageHolder {
    public void notifyImageChanged(final NotifyImageView thePosterImage, final int width, final int height);
}

import android.content.Context;
import android.graphics.Bitmap;
import android.graphics.drawable.Drawable;
import android.util.AttributeSet;
import android.view.ViewTreeObserver;
import android.widget.ImageView;

public class NotifyImageView extends ImageView {
    private boolean mImageChanged;
    private NotifyImageHolder mHolder;
    private boolean mImageFinished;

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

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

    public NotifyImageView(Context context, AttributeSet attrs, int defStyleAttr) {
        super(context, attrs, defStyleAttr);
        init();
    }

    protected void init() {
        mImageChanged = false;
        mImageFinished = false;
        mHolder = null;
        monitorPreDraw();
    }

    // so we can tell when the image finishes loading..
    protected void monitorPreDraw() {
        final NotifyImageView thePosterImage = this;
        getViewTreeObserver().addOnPreDrawListener(new ViewTreeObserver.OnPreDrawListener() {

            @Override
            public boolean onPreDraw() {
                try {
                    return true; //note, that "true" is important, since you don't want drawing pass to be canceled
                } finally {
                    getViewTreeObserver().removeOnPreDrawListener(this); // we don't need any further notifications
                    thePosterImage.buildDrawingCache();
                    mImageFinished = true;
                }
            }
        });
    }

    public void setNotifyImageHolder(NotifyImageHolder holder) {
        this.mHolder = holder;
    }

    public boolean isImageChanged() {
        return mImageChanged;
    }

    public boolean isImageFinished() {
        return mImageFinished;
    }

    public void notifyOff() {
        mHolder = null;
    }

    // the change notify happens here..
    @Override
    public void setImageDrawable(Drawable noPosterImage) {
        super.setImageDrawable(noPosterImage);
        if (mHolder != null && mImageFinished) {
            mImageFinished = false; // we send a single change-notification only
            final NotifyImageView theNotifyImageView = this;

            theNotifyImageView.post(new Runnable() {
                @Override
                public void run() {
                    if (mHolder != null) {
                        int width = getMeasuredWidth();
                        int height = getMeasuredHeight();
                        mImageChanged = true;
                        mHolder.notifyImageChanged(theNotifyImageView, width, height);
                    }
                }
            });

        }
    }

}

`