Using a ScaleGestureDetector
When learning a new concept I don't like using libraries or code dumps. I found a good description here and in the documentation of how to resize an image by pinching. This answer is a slightly modified summary. You will probably want to add more functionality later, but it will help you get started.
Layout
The ImageView just uses the app logo since it is already available. You can replace it with any image you like, though.
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent">
<ImageView
android:id="@+id/imageView"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:src="@mipmap/ic_launcher"
android:layout_centerInParent="true"/>
</RelativeLayout>
Activity
We use a ScaleGestureDetector on the activity to listen to touch events. When a scale (ie, pinch) gesture is detected, then the scale factor is used to resize the ImageView.
public class MainActivity extends AppCompatActivity {
private ScaleGestureDetector mScaleGestureDetector;
private float mScaleFactor = 1.0f;
private ImageView mImageView;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
// initialize the view and the gesture detector
mImageView = findViewById(R.id.imageView);
mScaleGestureDetector = new ScaleGestureDetector(this, new ScaleListener());
}
// this redirects all touch events in the activity to the gesture detector
@Override
public boolean onTouchEvent(MotionEvent event) {
return mScaleGestureDetector.onTouchEvent(event);
}
private class ScaleListener extends ScaleGestureDetector.SimpleOnScaleGestureListener {
// when a scale gesture is detected, use it to resize the image
@Override
public boolean onScale(ScaleGestureDetector scaleGestureDetector){
mScaleFactor *= scaleGestureDetector.getScaleFactor();
mImageView.setScaleX(mScaleFactor);
mImageView.setScaleY(mScaleFactor);
return true;
}
}
}
Notes
- Although the activity had the gesture detector in the example above, it could have also been set on the image view itself.
You can limit the size of the scaling with something like
mScaleFactor = Math.max(0.1f, Math.min(mScaleFactor, 5.0f));Thanks again to Pinch-to-zoom with multi-touch gestures In Android
- Documentation
- Use Ctrl + mouse drag to simulate a pinch gesture in the emulator.
Going on
You will probably want to do other things like panning and scaling to some focus point. You can develop these things yourself, but if you would like to use a pre-made custom view, copy TouchImageView.java into your project and use it like a normal ImageView. It worked well for me and I only ran into one bug. I plan to further edit the code to remove the warning and the parts that I don't need. You can do the same.