Scratchoff

A simple library for implementing scratchable Views.

Installation

repositories {
    jcenter()
}

dependencies {
    compile('com.jackpocket:scratchoff:1.3.1')
}

Usage

The goal of this library is to create a scratchoff interface. By storing and manipulating the drawing cache of a View, we can create the effect of scratching it away to reveal what's hidden below.

First, you need a RelativeLayout (to align layouts on top of one another) consisting of 2 sub-layouts, a behind-View and the View to be scratched away from the foreground. Here is a simple example using the ScratchableLinearLayout:

<RelativeLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent" >

    <RelativeLayout
        android:id="@+id/scratch_view_behind"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:background="#818B8D" >
        <ImageView
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:layout_centerInParent="true"
            android:layout_margin="25dip"
            android:adjustViewBounds="true"
            android:src="@drawable/some_drawable_to_be_revealed" />
    </RelativeLayout>

    <com.jackpocket.scratchoff.views.ScratchableLinearLayout
        android:id="@+id/scratch_view"
        android:layout_width="fill_parent"
        android:layout_height="fill_parent"
        android:background="#3C9ADF" >
        <ImageView
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:layout_gravity="center"
            android:adjustViewBounds="true"
            android:gravity="center"
            android:src="@drawable/some_top_drawable" />
    </com.jackpocket.scratchoff.views.ScratchableLinearLayout>
</RelativeLayout>

Note: be careful with the dimensions of both the behind-View and the foreground View. The ScratchableLayoutDrawer will attempt to set the foreground View's LayoutParam width and height attributes to match that of the behind-View so that their dimensions line up perfectly.

Now that you have a layout, we need to attach the ScratchoffController to it:

ScratchoffController controller = new ScratchoffController(context)
        .setThresholdPercent(0.40d)
        .setTouchRadiusDip(context, 30)
        .setFadeOnClear(true)
        .setClearOnThresholdReached(true)
        .setCompletionCallback(() -> {  })
        .attach(findViewById(R.id.scratch_view), findViewById(R.id.scratch_view_behind));

In this example, you only need the constructor and the attach(View, View) method to enable scratching. The default values for all the other methods are configurable by overriding the appropriate resources.

Since the foreground View in our example is a ScratchableLinearLayout (which implements ScratchableLayout), the ScratchoffController will automatically attach itself to the View and drawing will work correctly (the same goes for the ScratchableRelativeLayout).

Please note: If you're not using one of the supplied ScratchableLayouts, you must manually call ScratchoffController.draw(Canvas) from your custom View's onDraw(Canvas) method.

Re-using the ScratchController

With version 1.0.2, the ScratchController can be correctly reset using ScratchController.reset(), but you must set the background color of your ScratchableLayout back to something opaque before calling it, as the ScratchableLayoutDrawer must set it to transparent afterwards in order to efficiently process scratched paths. e.g.

controller = new ScratchoffController(this)
        .setCompletionCallback(() -> {
            findViewById(R.id.scratch_view)
                    .setBackgroundColor(0xFF3C9ADF); // Make sure to set the background. Don't worry, it's still hidden if it cleared

            new Handler(Looper.getMainLooper())
                    .postDelayed(() -> 
                            controller.reset(), 2000);
        })
        .setScratchValueChangedListener(percentChanged -> {
            // Do something on scratch percent value changed
        })
        .attach(findViewById(R.id.scratch_view), findViewById(R.id.scratch_view_behind));

Lifecycle

As a final note, if using the ScratchoffController in the context of an Activity, you may want to also ensure you call the correct lifecycle methods for onPause(), onResume(), and onDestroy() as needed, to ensure the processors will stop/restart and not run needlessly in the background. e.g.

@Override
public void onPause(){
    controller.onPause();
    controller.removeTouchObservers()

    super.onPause();
}

@Override
public void onResume(){
    super.onResume();

    controller.onResume();
    controller.addTouchObserver((view, event) -> {
        // Do something on MotionEvent?
    });
}

@Override
public void onDestroy(){
    controller.onDestroy();

    super.onDestroy();
}

Observing MotionEvents

As of version 1.3.1, you can add an OnTouchListener to the ScratchoffController to observe MotionEvents as they come in, regardless of enabled state. When adding these observers, it'd be a good idea to remove them in the appropriate lifecycle methods.

@Override
public void onPause(){
    controller.removeTouchObservers()

    ....
}

@Override
public void onResume(){
    ....

    controller.addTouchObserver((view, event) -> {
        // Do something on a particular MotionEvent?
    });
}