Monitoring State

How to change your template depending on the drag state

1. Take any connection object, like DropTarget
2. Call .listen() with a function that returns what information you want to monitor
3. Save the resulting Observable to an instance variable in your component
4. Use the observable via the | async pipe to render alternate content in your template.

For example:

<div [dropTarget]="target"
    class="cat-target"
    [class.cat-target--hovering]="hovering$ | async">
    drop cats here
</div>

.cat-target { background: #eee; }
.cat-target--hovering { background: lightgoldenrodyellow; }

target = this.dnd.dropTarget("CAT", {
    drop: monitor => {
        console.log('dropped a cat');
    }
});

// this is a very useful combination on a drop target,
// which expresses 'hovering over' + ('same type' + 'DropTargetSpec.canDrop returned true')
// you almost always want to use the combination rather than isOver on its own
hovering$ = this.target.listen(m => m.isOver() && m.canDrop());

What can you listen to?

Each type of connection gets a different set of information available on the monitor. They are laid out in these interfaces:

• DragSourceMonitor
• DropTargetMonitor
• DragLayerMonitor
• All three derive from MonitorBase

Optimisation

Note that all three monitors have very rapidly-changing information available on them, such as the current viewport-mouse offset. The .listen() functions will optimise component updates for you. You don't want to:

1. Subscribe to more properties than you need. This hinders performance.
2. Subscribe to the entire monitor object. It will only fire once, and then never again, because the monitor object itself is the same each time.

The implementation of listen() includes an implementation of RxJS' distinctUntilChanged that also optimises for JS Objects with the same keys but different values. The following two examples fire exactly as often:

isDraggingPlain$ = this.source.listen(m => m.isDragging());
isDraggingObj$   = this.source.listen(m => ({ isDragging: m.isDragging() }));

This is helpful if you have a lot of properties to listen to and wish to reduce the number of | async pipes in your code, or if you want to copy some react-dnd code.

Making decisions in the Spec based on current drag state

The other place you get access to a monitor is in the callbacks in each Spec. Monitors carry some information only relevant and usable inside these callbacks. Those methods are documented in the monitor interfaces, and particularly useful methods are highlighted in each of the Spec callbacks, such as DropTargetSpec.drop.