Drag Gesture Handler

[ShaMan123]

EDITED 22/05/21

Motivation #928

1. Drag and drop is a commonly used gesture in native platforms and is a top choice for app UI/UX.
2. There are many unique attributes and behaviors of the drag and drop gesture that are different than PanGestureHandler -> using PanGestureHandler requires a lot of boilerplate to get it to function as a synthetic drag and drop handler (I've tried it rn-drag-drop).
3. Without reanimated it is not responsive. With reanimated it causes a lot of overhead (lots of nodes created for each synthetic handler), which makes it not good enough of a solution (e.g. Using this as items of a FlatList is a KILLER).
4. Drag events between apps are now supported by both platforms. This can be handled only with native drag events. There is no way of achieving this currently in react-native. In this PR I've achieved it on android already.
5. The way I see it, react-native-gesture-handler aims to be a plug and play library, this is why I think drag and drop gesture handling is essential.

Changes

I've added 2 new handlers: DragGestureHandler and DropGestureHandler both of them extend PanGestureHandler.
I have made adaptions mainly to GestureHandlerOrchestrator in the android native code and added examples under Drag & Drop and Drag & Drop in FlatList.
In addition I've revived the native android example, this was done by adjusting the gesture handler registry and mocking a root view.

The only thing out of scope is this fix, which fixes passing down the wrong handlerTag to native making relations config wrong for NativeViewGestureHandler.
https://github.com/software-mansion/react-native-gesture-handler/pull/963/files#diff-6eb54a5e8a566ff2be948cdfe0abb5f4

Logic

Gesture handling is in progress.
PanGestureHandler super class of DragGestureHandler receives events. It is in charge of moving from State.UNDETERMINED to State.BEGAN and State.ACTIVE based on it's own logic.
Once active, DragGestureHandler begins dragging.
From this point on the GestureHandlerOrchestrator receives a DragEvent which is first adapted to a MotionEvent and delivered to all handlers. This is done to activate DropGestureHandlers (DropGestureHandler is the same as DragGestureHandler in terms of PanGestureHandler responsibilities), run simultaneous handlers and cancel the rest.
Then the DragEvent is delivered to all handlers, DropGestureHandlers first, then the rest.
Extracting DropGestureHandlers is done for each event to obtain the top most handler.
If DropGestureHandlers are extracted the top most will be activated notifying the other drag/drop handlers with an appropriate action.

Multi Window

Android supports multi window.
The AndroidManifest.xml file needs to be edited to get this working.
To persist the same behavior I had to add some logic to bridge the framework not dispatching drag events back to the app that started the drag gesture, leaving DragGestureHandler unaware of interactions with DropGestureHandlers. So, I've added a small broadcasting mechanism that broadcasts changes of the drag action to the source app.
To test this I suggest running both AndroidNativeExample and Example side by side.

Simultaneous Handlers

There is a caveat regarding simultaneous handlers. A drag event (on android) has only one pointer so simultaneous handling with RotationGestureHandler or PinchGestureHandler won't work unfortunately.
Passing DragGestureHandlers before the event begins will join them to the drag event and add them to the drag shadow. See example.

Props

1. types = number | number[] | null, if a DragGestureHandler and a DropGestureHandler share one type they can interact.

DragGestureHandler props

1. data - pass object to DropGestureHandler on drop via gesture event.
2. Drag Shadow
   There are several options here, all listed in react-native-gesture-handler.d.ts
   1. dragMode - handles the visibilty of DragGestureHandler while dragging
      1. move
      2. move-restore - after drop occurs, restores visibility to the DragGestureHandler's view
      3. copy
      4. none
   2. shadow = component | element | tag | null to render view while dragging
   3. shadowEnabled merged as dragMode='none'
   4. shadowConfig - control position, opacity, margin, etc.
      Drag shadow can update during a drag only for nougat and higher.

Events

Extending PanGestureHandler event.

  type Map = { [index: string]: any };
  type DragData<T extends Map> = T & { nativeProps?: Map };
    type DropData<T extends Map> = (DragData<T> | {
    /**This property will be available if an error occured while trying to parse the data */
    rawData: string
  }) & { readonly target: number };

  export enum DragState {
    BEGAN,
    ACTIVE,
    DROP,
    END,
    ENTERED,
    EXITED
  }

  export enum DragMode {
    MOVE,
    /** After drop occurs, restores visibility to the DragGestureHandler's view  */
    MOVE_RESTORE,
    COPY
  }

  interface DragGestureHandlerEventExtra extends PanGestureHandlerEventExtra {
    dragTarget: number,
    dragTargets: number[],
    dropTarget: number,
    dragState: DragState
  }

  interface DropGestureHandlerEventExtra<T extends Map> extends DragGestureHandlerEventExtra {
    /**
     * The data received from the DragGestureHandler
     */
    data: DropData<T>[] | null,
    /**
     * The id of the app the event originated from.
     * This property will be available once a drop occurs for an event that originated from a different app.
     */
    sourceAppID?: string
  }

#963 (comment)

I believe iOS will be much easier to implement.

Please consider adding this to the road map. Drag gesture handling is the missing piece of RNGH!

[ShaMan123]

fixed a bunch of edge case bugs

[ShaMan123]

I think android is ready for roll out

[nandorojo]

Would this support web too?

[ShaMan123]

Would this support web too?

@nandorojo Why not? It can wrap around react-dnd

[nandorojo]

That would be awesome

[nandorojo]

@ShaMan123 would it be possible to move this PR into a standalone npm package, which uses gesture handler as a peer dependency? That way, you could get community support and testing to help get this merged.

It doesn't seem likely that this will get merged as-is right now, but I'd love to get to use it in my app. I think making it a separate package (like native-dnd) would be awesome.

What do you think?

[ShaMan123]

I think it isn't feasible. The code can't be separated from the core. And can't be patched together. There is logic that changed. You could PR my fork and I'll merge it. I think this is worth working on to support ios. בתאריך יום ד׳, 19 במאי 2021, 15:31, מאת Fernando Rojo ‏< ***@***.***>:

…

@ShaMan123 <https://github.com/ShaMan123> would it be possible to move this PR into a standalone npm package, which uses gesture handler as a peer dependency? That way, you could get community support and testing to help get this merged. It doesn't seem likely that this will get merged as-is right now, but I'd love to get to use this. I think making it a separate package (like native-dnd) would be awesome. What do you think? — You are receiving this because you were mentioned. Reply to this email directly, view it on GitHub <#963 (comment)>, or unsubscribe <https://github.com/notifications/unsubscribe-auth/AIGAW4OW25HSYEEDKHAFZFDTOOVTDANCNFSM4KTI6WOQ> .

[nandorojo]

I see. How far along is it? Is it all working for you?

[ShaMan123]

Android works completely.
iOS isn't supported.
Clone the fork and run the example app

In Example/draggable index.js is a simple PanGestureHandler, not a DragGestureHandler.
list.js should work as should drag.js.
Are you running it in the example app or in your own app? I recommend you first check it works in the example to eliminate issues/wrong config.

[artemis-prime]

+1 Let's get this merged in!

[j-piasecki]

With the number of changes that happened in the repository since this PR has been opened, I don't think this is mergeable anymore.