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Manual Reference Pages  -  EZ_ITEMADDDNDDATAENCODER (3)


EZ_ItemAddDnDDataDecoder,EZ_ItemAddDnDDataEncoder, EZ_ItemDeleteDnDDataDecoder, EZ_ItemDeleteDnDDataEncoder EZ_ItemDeleteAllDnDDataDecoders, EZ_ItemDeleteAllDnDDataEncoders - register/remove drag and drop(DnD) handlers




#include <EZ.h> 

void EZ_ItemAddDnDDataDecoder(EZ_Item *item, Atom atom, unsigned int tag, EZ_DnDDecoder decoder, void *ddata, EZ_CallBack dcallback, void *cdata)

void EZ_ItemAddDnDDataEncoder(EZ_Item *item, Atom atom, unsigned int tag, EZ_DnDEncoder encoder, void *edata, EZ_CallBack ecallback, void *cdata)

void EZ_ItemDelteDnDDataDecoder(EZ_Item *item, EZ_DnDDecoder decoder, void *ddata)

void EZ_ItemDelteDnDDataEncoder(EZ_Item *item, EZ_DnDEncoder encoder, void *edata)

void EZ_ItemDelteAllDnDDataDecoders(EZ_Item *item)

void EZ_ItemDelteAllDnDDataEncoders(EZ_Item *item)


atom Specifies a DnD target.

tag Specifies a tag. It must be either ~0 or a value or’ed from ShifMask, ControlMask and Mod1Mask.

item Specifies a display item.

encoder Specify a DnD data encoder.

decoder Specify a DnD data decoder.

edata Specify a client data to be passed to encoder when invoked.

ddata Specify a client data to be passed to decoder when invoked.

ecallback Specify a procedure to be invoked when encoder executes succesfully.

dcallback Specify a procedure to be invoked when decoder executes succesfully.

.fIcdata Specify the argument to be passed to ecallback or dcallback when called.


EZ_ItemAddDnDDataEncoder registers a DnD encoder to a display item. A EZ_DnDEncoder is a data conversion procedure of the type
 int (*encoder)(void *object, void *data, char **message,int *length, int *needfree)

EZ_ItemDeleteDnDDataEncoder dissociate a DnD encoder with a display item. The specified decoder will be removed only if both the procedure and client data match.

EZ_ItemDeleteAllDnDDataEncoders removes all DnD encoders registered to a display item.

EZ_ItemAddDnDDataDecoder registers a DnD decoder to a item. A EZ_DnDDecoder is a data conversion procedure of the type

  int (*decoder)(void *object, void *data, char *message, int length)

EZ_ItemDeleteDnDDataDecoder dissociate a DnD decoder with a display item. The specified decoder will be removed only if both the procedure and client data match.

EZ_ItemDeleteAllDnDDataDecoders removes all DnD decoders registered to a display item.

The tag argument is used to choose conversion methods. It is useful for the following situations.

1. A drag source has multiple encoders for the same conversion target. For example, when transfering files, one may want either to copy a file or move a file to the destination specified by the drop site. This can be achieved by registering multiple encoders with different tags. For example, shift-press-drag-relase for copying the file and cntrl-press-drag-release for moving the file.

2. Sometimes it is preferable to enforce a more strict drag and drop policy so that a careless drag and drop will not do any harm. For example, if you write a file manager and want to use DnD to remove files, it may be better to enforce a control-press-drag-release policy for the trash can.

The tag value ~0 is reserved for a special purpose: drop at the root window of your display. Strictly speaking, the root window is not a valid drop site because it does not talk to the drag source. Nevertheless, for applications like file managers, it is useful to have a feature so a user can drop an item at the root window. Since the root window is not an official drop site, there will be no data transfers. The action is completely handled by the drag source. If you want to use this feature. Here are the tricks.

1. Write a special encoder which does not convert anything, but rather, acts like a callback. In the example of droping a file item over the background, you may want to create a Icon widget, set it up properly and display it. The encoder has to return EZ_DND_SUCCESS on success and EZ_DND_FAILURE on failure. The callbacks for this special encoder behave exactly the same as that of a regular encoder. It is invoked when the encoder returns EZ_DND_SUCCESS+.

2. Register your special encoder with a tag value ~0.


Atom MY_FILE_NAME_ATOM;      /* conversion type */

int encodeFileName(EZ_Item *item, void *data, char **message, /* place to return the data after conversion */ int *length, /* return the length of message */ int *needFree) /* signal whether to free *message when done */ { if(item) { /* file name is stored in the client data slots of item */ char *ptr = (char *)EZ_GetItemPtrData(item); int len = EZ_GetItemIntData(item); if(len > 0) { *length = len; *message = ptr; *needFree = 0; return(EZ_DND_SUCCESS); } } return(EZ_DND_FAILURE); }

int encodeFileContents(EZ_Item *item, void *data, char **message, int *length, int *needFree) { if(item) { char *ptr = (char *)EZ_GetItemPtrData(item); int len = EZ_GetItemIntData(item); if(len > 0) { char *msg; int c, totalLength = 0; FILE *fp = fopen(ptr, "r"); if(fp) while(fgetc(fp) != EOF) totalLength++; /* length of file */ (void)rewind(fp); msg = (char *)malloc( (totalLength + 1)*sizeof(char)); ptr = msg; while((c = fgetc(fp)) != EOF) *ptr++ =c; fclose(fp); *length = totalLength; *message = msg; *needFree = 1; /* ask EZWGL to free msg when done with it */ return(EZ_DND_SUCCESS); } } return(EZ_DND_FAILURE); }

int decodeFileName(EZ_Item *item, void *data,                  char *message, int length) { if(item) { if(length > 0)         {          FILE *fp = fopen(message, "r");          if(fp)          {          int c;          while( (c = getc(fp)) != EOF) putchar(c);          fclose(fp);          return(EZ_DND_SUCCESS);          }         } } return(EZ_DND_FAILURE); }

int decodeFileContents(EZ_Item *item, void *data,                  char *message, int length) { if(item) { if(length > 0)         {          printf("%s", message);          return(EZ_DND_SUCCESS);         } } return(EZ_DND_FAILURE); }



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