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Clipboard Operations

  • Стаття

A window should use the clipboard when cutting, copying, or pasting data. A window places data on the clipboard for cut and copy operations and retrieves data from the clipboard for paste operations. The following sections describe these operations and related issues.

To place data on or retrieve data from the clipboard, a window must first open the clipboard by using the OpenClipboard function. Only one window can have the clipboard open at a time. To find out which window has the clipboard open, call the GetOpenClipboardWindow function. When it has finished, the window must close the clipboard by calling the CloseClipboard function.

The following topics are discussed in this section.

Cut and Copy Operations

To place information on the clipboard, a window first clears any previous clipboard content by using the EmptyClipboard function. This function sends the WM_DESTROYCLIPBOARD message to the previous clipboard owner, frees resources associated with data on the clipboard, and assigns clipboard ownership to the window that has the clipboard open. To find out which window owns the clipboard, call the GetClipboardOwner function.

After emptying the clipboard, the window places data on the clipboard in as many clipboard formats as possible, ordered from the most descriptive clipboard format to the least descriptive. For each format, the window calls the SetClipboardData function, specifying the format identifier and a global memory handle. The memory handle can be NULL, indicating that the window renders the data on request. For more information, see Delayed Rendering.

Paste Operations

To retrieve paste information from the clipboard, a window first determines the clipboard format to retrieve. Typically, a window enumerates the available clipboard formats by using the EnumClipboardFormats function and uses the first format it recognizes. This method selects the best available format according to the priority set when the data was placed on the clipboard.

Alternatively, a window can use the GetPriorityClipboardFormat function. This function identifies the best available clipboard format according to a specified priority. A window that recognizes only one clipboard format can simply determine whether that format is available by using the IsClipboardFormatAvailable function.

After determining the clipboard format to use, a window calls the GetClipboardData function. This function returns the handle to a global memory object containing data in the specified format. A window can briefly lock the memory object in order to examine or copy the data. However, a window should not free the object or leave it locked for a long period of time.

Clipboard Ownership

The clipboard owner is the window associated with the information on the clipboard. A window becomes the clipboard owner when it places data on the clipboard, specifically, when it calls the EmptyClipboard function. The window remains the clipboard owner until it is closed or another window empties the clipboard.

When the clipboard is emptied, the clipboard owner receives a WM_DESTROYCLIPBOARD message. Following are some reasons why a window might process this message:

  • The window delayed rendering of one or more clipboard formats. In response to the WM_DESTROYCLIPBOARD message, the window might free resources it had allocated in order to render data on request. For more information about the rendering of data, see Delayed Rendering.
  • The window placed data on the clipboard in a private clipboard format. The data for private clipboard formats is not freed by the system when the clipboard is emptied. Therefore, the clipboard owner should free the data upon receiving the WM_DESTROYCLIPBOARD message. For more information about private clipboard formats, see Clipboard Formats.
  • The window placed data on the clipboard using the CF_OWNERDISPLAY clipboard format. In response to the WM_DESTROYCLIPBOARD message, the window might free resources it had used to display information in the clipboard viewer window. For more information about this alternative format, see Owner Display Format.

Delayed Rendering

When placing a clipboard format on the clipboard, a window can delay rendering the data in that format until the data is needed. To do so, an application can specify NULL for the hData parameter of the SetClipboardData function. This is useful if the application supports several clipboard formats, some or all of which are time-consuming to render. By passing a NULL handle, a window renders complex clipboard formats only when and if they are needed.

If a window delays rendering a clipboard format, it must be prepared to render the format upon request for as long as it is the clipboard owner. The system sends the clipboard owner a WM_RENDERFORMAT message when a request is received for a specific format that has not been rendered. Upon receiving this message, the window should call the SetClipboardData function to place a global memory handle on the clipboard in the requested format.

An application must not open the clipboard before calling SetClipboardData in response to the WM_RENDERFORMAT message. Opening the clipboard is not necessary, and any attempt to do so will fail because the clipboard is currently being held open by the application that requested the format to be rendered.

If the clipboard owner is about to be destroyed and has delayed rendering some or all clipboard formats, it receives the WM_RENDERALLFORMATS message. Upon receiving this message, the window should open the clipboard, check that it is still the clipboard owner with the GetClipboardOwner function, and then place valid memory handles on the clipboard for all clipboard formats that it provides. This ensures that these formats remain available after the clipboard owner is destroyed.

Unlike with WM_RENDERFORMAT, an application responding to WM_RENDERALLFORMATS should open the clipboard before calling SetClipboardData to place any global memory handles on the clipboard.

Any clipboard formats that are not rendered in response to the WM_RENDERALLFORMATS message cease to be available to other applications and are no longer enumerated by the clipboard functions.

Delayed Rendering Guidance

Delayed rendering is a performance feature, enabling an application to avoid doing work to render clipboard data in a format that may never be requested. However, using delayed rendering involves the following tradeoffs that should be taken into consideration:

  • Using delayed rendering adds some complexity to the application, requiring it to handle two rendering window messages, as described above.
  • Using delayed rendering means that the application loses the option to keep the UI responsive if rendering the data takes enough time that it is noticeable to the user. With delayed rendering, if the data is eventually needed, the window must render the data while processing a rendering window message, as described above. As a result, if the data is very time-consuming to render, the application may become visibly unresponsive (hung) while rendering occurs, as no other window messages can be processed while the rendering window message is being processed. An application that does not use delayed rendering might instead choose to render data on a background thread in order to keep the UI response while rendering occurs, perhaps providing progress or cancellation options, which are not available when using delayed rendering.
  • Using delayed rendering add a small amount of overhead if the data is eventually needed. When using delay rendering, a window initially calls the SetClipboardData function with a NULL handle, and if the data is later needed, the window must respond to a window message and call the SetClipboardData function a second time with a handle to the rendered data, as described above. As a result, if the data is eventually needed, using delayed rendering adds the cost of processing a window message and calling the SetClipboardData function a second time. This cost is small but not zero. If an application only supports a single clipboard format, and if the data is always eventually requested, using delayed rendering just adds this small amount of overhead (the cost varies by hardware; an estimate is between 10 and 100 microseconds). However, if the data is small, the overhead of using delayed rendering may exceed the cost to render the data, which could defeat the purpose of using delayed rendering to improve performance. (In testing, for data that is already in its final form, the overhead of using delayed rendering consistently exceeded the cost to copy the data to the clipboard if the data was 100 KiB or less. This testing does not include the cost to render data, just to copy it once it is rendered.)
  • Delayed rendering is a net performance advantage if it saves more time than it adds in overhead. To determine the overhead of delayed rendering, measuring is best, but 10 to 100 microseconds is an estimate. To calculate the savings of using delayed rendering for each clipboard format, measure the cost to render the data in that format and determine how frequently that format is eventually requested (based on the window messages described above). Multiply the cost of rendering the data by the percentage of time that the data is not eventually requested (before the clipboard is emptied or its contents change) to determine the savings of delayed rendering for each clipboard format. Delayed rendering is a net performance advantage if the savings exceeds the overhead cost.
  • As a concrete guideline, for applications that only support a single clipboard format, such as text, where the data is not significantly expensive to render, consider placing the data directly on the clipboard if the size of the data is 4 KiB or less.

Memory and the Clipboard

A memory object that is to be placed on the clipboard should be allocated by using the GlobalAlloc function with the GMEM_MOVEABLE flag.

After a memory object is placed on the clipboard, ownership of that memory handle is transferred to the system. When the clipboard is emptied and the memory object has one of the following clipboard formats, the system frees the memory object by calling the specified function:

Function to free object Clipboard format
DeleteMetaFile
 CF_DSPENHMETAFILE
CF_DSPMETAFILEPICT
CF_ENHMETAFILE
CF_METAFILEPICT
DeleteObject
 CF_BITMAP
CF_DSPBITMAP
CF_PALETTE
GlobalFree
 CF_DIB
CF_DIBV5
CF_DSPTEXT
CF_OEMTEXT
CF_TEXT
CF_UNICODETEXT
none
 CF_OWNERDISPLAY
When the clipboard is emptied of a CF_OWNERDISPLAY object, the application itself must free the memory object.