undo: redesign the interface

- the plugin interfaces have been separated into 2 independent ones
- the plugin interfaces are optional
- simplification of the design

include/clap/ext/draft/undo.h

@@ -1,8 +1,11 @@
 #pragma once
 
 #include "../../plugin.h"
+#include "../../stream.h"
 
-static CLAP_CONSTEXPR const char CLAP_EXT_UNDO[] = "clap.undo/2";
+static CLAP_CONSTEXPR const char CLAP_EXT_UNDO[] = "clap.undo/4";
+static CLAP_CONSTEXPR const char CLAP_EXT_UNDO_CONTEXT[] = "clap.undo_context/4";
+static CLAP_CONSTEXPR const char CLAP_EXT_UNDO_DELTA[] = "clap.undo_delta/4";
 
 #ifdef __cplusplus
 extern "C" {
@@ -15,7 +18,7 @@ extern "C" {
 ///
 /// Calling host->undo() or host->redo() is equivalent to clicking undo/redo within the host's GUI.
 ///
-/// If the plugin implements this interface then its undo and redo should be entirely delegated to
+/// If the plugin uses this interface then its undo and redo should be entirely delegated to
 /// the host; clicking in the plugin's UI undo or redo is equivalent to clicking undo or redo in the
 /// host's UI.
 ///
@@ -40,31 +43,29 @@ extern "C" {
 /// history. This simplifies the host implementation, leading to less bugs, a more robust design
 /// and maybe an easier experience for the user because there's a single undo context versus one
 /// for the host and one for each plugin instance.
+///
+/// This extension tries to make it as easy as possible for the plugin to hook into the host undo
+/// and make it efficient when possible by using deltas. The plugin interfaces are all optional, and
+/// the plugin can for a minimal implementation, just use the host interface and call
+/// host->change_made() without providing a delta. This is enough for the host to know that it can
+/// capture a plugin state for the undo step.
 
-enum clap_undo_context_flags {
-   // While the host is within a change, it is impossible to perform undo or redo.
-   CLAP_UNDO_IS_WITHIN_CHANGE = 1 << 0,
-};
-
-enum clap_undo_delta_properties_flags {
-   // If not set, then all clap_undo_delta_properties's attributes become irrelevant.
+typedef struct clap_undo_delta_properties {
+   // If false, then all clap_undo_delta_properties's attributes become irrelevant.
    // If set, then the plugin will provide deltas in host->change_made().
-   CLAP_UNDO_DELTA_PROPERTIES_HAS_DELTA = 1 << 0,
+   bool has_delta;
 
    // If set, then the delta will be reusable in the future as long as the plugin is
    // compatible with the given format_version.
-   CLAP_UNDO_DELTA_PROPERTIES_IS_PERSISTENT = 1 << 1,
-};
-
-typedef struct clap_undo_delta_properties {
-   // Bitmask of clap_undo_delta_properties_flags
-   uint64_t flags;
+   bool are_deltas_persistent;
 
-   // This represents the delta format version that the plugin is using.
+   // This represents the delta format version that the plugin is currently using.
    uint32_t format_version;
 } clap_undo_delta_properties_t;
 
-typedef struct clap_plugin_undo {
+// Use CLAP_EXT_UNDO_DELTA.
+// This is an optional interface, using deltas is an optimization versus making a state snapshot.
+typedef struct clap_plugin_undo_delta {
    // Asks the plugin the delta properties.
    // [main-thread]
    void(CLAP_ABI *get_delta_properties)(const clap_plugin_t          *plugin,
@@ -76,25 +77,42 @@ typedef struct clap_plugin_undo {
    bool(CLAP_ABI *can_use_delta_format_version)(const clap_plugin_t *plugin,
                                                 clap_id              format_version);
 
-   // Applies synchronously a delta.
+   // Undo using the delta.
    // Returns true on success.
    //
    // [main-thread]
-   bool(CLAP_ABI *apply_delta)(const clap_plugin_t *plugin,
-                               clap_id              format_version,
-                               const void          *delta,
-                               size_t               delta_size);
-
-   // Sets the undo context.
-   // flags: bitmask of clap_undo_context_flags values
-   // names: null terminated string if an redo/undo step exists, null otherwise.
-   // [main-thread]
-   void(CLAP_ABI *set_context_info)(const clap_plugin_t *plugin,
-                                    uint64_t             flags,
-                                    const char          *undo_name,
-                                    const char          *redo_name);
-} clap_plugin_undo_t;
+   bool(CLAP_ABI *undo)(const clap_plugin_t *plugin,
+                        clap_id              format_version,
+                        const void          *delta,
+                        size_t               delta_size);
 
+   // Redo using the delta.
+   // Returns true on success.
+   //
+   // [main-thread]
+   bool(CLAP_ABI *redo)(const clap_plugin_t *plugin,
+                        clap_id              format_version,
+                        const void          *delta,
+                        size_t               delta_size);
+} clap_plugin_undo_delta_t;
+
+// Use CLAP_EXT_UNDO_CONTEXT.
+// This is an optional interface, that the plugin can implement in order to know about
+// the current undo context.
+typedef struct clap_plugin_undo_context {
+   // Indicate if it is currently possible to perform an undo or redo operation.
+   // [main-thread & plugin-subscribed-to-undo-context]
+   void(CLAP_ABI *set_can_undo)(const clap_plugin_t *plugin, bool can_undo);
+   void(CLAP_ABI *set_can_redo)(const clap_plugin_t *plugin, bool can_redo);
+
+   // Sets the name of the next undo or redo step.
+   // name: null terminated string.
+   // [main-thread & plugin-subscribed-to-undo-context]
+   void(CLAP_ABI *set_undo_name)(const clap_plugin_t *plugin, const char *name);
+   void(CLAP_ABI *set_redo_name)(const clap_plugin_t *plugin, const char *name);
+} clap_plugin_undo_context_t;
+
+// Use CLAP_EXT_UNDO.
 typedef struct clap_host_undo {
    // Begins a long running change.
    // The plugin must not call this twice: there must be either a call to cancel_change() or
@@ -112,36 +130,51 @@ typedef struct clap_host_undo {
    //
    // name: mandatory null terminated string describing the change, this is displayed to the user
    //
-   // deltas: optional, they are binary blobs used to perform the undo and redo. When not available
+   // delta: optional, it is a binary blobs used to perform the undo and redo. When not available
    // the host will save the plugin state and use state->load() to perform undo and redo.
+   // The plugin must be able to perform a redo operation using the delta, though the undo operation
+   // is only possible if delta_can_undo is true.
    //
    // Note: the provided delta may be used for incremental state saving and crash recovery. The
    // plugin can indicate a format version id and the validity lifetime for the binary blobs.
    // The host can use these to verify the compatibility before applying the delta.
    // If the plugin is unable to use a delta, a notification should be provided to the user and
-   // the crash recovery should perform a best effort job, at least restoring the latest saved state.
+   // the crash recovery should perform a best effort job, at least restoring the latest saved
+   // state.
    //
    // Special case: for objects with shared and synchronized state, changes shouldn't be reported
    // as the host already knows about it.
    // For example, plugin parameter changes shouldn't produce a call to change_made().
    //
+   // Note: if the plugin asked for this interface, then host_state->mark_dirty() will not create an
+   // implicit undo step.
+   //
+   // Note: if the plugin did load a preset or did something that leads to a large delta,
+   // it may consider not producing a delta (pass null) and let the host make a state snapshot
+   // instead.
+   //
+   // Note: if a plugin is producing a lot of changes within a small amount of time, the host
+   // may merge them into a single undo step.
+   //
    // [main-thread]
    void(CLAP_ABI *change_made)(const clap_host_t *host,
                                const char        *name,
-                               const void        *redo_delta,
-                               size_t             redo_delta_size,
-                               const void        *undo_delta,
-                               size_t             undo_delta_size);
-
-   // Asks the host to perform the next undo step.
-   // This operation may be asynchronous.
-   // [main-thread]
-   void(CLAP_ABI *undo)(const clap_host_t *host);
+                               const void        *delta,
+                               size_t             delta_size,
+                               bool               delta_can_undo);
 
-   // Asks the host to perform the next redo step.
-   // This operation may be asynchronous.
+   // Asks the host to perform the next undo or redo step.
+   //
+   // Note: this maybe a complex and asynchronous operation, which may complete after
+   // this function returns.
+   //
+   // Note: the host may ignore this request if there is no undo/redo step to perform,
+   // or if the host is unable to perform undo/redo at the time (eg: a long running
+   // change is going on).
+   //
    // [main-thread]
-   void(CLAP_ABI *redo)(const clap_host_t *host);
+   void(CLAP_ABI *request_undo)(const clap_host_t *host);
+   void(CLAP_ABI *request_redo)(const clap_host_t *host);
 
    // Subscribes to or unsubscribes from undo context info.
    //
@@ -154,8 +187,10 @@ typedef struct clap_host_undo {
    //
    // is_subscribed: set to true to receive context info
    //
+   // It is mandatory for the plugin to implement CLAP_EXT_UNDO_CONTEXT when using this method.
+   //
    // [main-thread]
-   void(CLAP_ABI *set_context_info_subscription)(const clap_host_t *host, bool is_subscribed);
+   void(CLAP_ABI *set_wants_context_updates)(const clap_host_t *host, bool is_subscribed);
 } clap_host_undo_t;
 
 #ifdef __cplusplus