From 4debb110d70691a405a50272b3ca5d0a264e0010 Mon Sep 17 00:00:00 2001
From: Po Lu
Date: Sun, 3 Sep 2023 10:04:44 +0800
Subject: Move Android port internals documentation to admin/notes
* admin/notes/java: New file. Move most of its contents from
README, and introduce a section on compatibility.
* java/README: Move internals to admin/notes/java.
---
java/README | 1027 +----------------------------------------------------------
1 file changed, 3 insertions(+), 1024 deletions(-)
(limited to 'java/README')
diff --git a/java/README b/java/README
index e518e9fbb2f..a909cdd22ef 100644
--- a/java/README
+++ b/java/README
@@ -22,1027 +22,6 @@ Please keep the Java code indented with tabs and formatted according
to the rules for C code in the GNU coding standards. Always use
C-style comments.
-======================================================================
-
-OVERVIEW OF JAVA
-
-Emacs developers do not know Java, and there is no reason they should
-have to. Thus, the code in this directory is confined to what is
-strictly necessary to support Emacs, and only uses a subset of Java
-written in a way that is easily understandable to C programmers.
-
-Java is required because the entire Android runtime is based around
-Java, and there is no way to write an Android program which runs
-without Java.
-
-This text exists to prime other Emacs developers, already familar with
-C, on the basic architecture of the Android port, and to teach them
-how to read and write the Java code found in this directory.
-
-Java is an object oriented language with automatic memory management
-compiled down to bytecode, which is then subject to interpretation by
-a Java virtual machine.
-
-What that means, is that:
-
-struct emacs_window
-{
- int some_fields;
- int of_emacs_window;
-};
-
-static void
-do_something_with_emacs_window (struct emacs_window *a, int n)
-{
- a->some_fields = a->of_emacs_window + n;
-}
-
-would be written:
-
-public class EmacsWindow
-{
- public int someFields;
- public int ofEmacsWindow;
-
- public void
- doSomething (int n)
- {
- someFields = ofEmacsWindow + n;
- }
-}
-
-and instead of doing:
-
-do_something_with_emacs_window (my_window, 1);
-
-you say:
-
-myWindow.doSomething (1);
-
-In addition to functions associated with an object of a given class
-(such as EmacsWindow), Java also has two other kinds of functions.
-
-The first are so-called ``static'' functions (the static means
-something entirely different from what it does in C.)
-
-A static function, while still having to be defined within a class,
-can be called without any object. Instead of the object, you write
-the name of the Java class within which it is defined. For example,
-the following C code:
-
-int
-multiply_a_with_b_and_then_add_c (int a, int b, int c)
-{
- return a * b + c;
-}
-
-would be:
-
-public class EmacsSomething
-{
- public static int
- multiplyAWithBAndThenAddC (int a, int b, int c)
- {
- return a * b + c;
- }
-};
-
-Then, instead of calling:
-
-int foo;
-
-foo = multiply_a_with_b_then_add_c (1, 2, 3);
-
-you say:
-
-int foo;
-
-foo = EmacsSomething.multiplyAWithBAndThenAddC (1, 2, 3);
-
-In Java, ``static'' does not mean that the function is only used
-within its compilation unit! Instead, the ``private'' qualifier is
-used to mean more or less the same thing:
-
-static void
-this_procedure_is_only_used_within_this_file (void)
-{
- do_something ();
-}
-
-becomes
-
-public class EmacsSomething
-{
- private static void
- thisProcedureIsOnlyUsedWithinThisClass ()
- {
-
- }
-}
-
-the other kind are called ``constructors''. They are functions that
-must be called to allocate memory to hold a class:
-
-public class EmacsFoo
-{
- int bar;
-
- public
- EmacsFoo (int tokenA, int tokenB)
- {
- bar = tokenA + tokenB;
- }
-}
-
-now, the following statement:
-
-EmacsFoo foo;
-
-foo = new EmacsFoo (1, 2);
-
-becomes more or less equivalent to the following C code:
-
-struct emacs_foo
-{
- int bar;
-};
-
-struct emacs_foo *
-make_emacs_foo (int token_a, int token_b)
-{
- struct emacs_foo *foo;
-
- foo = xmalloc (sizeof *foo);
- foo->bar = token_a + token_b;
-
- return foo;
-}
-
-/* ... */
-
-struct emacs_foo *foo;
-
-foo = make_emacs_foo (1, 2);
-
-A class may have any number of constructors, or no constructors at
-all, in which case the compiler inserts an empty constructor.
-
-
-
-Sometimes, you will see Java code that looks like this:
-
- allFiles = filesDirectory.listFiles (new FileFilter () {
- @Override
- public boolean
- accept (File file)
- {
- return (!file.isDirectory ()
- && file.getName ().endsWith (".pdmp"));
- }
- });
-
-This is Java's version of GCC's nested function extension. The major
-difference is that the nested function may still be called even after
-it goes out of scope, and always retains a reference to the class and
-local variables around where it was called.
-
-Being an object-oriented language, Java also allows defining that a
-class ``extends'' another class. The following C code:
-
-struct a
-{
- long thirty_two;
-};
-
-struct b
-{
- struct a a;
- long long sixty_four;
-};
-
-extern void do_something (struct a *);
-
-void
-my_function (struct b *b)
-{
- do_something (&b->a);
-}
-
-is roughly equivalent to the following Java code, split into two
-files:
-
- A.java
-
-public class A
-{
- int thirtyTwo;
-
- public void
- doSomething ()
- {
- etcEtcEtc ();
- }
-};
-
- B.java
-
-public class B extends A
-{
- long sixty_four;
-
- public static void
- myFunction (B b)
- {
- b.doSomething ();
- }
-}
-
-the Java runtime has transformed the call to ``b.doSomething'' to
-``((A) b).doSomething''.
-
-However, Java also allows overriding this behavior, by specifying the
-@Override keyword:
-
-public class B extends A
-{
- long sixty_four;
-
- @Override
- public void
- doSomething ()
- {
- Something.doSomethingTwo ();
- super.doSomething ();
- }
-}
-
-now, any call to ``doSomething'' on a ``B'' created using ``new B ()''
-will end up calling ``Something.doSomethingTwo'', before calling back
-to ``A.doSomething''. This override also applies in reverse; that is
-to say, even if you write:
-
- ((A) b).doSomething ();
-
-B's version of doSomething will still be called, if ``b'' was created
-using ``new B ()''.
-
-This mechanism is used extensively throughout the Java language and
-Android windowing APIs.
-
-Elsewhere, you will encounter Java code that defines arrays:
-
-public class EmacsFrobinicator
-{
- public static void
- emacsFrobinicate (int something)
- {
- int[] primesFromSomething;
-
- primesFromSomething = new int[numberOfPrimes];
- /* ... */
- }
-}
-
-Java arrays are similar to C arrays in that they can not grow. But
-they are very much unlike C arrays in that they are always references
-(as opposed to decaying into pointers in only some situations), and
-contain information about their length.
-
-If another function named ``frobinicate1'' takes an array as an
-argument, then it need not take the length of the array.
-
-Instead, it may simply iterate over the array like so:
-
-int i, k;
-
-for (i = 0; i < array.length; ++i)
- {
- k = array[i];
-
- Whatever.doSomethingWithK (k);
- }
-
-The syntax used to define arrays is also slightly different. As
-arrays are always references, there is no way for you to tell the
-runtime to allocate an array of size N in a structure (class.)
-
-Instead, if you need an array of that size, you must declare a field
-with the type of the array, and allocate the array inside the class's
-constructor, like so:
-
-public class EmacsArrayContainer
-{
- public int[] myArray;
-
- public
- EmacsArrayContainer ()
- {
- myArray = new array[10];
- }
-}
-
-while in C, you could just have written:
-
-struct emacs_array_container
-{
- int my_array[10];
-};
-
-or, possibly even better,
-
-typedef int emacs_array_container[10];
-
-Alas, Java has no equivalent of `typedef'.
-
-Like in C, Java string literals are delimited by double quotes.
-Unlike C, however, strings are not NULL-terminated arrays of
-characters, but a distinct type named ``String''. They store their
-own length, characters in Java's 16-bit ``char'' type, and are capable
-of holding NULL bytes.
-
-Instead of writing:
-
-wchar_t character;
-extern char *s;
-size_t s;
-
- for (/* determine n, s in a loop. */)
- s += mbstowc (&character, s, n);
-
-or:
-
-const char *byte;
-
-for (byte = my_string; *byte; ++byte)
- /* do something with *byte. */;
-
-or perhaps even:
-
-size_t length, i;
-char foo;
-
-length = strlen (my_string);
-
-for (i = 0; i < length; ++i)
- foo = my_string[i];
-
-you write:
-
-char foo;
-int i;
-
-for (i = 0; i < myString.length (); ++i)
- foo = myString.charAt (0);
-
-Java also has stricter rules on what can be used as a truth value in a
-conditional. While in C, any non-zero value is true, Java requires
-that every truth value be of the boolean type ``boolean''.
-
-What this means is that instead of simply writing:
-
- if (foo || bar)
-
-where foo can either be 1 or 0, and bar can either be NULL or a
-pointer to something, you must explicitly write:
-
- if (foo != 0 || bar != null)
-
-in Java.
-
-JAVA NATIVE INTERFACE
-
-Java also provides an interface for C code to interface with Java.
-
-C functions exported from a shared library become static Java
-functions within a class, like so:
-
-public class EmacsNative
-{
- /* Obtain the fingerprint of this build of Emacs. The fingerprint
- can be used to determine the dump file name. */
- public static native String getFingerprint ();
-
- /* Set certain parameters before initializing Emacs.
-
- assetManager must be the asset manager associated with the
- context that is loading Emacs. It is saved and remains for the
- remainder the lifetime of the Emacs process.
-
- filesDir must be the package's data storage location for the
- current Android user.
-
- libDir must be the package's data storage location for native
- libraries. It is used as PATH.
-
- cacheDir must be the package's cache directory. It is used as
- the `temporary-file-directory'.
-
- pixelDensityX and pixelDensityY are the DPI values that will be
- used by Emacs.
-
- classPath must be the classpath of this app_process process, or
- NULL.
-
- emacsService must be the EmacsService singleton, or NULL. */
- public static native void setEmacsParams (AssetManager assetManager,
- String filesDir,
- String libDir,
- String cacheDir,
- float pixelDensityX,
- float pixelDensityY,
- String classPath,
- EmacsService emacsService);
-}
-
-Where the corresponding C functions are located in android.c, and
-loaded by the special invocation:
-
- static
- {
- System.loadLibrary ("emacs");
- };
-
-where ``static'' defines a section of code which will be run upon the
-object (containing class) being loaded. This is like:
-
- __attribute__((constructor))
-
-on systems where shared object constructors are supported.
-
-See http://docs.oracle.com/en/java/javase/19/docs/specs/jni/intro.html
-for more details.
-
-
-
-OVERVIEW OF ANDROID
-
-When the Android system starts an application, it does not actually
-call the application's ``main'' function. It may not even start the
-application's process if one is already running.
-
-Instead, Android is organized around components. When the user opens
-the ``Emacs'' icon, the Android system looks up and starts the
-component associated with the ``Emacs'' icon. In this case, the
-component is called an activity, and is declared in
-the AndroidManifest.xml in this directory:
-
-
-
-
-
-
-
-
-
-This tells Android to start the activity defined in ``EmacsActivity''
-(defined in org/gnu/emacs/EmacsActivity.java), a class extending the
-Android class ``Activity''.
-
-To do so, the Android system creates an instance of ``EmacsActivity''
-and the window system window associated with it, and eventually calls:
-
- Activity activity;
-
- activity.onCreate (...);
-
-But which ``onCreate'' is really called?
-It is actually the ``onCreate'' defined in EmacsActivity.java, as
-it overrides the ``onCreate'' defined in Android's own Activity class:
-
- @Override
- public void
- onCreate (Bundle savedInstanceState)
- {
- FrameLayout.LayoutParams params;
- Intent intent;
-
-Then, this is what happens step-by-step within the ``onCreate''
-function:
-
- /* See if Emacs should be started with -Q. */
- intent = getIntent ();
- EmacsService.needDashQ
- = intent.getBooleanExtra ("org.gnu.emacs.START_DASH_Q",
- false);
-
-Here, Emacs obtains the intent (a request to start a component) which
-was used to start Emacs, and sets a special flag if it contains a
-request for Emacs to start with the ``-Q'' command-line argument.
-
- /* Set the theme to one without a title bar. */
-
- if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.ICE_CREAM_SANDWICH)
- setTheme (android.R.style.Theme_DeviceDefault_NoActionBar);
- else
- setTheme (android.R.style.Theme_NoTitleBar);
-
-Next, Emacs sets an appropriate theme for the activity's associated
-window decorations.
-
- params = new FrameLayout.LayoutParams (LayoutParams.MATCH_PARENT,
- LayoutParams.MATCH_PARENT);
-
- /* Make the frame layout. */
- layout = new FrameLayout (this);
- layout.setLayoutParams (params);
-
- /* Set it as the content view. */
- setContentView (layout);
-
-Then, Emacs creates a ``FrameLayout'', a widget that holds a single
-other widget, and makes it the activity's ``content view''.
-
-The activity itself is a ``FrameLayout'', so the ``layout parameters''
-here apply to the FrameLayout itself, and not its children.
-
- /* Maybe start the Emacs service if necessary. */
- EmacsService.startEmacsService (this);
-
-And after that, Emacs calls the static function ``startEmacsService'',
-defined in the class ``EmacsService''. This starts the Emacs service
-component if necessary.
-
- /* Add this activity to the list of available activities. */
- EmacsWindowAttachmentManager.MANAGER.registerWindowConsumer (this);
-
- super.onCreate (savedInstanceState);
-
-Finally, Emacs registers that this activity is now ready to receive
-top-level frames (windows) created from Lisp.
-
-Activities come and go, but Emacs has to stay running in the mean
-time. Thus, Emacs also defines a ``service'', which is a long-running
-component that the Android system allows to run in the background.
-
-Let us go back and review the definition of ``startEmacsService'':
-
- public static void
- startEmacsService (Context context)
- {
- if (EmacsService.SERVICE == null)
- {
- if (Build.VERSION.SDK_INT < Build.VERSION_CODES.O)
- /* Start the Emacs service now. */
- context.startService (new Intent (context,
- EmacsService.class));
- else
- /* Display the permanant notification and start Emacs as a
- foreground service. */
- context.startForegroundService (new Intent (context,
- EmacsService.class));
- }
- }
-
-If ``EmacsService.SERVICE'' does not yet exist, what this does is to
-tell the ``context'' (the equivalent of an Xlib Display *) to start a
-service defined by the class ``EmacsService''. Eventually, this
-results in ``EmacsService.onCreate'' being called:
-
- @Override
- public void
- onCreate ()
- {
- AssetManager manager;
- Context app_context;
- String filesDir, libDir, cacheDir, classPath;
- double pixelDensityX;
- double pixelDensityY;
-
-Here is what this function does, step-by-step:
-
- SERVICE = this;
-
-First, it sets the special static variable ``SERVICE'' to ``this'',
-which is a pointer to the ``EmacsService' object that was created.
-
- handler = new Handler (Looper.getMainLooper ());
-
-Next, it creates a ``Handler'' object for the ``main looper''.
-This is a helper structure which allows executing code on the Android
-user interface thread.
-
- manager = getAssets ();
- app_context = getApplicationContext ();
- metrics = getResources ().getDisplayMetrics ();
- pixelDensityX = metrics.xdpi;
- pixelDensityY = metrics.ydpi;
-
-Finally, it obtains:
-
- - the asset manager, which is used to retrieve assets packaged
- into the Emacs application package.
-
- - the application context, used to obtain application specific
- information.
-
- - the display metrics, and from them, the X and Y densities in dots
- per inch.
-
-Then, inside a ``try'' block:
-
- try
- {
- /* Configure Emacs with the asset manager and other necessary
- parameters. */
- filesDir = app_context.getFilesDir ().getCanonicalPath ();
- libDir = getLibraryDirectory ();
- cacheDir = app_context.getCacheDir ().getCanonicalPath ();
-
-It obtains the names of the Emacs home, shared library, and temporary
-file directories.
-
- /* Now provide this application's apk file, so a recursive
- invocation of app_process (through android-emacs) can
- find EmacsNoninteractive. */
- classPath = getApkFile ();
-
-The name of the Emacs application package.
-
- Log.d (TAG, "Initializing Emacs, where filesDir = " + filesDir
- + ", libDir = " + libDir + ", and classPath = " + classPath);
-
-Prints a debug message to the Android system log with this
-information.
-
- EmacsNative.setEmacsParams (manager, filesDir, libDir,
- cacheDir, (float) pixelDensityX,
- (float) pixelDensityY,
- classPath, this);
-
-And calls the native function ``setEmacsParams'' (defined in
-android.c) to configure Emacs with this information.
-
- /* Start the thread that runs Emacs. */
- thread = new EmacsThread (this, needDashQ);
- thread.start ();
-
-Then, it allocates an ``EmacsThread'' object, and starts that thread.
-Inside that thread is where Emacs's C code runs.
-
- }
- catch (IOException exception)
- {
- EmacsNative.emacsAbort ();
- return;
-
-And here is the purpose of the ``try'' block. Functions related to
-file names in Java will signal errors of various types upon failure.
-
-This ``catch'' block means that the Java virtual machine will abort
-execution of the contents of the ``try'' block as soon as an error of
-type ``IOException'' is encountered, and begin executing the contents
-of the ``catch'' block.
-
-Any failure of that type here is a crash, and
-``EmacsNative.emacsAbort'' is called to quickly abort the process to
-get a useful backtrace.
- }
- }
-
-Now, let us look at the definition of the class ``EmacsThread'', found
-in org/gnu/emacs/EmacsThread.java:
-
-public class EmacsThread extends Thread
-{
- /* Whether or not Emacs should be started -Q. */
- private boolean startDashQ;
-
- public
- EmacsThread (EmacsService service, boolean startDashQ)
- {
- super ("Emacs main thread");
- this.startDashQ = startDashQ;
- }
-
- @Override
- public void
- run ()
- {
- String args[];
-
- if (!startDashQ)
- args = new String[] { "libandroid-emacs.so", };
- else
- args = new String[] { "libandroid-emacs.so", "-Q", };
-
- /* Run the native code now. */
- EmacsNative.initEmacs (args, EmacsApplication.dumpFileName);
- }
-};
-
-The class itself defines a single field, ``startDashQ'', a constructor
-with an unused argument of the type ``EmacsService'' (which is useful
-while debugging) and a flag ``startDashQ'', and a single function
-``run'', overriding the same function in the class ``Thread''.
-
-When ``thread.start'' is called, the Java virtual machine creates a
-new thread, and then calls the function ``run'' within that thread.
-
-This function then computes a suitable argument vector, and calls
-``EmacsNative.initEmacs'' (defined in android.c), which then calls a
-modified version of the regular Emacs ``main'' function.
-
-At that point, Emacs initialization proceeds as usual:
-Vinitial_window_system is set, loadup.el calls `normal-top-level',
-which calls `command-line', and finally
-`window-system-initialization', which initializes the `android'
-terminal interface as usual.
-
-What happens here is the same as on other platforms. Now, here is
-what happens when the initial frame is created: Fx_create_frame calls
-`android_create_frame_window' to create a top level window:
-
-static void
-android_create_frame_window (struct frame *f)
-{
- struct android_set_window_attributes attributes;
- enum android_window_value_mask attribute_mask;
-
- attributes.background_pixel = FRAME_BACKGROUND_PIXEL (f);
- attribute_mask = ANDROID_CW_BACK_PIXEL;
-
- block_input ();
- FRAME_ANDROID_WINDOW (f)
- = android_create_window (FRAME_DISPLAY_INFO (f)->root_window,
- f->left_pos,
- f->top_pos,
- FRAME_PIXEL_WIDTH (f),
- FRAME_PIXEL_HEIGHT (f),
- attribute_mask, &attributes);
- unblock_input ();
-}
-
-This calls the function `android_create_window' with some arguments
-whose meanings are identical to the arguments to `XCreateWindow'.
-
-Here is the definition of `android_create_window', in android.c:
-
-android_window
-android_create_window (android_window parent, int x, int y,
- int width, int height,
- enum android_window_value_mask value_mask,
- struct android_set_window_attributes *attrs)
-{
- static jclass class;
- static jmethodID constructor;
- jobject object, parent_object, old;
- android_window window;
- android_handle prev_max_handle;
- bool override_redirect;
-
-What does it do? First, some context:
-
-At any time, there can be at most 65535 Java objects referred to by
-the rest of Emacs through the Java native interface. Each such object
-is assigned a ``handle'' (similar to an XID on X) and given a unique
-type. The function `android_resolve_handle' returns the JNI `jobject'
-associated with a given handle.
-
- parent_object = android_resolve_handle (parent, ANDROID_HANDLE_WINDOW);
-
-Here, it is being used to look up the `jobject' associated with the
-`parent' handle.
-
- prev_max_handle = max_handle;
- window = android_alloc_id ();
-
-Next, `max_handle' is saved, and a new handle is allocated for
-`window'.
-
- if (!window)
- error ("Out of window handles!");
-
-An error is signalled if Emacs runs out of available handles.
-
- if (!class)
- {
- class = (*android_java_env)->FindClass (android_java_env,
- "org/gnu/emacs/EmacsWindow");
- assert (class != NULL);
-
-Then, if this initialization has not yet been completed, Emacs
-proceeds to find the Java class named ``EmacsWindow''.
-
- constructor
- = (*android_java_env)->GetMethodID (android_java_env, class, "",
- "(SLorg/gnu/emacs/EmacsWindow;"
- "IIIIZ)V");
- assert (constructor != NULL);
-
-And it tries to look up the constructor, which should take seven
-arguments:
-
- S - a short. (the handle ID)
- Lorg/gnu/Emacs/EmacsWindow; - an instance of the EmacsWindow
- class. (the parent)
- IIII - four ints. (the window geometry.)
- Z - a boolean. (whether or not the
- window is override-redirect; see
- XChangeWindowAttributes.)
-
- old = class;
- class = (*android_java_env)->NewGlobalRef (android_java_env, class);
- (*android_java_env)->ExceptionClear (android_java_env);
- ANDROID_DELETE_LOCAL_REF (old);
-
-Next, it saves a global reference to the class and deletes the local
-reference. Global references will never be deallocated by the Java
-virtual machine as long as they still exist.
-
- if (!class)
- memory_full (0);
- }
-
- /* N.B. that ANDROID_CW_OVERRIDE_REDIRECT can only be set at window
- creation time. */
- override_redirect = ((value_mask
- & ANDROID_CW_OVERRIDE_REDIRECT)
- && attrs->override_redirect);
-
- object = (*android_java_env)->NewObject (android_java_env, class,
- constructor, (jshort) window,
- parent_object, (jint) x, (jint) y,
- (jint) width, (jint) height,
- (jboolean) override_redirect);
-
-Then, it creates an instance of the ``EmacsWindow'' class with the
-appropriate arguments and previously determined constructor.
-
- if (!object)
- {
- (*android_java_env)->ExceptionClear (android_java_env);
-
- max_handle = prev_max_handle;
- memory_full (0);
-
-If creating the object fails, Emacs clears the ``pending exception''
-and signals that it is out of memory.
- }
-
- android_handles[window].type = ANDROID_HANDLE_WINDOW;
- android_handles[window].handle
- = (*android_java_env)->NewGlobalRef (android_java_env,
- object);
- (*android_java_env)->ExceptionClear (android_java_env);
- ANDROID_DELETE_LOCAL_REF (object);
-
-Otherwise, it associates a new global reference to the object with the
-handle, and deletes the local reference returned from the JNI
-NewObject function.
-
- if (!android_handles[window].handle)
- memory_full (0);
-
-If allocating the global reference fails, Emacs signals that it is out
-of memory.
-
- android_change_window_attributes (window, value_mask, attrs);
- return window;
-
-Otherwise, it applies the specified window attributes and returns the
-handle of the new window.
-}
-
-
-
-DRAWABLES, CURSORS AND HANDLES
-
-Each widget created by Emacs corresponds to a single ``window'', which
-has its own backing store. This arrangement is quite similar to X.
-
-C code does not directly refer to the EmacsView widgets that implement
-the UI logic behind windows. Instead, its handles refer to
-EmacsWindow structures, which contain the state necessary to interact
-with the widgets in an orderly and synchronized manner.
-
-Like X, both pixmaps and windows are drawable resources, and the same
-graphics operations can be applied to both. Thus, a separate
-EmacsPixmap structure is used to wrap around Android Bitmap resources,
-and the Java-level graphics operation functions are capable of
-operating on them both.
-
-Finally, graphics contexts are maintained on both the C and Java
-levels; the C state recorded in `struct android_gc' is kept in sync
-with the Java state in the GContext handle's corresponding EmacsGC
-structure, and cursors are used through handles that refer to
-EmacsCursor structures that hold system PointerIcons.
-
-In all cases, the interfaces provided are identical to X.
-
-
-
-EVENT LOOP
-
-In a typical Android application, the event loop is managed by the
-operating system, and callbacks (implemented through overriding
-separate functions in widgets) are run by the event loop wherever
-necessary. The thread which runs the event loop is also the only
-thread capable of creating and manipulating widgets and activities,
-and is referred to as the ``UI thread''.
-
-These callbacks are used by Emacs to write representations of X-like
-events to a separate event queue, which are then read from Emacs's own
-event loop running in a separate thread. This is accomplished through
-replacing `select' by a function which waits for the event queue to be
-occupied, in addition to any file descriptors that `select' would
-normally wait for.
-
-Conversely, Emacs's event loop sometimes needs to send events to the
-UI thread. These events are implemented as tiny fragments of code,
-which are run as they are received by the main thread.
-
-A typical example is `displayToast', which is implemented in
-EmacsService.java:
-
- public void
- displayToast (final String string)
- {
- runOnUiThread (new Runnable () {
- @Override
- public void
- run ()
- {
- Toast toast;
-
- toast = Toast.makeText (getApplicationContext (),
- string, Toast.LENGTH_SHORT);
- toast.show ();
- }
- });
- }
-
-Here, the variable `string' is used by a nested function. This nested
-function contains a copy of that variable, and is run on the main
-thread using the function `runOnUiThread', in order to display a short
-status message on the display.
-
-When Emacs needs to wait for the nested function to finish, it uses a
-mechanism implemented in `syncRunnable'. This mechanism first calls a
-deadlock avoidance mechanism, then runs a nested function on the UI
-thread, which is expected to signal itself as a condition variable
-upon completion. It is typically used to allocate resources that can
-only be allocated from the UI thread, or to obtain non-thread-safe
-information. The following function is an example; it returns a new
-EmacsView widget corresponding to the provided window:
-
- public EmacsView
- getEmacsView (final EmacsWindow window, final int visibility,
- final boolean isFocusedByDefault)
- {
- Runnable runnable;
- final EmacsHolder view;
-
- view = new EmacsHolder ();
-
- runnable = new Runnable () {
- public void
- run ()
- {
- synchronized (this)
- {
- view.thing = new EmacsView (window);
- view.thing.setVisibility (visibility);
-
- /* The following function is only present on Android 26
- or later. */
- if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.O)
- view.thing.setFocusedByDefault (isFocusedByDefault);
-
- notify ();
- }
- }
- };
-
- syncRunnable (runnable);
- return view.thing;
- }
-
-As no value can be directly returned from the nested function, a
-separate container object is used to hold the result after the
-function finishes execution. Note the type name inside the angle
-brackets: this type is substituted into the class definition as it is
-used; a definition such as:
-
-public class Foo
-{
- T bar;
-};
-
-can not be used alone:
-
- Foo holder; /* Error! */
-
-but must have a type specified:
-
- Foo