emacs/java/README

This directory holds the Java sources of the port of GNU Emacs to
Android-like systems, along with files needed to create an application
package out of them.  If you need to build this port, please read the
file INSTALL in this directory.

The ``org/gnu/emacs'' subdirectory contains the Java sources under the
``org.gnu.emacs'' package identifier.

``AndroidManifest.xml'' contains a manifest describing the Java
sources to the system.

The ``res'' directory contains resources, mainly the Emacs icon and
several ``boolean resources'' which are used as a form of conditional
evaluation for manifest entries.

`emacs.keystore' is the signing key used to build Emacs.  It is kept
here, and we encourage all people redistributing Emacs to use this
key.  It holds no security value, and otherwise it will be impossible
to install different builds of Emacs on top of each other.

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:

    <activity android:name="org.gnu.emacs.EmacsActivity"
	      android:launchMode="singleTop"
	      android:windowSoftInputMode="adjustResize"
	      android:exported="true"
	      android:configChanges="orientation|screenSize|screenLayout|keyboardHidden">
      <intent-filter>
	<action android:name="android.intent.action.MAIN" />
	<category android:name="android.intent.category.DEFAULT" />
	<category android:name="android.intent.category.LAUNCHER" />
      </intent-filter>
    </activity>

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, "<init>",
					    "(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<EmacsView> view;

    view = new EmacsHolder<EmacsView> ();

    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>
{
  T bar;
};

can not be used alone:

  Foo holder; /* Error! */

but must have a type specified:

  Foo<Object> holder;

in which case the effective definition is:

public class Foo
{
  Object bar;
};