diff options
author | Po Lu <luangruo@yahoo.com> | 2023-09-03 10:04:44 +0800 |
---|---|---|
committer | Po Lu <luangruo@yahoo.com> | 2023-09-03 10:05:15 +0800 |
commit | 4debb110d70691a405a50272b3ca5d0a264e0010 (patch) | |
tree | b6b3fa743030a6b31e1f15beaf246d78d85418bb /admin | |
parent | 8ecc73f47a7d5473d0e3f070d85051fc85580f9d (diff) | |
download | emacs-4debb110d70691a405a50272b3ca5d0a264e0010.tar.gz |
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.
Diffstat (limited to 'admin')
-rw-r--r-- | admin/notes/java | 1097 |
1 files changed, 1097 insertions, 0 deletions
diff --git a/admin/notes/java b/admin/notes/java new file mode 100644 index 00000000000..125ac0aad67 --- /dev/null +++ b/admin/notes/java @@ -0,0 +1,1097 @@ +Installation instructions for Android +Copyright (C) 2023 Free Software Foundation, Inc. +See the end of the file for license conditions. + + + +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; +}; + + + +COMPATIBILITY + +There are three variables set within every Android application that +extert influence over the set of Android systems it supports, and the +measures it must take to function faithfully on each of those systems: +the minimum API level, compile SDK version and target API level. + +The minimum API level is the earliest version of Android that is +permitted to install and run the application. For Emacs, this is +established by detecting the __ANDROID_API__ preprocessor macro +defined within the Android C compiler. + +Before Java code executes any Android API calls that are not present +within Android 2.2 (API level 8), the lowest API level supported by +Emacs as a whole, it must first check the value of the: + + Build.VERSION.SDK_INT + +variable, which is always set to the API level of the system Emacs is +presently installed within. For example, before calling +`dispatchKeyEventFromInputMethod', a function absent from Android 6.0 +(API level 23) or earlier, check: + + if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.N) + view.imManager.dispatchKeyEventFromInputMethod (view, key); + else + { + +where `N' is a constant defined to 24. + +The compile SDK version is the version of the Android SDK headers Java +code is compiled against. Because Java does not provide conditional +compilation constructs, Emacs can't be compiled with any version of +these headers other than the version mentioned in `java/INSTALL', but +the headers used do not affect the set of supported systems provided +that the version checks illustrated above are performed where +necessary. + +The target API level is a number within java/AndroidManifest.xml.in +the system refers to when deciding whether to enable +backwards-incompatible modifications to the behavior of various system +APIs. For any given Android version, backwards incompatible changes +in that version will be disabled for applications whose target API +levels don't exceed its own. + +The target API should nevertheless be updated to match every major +Android update, as Google has stated their intentions to prohibit +users from installing applications targeting ``out-of-date'' versions +of Android, though this threat has hitherto been made good on. + + + +This file is part of GNU Emacs. + +GNU Emacs is free software: you can redistribute it and/or modify +it under the terms of the GNU General Public License as published by +the Free Software Foundation, either version 3 of the License, or +(at your option) any later version. + +GNU Emacs is distributed in the hope that it will be useful, +but WITHOUT ANY WARRANTY; without even the implied warranty of +MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the +GNU General Public License for more details. + +You should have received a copy of the GNU General Public License +along with GNU Emacs. If not, see <https://www.gnu.org/licenses/>. |