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GStreamer iOS 教程2 —— 運行pipeline

編輯：IOS開發綜合

1. 目標

在Basic和Playback的教程中（注：另外兩個教程），GStreamer可以和GLib的主循環完美的集成起來，這樣能用一種簡單的方法同時監控pipeline的操作和UI的處理。而在不支持GLib的iOS和Android平台上就必須小心對於pipeline的操作——不能阻塞住UI。

這份教程講述了：

如何把GStreamer的相關處理代碼放到其他的線程（DispatchQueue），保持UI仍然保留在主線程（MainDispatchQueue）在ObjC的UI代碼中如何和GStreamer的C代碼通信 2. 介紹當由UI界面的時候，如果應用等待GStreamer的回傳消息然後進行UI的處理是非常悲催的。通常的做法是（用GTK＋toolkit做例子哈）讓GMainLoop（GLib）來處理收到的event，無論是UI的還是GStreamer發出的。悲催的是這個方法不適合其他非基於GLib的圖形系統（比如iOS的GocoaTouch框架），我們的方法是在另一個線程裡面簡單的調用GMainLoop，確保不會阻塞UI主線程。這個教程還會指出幾個ObjC和C互相調用時需要注意的幾個地方。下面的代碼使用的pipeline僅僅使用了audiotestsrc和autoaudiosink兩個element。UI上包含兩個按鈕，用來設置pipeline的狀態PLAYING/ PAUSED。還有一個Label用來顯示一些信息（錯誤信息或狀態改變）。
3. UI 這個界面底下包含了一個工具條，工具條上放了Play和Pause兩個按鈕。工具條上方就是一個Label，用來顯示GStreamer的信息。本次教程就包含這麼多內容，後面的教程會在這個基礎上逐漸增加內容。 ViewController.m

#import "ViewController.h"
#import "GStreamerBackend.h"
#import 

@interface ViewController () {
    GStreamerBackend *gst_backend;
}

@end

@implementation ViewController

/*
 * Methods from UIViewController
 */

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    play_button.enabled = FALSE;
    pause_button.enabled = FALSE;

    gst_backend = [[GStreamerBackend alloc] init:self];
}

- (void)didReceiveMemoryWarning
{
    [super didReceiveMemoryWarning];
    // Dispose of any resources that can be recreated.
}

/* Called when the Play button is pressed */
-(IBAction) play:(id)sender
{
    [gst_backend play];
}

/* Called when the Pause button is pressed */
-(IBAction) pause:(id)sender
{
    [gst_backend pause];
}

/*
 * Methods from GstreamerBackendDelegate
 */

-(void) gstreamerInitialized
{
    dispatch_async(dispatch_get_main_queue(), ^{
        play_button.enabled = TRUE;
        pause_button.enabled = TRUE;
        message_label.text = @"Ready";
    });
}

-(void) gstreamerSetUIMessage:(NSString *)message
{
    dispatch_async(dispatch_get_main_queue(), ^{
        message_label.text = message;
    });
}

@end

在這個類裡面包含了一個GStreamerBackend的實例，

@interface ViewController () {
    GStreamerBackend *gst_backend;
}

在viewDidLoad方法裡面創建並調用了自定義的init方法

- (void)viewDidLoad
{
    [super viewDidLoad];
    
    play_button.enabled = FALSE;
    pause_button.enabled = FALSE;

    gst_backend = [[GStreamerBackend alloc] init:self];
}

這個自定義的init方法必須傳入一個對象作為UI的delegate（本例是使用了self）
在viewDidLoad的時候，Play/Pause兩個按鈕都是不能使用的，直到GStreamerBackend通知初始化結束為止。

/* Called when the Play button is pressed */
-(IBAction) play:(id)sender
{
    [gst_backend play];
}

/* Called when the Pause button is pressed */
-(IBAction) pause:(id)sender
{
    [gst_backend pause];
}
      在用戶按下Play/Pause按鈕時，上面的方法會被調用。我們看到僅僅就是簡單的調用了GStreamerBackend裡面對應的方法。


-(void) gstreamerInitialized
{
    dispatch_async(dispatch_get_main_queue(), ^{
        play_button.enabled = TRUE;
        pause_button.enabled = TRUE;
        message_label.text = @"Ready";
    });
}     
gstreamerInitialized方法是定義在GStreamerBackendDelegate協議裡面的，用來標識後台已經准備好，可以接受命令了。在這個例子中我們把Play/Pause按鈕激活，並顯示“Ready”信息。這個方法不是在UI線程裡面運行的，所以要用dispatch_async()方法把UI的內容封起來。
-(void) gstreamerSetUIMessage:(NSString *)message
{
    dispatch_async(dispatch_get_main_queue(), ^{
        message_label.text = message;
    });
}     
gstreamerSetUIMessage:方法同樣是定義在GStreamerBackendDelegate協議裡面的，當後台有消息通知UI的時候會調用這個方法。在這個例子裡面消息會拷貝到界面的Label控件上，當然，也同樣要用dispatch_async()方法來封裝。



4. GStreamer後端
      GStreamerBackend類處理了所有和GStreamer相關的內容，並給應用提供了一個簡單的接口。這個接口並不需要實現所有的GStreamer的細節，當需要引起UI的變化的時候，調用GSreamerBackendDelegate協議來解決。
GStreamerBackend.m

#import "GStreamerBackend.h"

#include 

GST_DEBUG_CATEGORY_STATIC (debug_category);
#define GST_CAT_DEFAULT debug_category

@interface GStreamerBackend()
-(void)setUIMessage:(gchar*) message;
-(void)app_function;
-(void)check_initialization_complete;
@end

@implementation GStreamerBackend {
    id ui_delegate;        /* Class that we use to interact with the user interface */
    GstElement *pipeline;  /* The running pipeline */
    GMainContext *context; /* GLib context used to run the main loop */
    GMainLoop *main_loop;  /* GLib main loop */
    gboolean initialized;  /* To avoid informing the UI multiple times about the initialization */
}

/*
 * Interface methods
 */

-(id) init:(id) uiDelegate
{
    if (self = [super init])
    {
        self->ui_delegate = uiDelegate;

        GST_DEBUG_CATEGORY_INIT (debug_category, "tutorial-2", 0, "iOS tutorial 2");
        gst_debug_set_threshold_for_name("tutorial-2", GST_LEVEL_DEBUG);

        /* Start the bus monitoring task */
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            [self app_function];
        });
    }

    return self;
}

-(void) dealloc
{
    if (pipeline) {
        GST_DEBUG("Setting the pipeline to NULL");
        gst_element_set_state(pipeline, GST_STATE_NULL);
        gst_object_unref(pipeline);
        pipeline = NULL;
    }
}

-(void) play
{
    if(gst_element_set_state(pipeline, GST_STATE_PLAYING) == GST_STATE_CHANGE_FAILURE) {
        [self setUIMessage:"Failed to set pipeline to playing"];
    }
}

-(void) pause
{
    if(gst_element_set_state(pipeline, GST_STATE_PAUSED) == GST_STATE_CHANGE_FAILURE) {
        [self setUIMessage:"Failed to set pipeline to paused"];
    }
}

/*
 * Private methods
 */

/* Change the message on the UI through the UI delegate */
-(void)setUIMessage:(gchar*) message
{
    NSString *string = [NSString stringWithUTF8String:message];
    if(ui_delegate && [ui_delegate respondsToSelector:@selector(gstreamerSetUIMessage:)])
    {
        [ui_delegate gstreamerSetUIMessage:string];
    }
}

/* Retrieve errors from the bus and show them on the UI */
static void error_cb (GstBus *bus, GstMessage *msg, GStreamerBackend *self)
{
    GError *err;
    gchar *debug_info;
    gchar *message_string;
    
    gst_message_parse_error (msg, &err, &debug_info);
    message_string = g_strdup_printf ("Error received from element %s: %s", GST_OBJECT_NAME (msg->src), err->message);
    g_clear_error (&err);
    g_free (debug_info);
    [self setUIMessage:message_string];
    g_free (message_string);
    gst_element_set_state (self->pipeline, GST_STATE_NULL);
}

/* Notify UI about pipeline state changes */
static void state_changed_cb (GstBus *bus, GstMessage *msg, GStreamerBackend *self)
{
    GstState old_state, new_state, pending_state;
    gst_message_parse_state_changed (msg, &old_state, &new_state, &pending_state);
    /* Only pay attention to messages coming from the pipeline, not its children */
    if (GST_MESSAGE_SRC (msg) == GST_OBJECT (self->pipeline)) {
        gchar *message = g_strdup_printf("State changed to %s", gst_element_state_get_name(new_state));
        [self setUIMessage:message];
        g_free (message);
    }
}

/* Check if all conditions are met to report GStreamer as initialized.
 * These conditions will change depending on the application */
-(void) check_initialization_complete
{
    if (!initialized && main_loop) {
        GST_DEBUG ("Initialization complete, notifying application.");
        if (ui_delegate && [ui_delegate respondsToSelector:@selector(gstreamerInitialized)])
        {
            [ui_delegate gstreamerInitialized];
        }
        initialized = TRUE;
    }
}

/* Main method for the bus monitoring code */
-(void) app_function
{
    GstBus *bus;
    GSource *bus_source;
    GError *error = NULL;

    GST_DEBUG ("Creating pipeline");

    /* Create our own GLib Main Context and make it the default one */
    context = g_main_context_new ();
    g_main_context_push_thread_default(context);
    
    /* Build pipeline */
    pipeline = gst_parse_launch("audiotestsrc ! audioconvert ! audioresample ! autoaudiosink", &error);
    if (error) {
        gchar *message = g_strdup_printf("Unable to build pipeline: %s", error->message);
        g_clear_error (&error);
        [self setUIMessage:message];
        g_free (message);
        return;
    }
    
    /* Instruct the bus to emit signals for each received message, and connect to the interesting signals */
    bus = gst_element_get_bus (pipeline);
    bus_source = gst_bus_create_watch (bus);
    g_source_set_callback (bus_source, (GSourceFunc) gst_bus_async_signal_func, NULL, NULL);
    g_source_attach (bus_source, context);
    g_source_unref (bus_source);
    g_signal_connect (G_OBJECT (bus), "message::error", (GCallback)error_cb, (__bridge void *)self);
    g_signal_connect (G_OBJECT (bus), "message::state-changed", (GCallback)state_changed_cb, (__bridge void *)self);
    gst_object_unref (bus);
    
    /* Create a GLib Main Loop and set it to run */
    GST_DEBUG ("Entering main loop...");
    main_loop = g_main_loop_new (context, FALSE);
    [self check_initialization_complete];
    g_main_loop_run (main_loop);
    GST_DEBUG ("Exited main loop");
    g_main_loop_unref (main_loop);
    main_loop = NULL;
    
    /* Free resources */
    g_main_context_pop_thread_default(context);
    g_main_context_unref (context);
    gst_element_set_state (pipeline, GST_STATE_NULL);
    gst_object_unref (pipeline);
    
    return;
}

@end
      其中，接口方法是：
-(id) init:(id) uiDelegate
{
    if (self = [super init])
    {
        self->ui_delegate = uiDelegate;

        GST_DEBUG_CATEGORY_INIT (debug_category, "tutorial-2", 0, "iOS tutorial 2");
        gst_debug_set_threshold_for_name("tutorial-2", GST_LEVEL_DEBUG);

        /* Start the bus monitoring task */
        dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
            [self app_function];
        });
    }

    return self;
}      這個init方法通過調用［super init］來生成實例，保存delegate的對象用來做UI互動，接著調用app_function並運行在一個獨立的並發的線程裡面，app_function會一直監聽GStreamer總線，看看有沒有應用需要處理的消息或者警告發生。
      init:方法同樣注冊了一個新的GStreamer調試類別並設置了吐出的信息的等級，我們就可以在Xcode裡面看到打印信息了。

-(void) dealloc
{
    if (pipeline) {
        GST_DEBUG("Setting the pipeline to NULL");
        gst_element_set_state(pipeline, GST_STATE_NULL);
        gst_object_unref(pipeline);
        pipeline = NULL;
    }
}
      dealloc方法把pipeline置成NULL狀態並釋放它。

-(void) play
{
    if(gst_element_set_state(pipeline, GST_STATE_PLAYING) == GST_STATE_CHANGE_FAILURE) {
        [self setUIMessage:"Failed to set pipeline to playing"];
    }
}

-(void) pause
{
    if(gst_element_set_state(pipeline, GST_STATE_PAUSED) == GST_STATE_CHANGE_FAILURE) {
        [self setUIMessage:"Failed to set pipeline to paused"];
    }
}    play/pause方法僅僅簡單的設置pipeline狀態的該變，並在出錯的時候通知UI
    下面是文件中幾個私有方法：
-(void)setUIMessage:(gchar*) message
{
    NSString *string = [NSString stringWithUTF8String:message];
    if(ui_delegate && [ui_delegate respondsToSelector:@selector(gstreamerSetUIMessage:)])
    {
        [ui_delegate gstreamerSetUIMessage:string];
    }
}      setUIMessage:方法是把GStreamer使用的C的字符串轉變成NSString＊字符串，然後調用GStreamerBackendProtocal協議裡面的gstreamerSetUIMessage:方法來在屏幕上顯示出來。
      因為這個方法是optional的，所以需要用respondsToSelector來判一下是否存在。

/* Retrieve errors from the bus and show them on the UI */
static void error_cb (GstBus *bus, GstMessage *msg, GStreamerBackend *self)
{
    GError *err;
    gchar *debug_info;
    gchar *message_string;
    
    gst_message_parse_error (msg, &err, &debug_info);
    message_string = g_strdup_printf ("Error received from element %s: %s", GST_OBJECT_NAME (msg->src), err->message);
    g_clear_error (&err);
    g_free (debug_info);
    [self setUIMessage:message_string];
    g_free (message_string);
    gst_element_set_state (self->pipeline, GST_STATE_NULL);
}

/* Notify UI about pipeline state changes */
static void state_changed_cb (GstBus *bus, GstMessage *msg, GStreamerBackend *self)
{
    GstState old_state, new_state, pending_state;
    gst_message_parse_state_changed (msg, &old_state, &new_state, &pending_state);
    /* Only pay attention to messages coming from the pipeline, not its children */
    if (GST_MESSAGE_SRC (msg) == GST_OBJECT (self->pipeline)) {
        gchar *message = g_strdup_printf("State changed to %s", gst_element_state_get_name(new_state));
        [self setUIMessage:message];
        g_free (message);
    }
}      error_cb()和state_changed_cb()是注冊的兩個回調，分別在GStreamer出錯和狀態變化的時候被調用。這兩個回調的目的是當事件發生時能通知到用戶。
      這兩個回調函數在Base的教程中出現了多次，實現起來也除了下面2點之外基本一致：一個是消息使用私有方法setUIMessage:來傳遞到UI；第二個是要調用setUIMessage:就需要一個GStreamerBackend的實例，通過callback的userdata來傳遞，這個在下面討論app_function裡回調的注冊時可以看到
-(void) check_initialization_complete
{
    if (!initialized && main_loop) {
        GST_DEBUG ("Initialization complete, notifying application.");
        if (ui_delegate && [ui_delegate respondsToSelector:@selector(gstreamerInitialized)])
        {
            [ui_delegate gstreamerInitialized];
        }
        initialized = TRUE;
    }
}      check_initialization_complete()方法確認滿足所有的條件之後通知UI後台GStreamer准備完成。在這個教程裡面這個條件非常簡單，僅僅是主循環存在並且沒有通知過UI。後續的教程這裡會更加復雜。
      絕大部分的GStreamer的行為都是在app_function裡面實現的，這些代碼和android的教程幾乎一致。

    /* Create our own GLib Main Context and make it the default one */
    context = g_main_context_new ();
    g_main_context_push_thread_default(context);      
這裡第一次創建了一個GLib的上下文，使用g_main_context_new()，然後用g_main_context_push_thread_default()來創建了一個線程





    /* Build pipeline */
    pipeline = gst_parse_launch("audiotestsrc ! audioconvert ! audioresample ! autoaudiosink", &error);
    if (error) {
        gchar *message = g_strdup_printf("Unable to build pipeline: %s", error->message);
        g_clear_error (&error);
        [self setUIMessage:message];
        g_free (message);
        return;
    }      這裡用gst_arse_launch()方法很輕易的創建了一個pipeline。在這個教程裡面僅僅audiotestsrc和autoaudiosink兩個element需要完成適配。



    /* Instruct the bus to emit signals for each received message, and connect to the interesting signals */
    bus = gst_element_get_bus (pipeline);
    bus_source = gst_bus_create_watch (bus);
    g_source_set_callback (bus_source, (GSourceFunc) gst_bus_async_signal_func, NULL, NULL);
    g_source_attach (bus_source, context);
    g_source_unref (bus_source);
    g_signal_connect (G_OBJECT (bus), "message::error", (GCallback)error_cb, (__bridge void *)self);
    g_signal_connect (G_OBJECT (bus), "message::state-changed", (GCallback)state_changed_cb, (__bridge void *)self);
    gst_object_unref (bus);      這幾行創建了一個總線信號的監視器，並和設置了需要監視的信號，這些和Basic教程裡面的做法也是一致的。這個監視器我們這裡是一步一步創建的，並非調用gst_bus_add_signal_watch()來創建，這樣可以看清如何使用GLib的一些內容。這裡需要指出的是使用了__bridge來把一個ObjC的對象指針轉換成C語言裡面的指針。


      這裡我們不需要太擔心對象的所有權的轉移問題，因為在回調裡面userdata會把這個指針帶回來，重新轉換成GStreamerBackend的對象指針


    /* Create a GLib Main Loop and set it to run */
    GST_DEBUG ("Entering main loop...");
    main_loop = g_main_loop_new (context, FALSE);
    [self check_initialization_complete];
    g_main_loop_run (main_loop);
    GST_DEBUG ("Exited main loop");
    g_main_loop_unref (main_loop);
    main_loop = NULL;      最後，主循環創建並開始運行，在進入主循環之前，我們調用了check_initialization_complete()方法。主循環會一直運行，直到退出為止。


      這篇教程有點長了，主要是需要講清楚一系列的基礎內容。在這個基礎之上，後面的會比較短一些，並且會只關注新的內容。



5. 結論

      這篇教程主要講述了：



使用DispatchQueue如何讓GStreamer的代碼單獨運行在子線程中如何在ObjC的UI代碼和GStreamer的C代碼中傳遞對象


      在這篇教程裡面的方法，象check_initialization_complete()和app_function()，init:，play;，pause:，gstreamerInitialized:和setUIMessage:等接口後續會簡單修改一下繼續使用，所以最好熟悉一下。

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